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This is to certify that the

dissertation entitled

Distribution of Cover Crop N Retained by Soil
Aggregates Within a Rye-Corn Agroecosystem.

presented by

Yasemin Kavdir

has been accepted towards fulfillment
of the requirements for

Ph.D. , Soil Biophysics
degree m

 

 

 
 
  

 
 

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DISTRIBUTION OF COVER CROP NITROGEN RETAINED BY SOIL
AGGREGATES WITHIN A RYE-CORN AGROECOSYSTEM

By

Yasemin Kavdir

A DISSERTATION
Submitted to
MICHIGAN STATE UNIVERSITY
in partial fulfillment of the requirements
for the degree
of
DOCTOR OF PHILOSOPHY

Crops and Soil Sciences Department

2000

ABSTRACT

DISTRIBUTION OF COVER CROP NITROGEN RETAINED BY SOIL
AGGREGATES WITHIN A RYE-CORN AGROECOSYSTEM

By

Yasemin Kavdir

Decomposition of rye root and shoots and their contributions to soil N, its
location within soil aggregates, and uptake by succeeding com were monitored
using two different field experiments in a Kalamazoo loam soil. Experiments were
conducted at the LTER Interactions Sites and Microplots at the Kellogg Biological
Station in Southwest Michigan from November 1997 to November 1999. Four
main treatments were considered at the Interactions sites: Conventionally tilled
with N fertilization (CT-F), conventionally tilled with no N fertilization (CT-NF), no
tillage with N fertilization (NT-F) and no tillage with no N fertilization (NT-NF).
Each plot was split in half with a rye cover crop planted on the west one half of
the 16 plots. Sixteen microplots consisted of four treatments: bare fallow (C),
bare fallow with rye shoot (RS), rye root (RR), and rye root plus shoot (RRS).

Rye plants were labeled with 15N by foliar applications of solutions
containing 6.39 g (15NH4)2SO4. 15N enrichments of soil aggregates between 2.0—
4.0, 4.0-6.3 and 6.3-9.5 mm across were determined after residue application.
Concentric layers of aggregates were removed from each aggregate by newly

designed meso soil aggregate erosion (SAE) chambers.

Deep soil samples (to 150 cm) were collected to determine extractable
inorganic N contents. Volumetric soil water contents were measured by time
domain reflectometry (TDR). Non-destructive development of roots was
monitored by minirhizotron technology. Suction Iysimeter samples for soluble N
content. Destructive soil samples were collected to extract roots to determine
root biomass, N and C. Corn yield and rye biomass were monitored each year.

Rye root uptake of residual soil N reduced inorganic N leaching from the
soil profile. During the 2-year study, cover crop management greatly reduced the
quantities of inorganic N lost below the rooting zone of a Kalamazoo loam soil.
Dual spray applications of Roundup herbicide to Roundup-ready com, planted
directly into decomposing rye cover crops, resulted in the contributions of at least
28 kg N per ha (NT-NF) to 41 kg N per ha (NT-F) to the 1998 corn crop.
Reducing nitrogen losses to groundwater beyond the root zone by 27 to 73 kg N
per ha during the year. Leaching losses of N from fertilized CT-F treatments were
60 kg N per ha greater than N losses from NT-F treatments. Negative linear
correlations were observed between extractable inorganic N contents and root
length, volume and surface area in Ap horizons of all treatments. Results
suggested that rye roots reduce N leaching from the soil profile primarily by
absorbing N and plugging macropores reducing leaching losses from the Ap
horizon of a Kalamazoo loam soil.

Rye roots deposited N to the surfaces of soil aggregates more rapidly than
did rye shoots. Concentration gradients of recently derived rye N, within soil

aggregates increased with aggregate size. Recently generated rye plant N was

retained by exterior layers with little accumulating within interior regions of soil
aggregates larger than 4 mm. Yields became more dependent on location of N
within aggregates when soil aggregate size and N gradients increased. Highly
significant correlations were observed between changes in the ratios of N,
comparing N located on exterior layers to N located within interior regions of soil
aggregates, 6.3-9.5 mm across, and corn biomass, r2=0.88 for no cover crop and
0.71 for rye cover crops in 1999. Recovery of rye shoot-derived 15N by com
averaged 8% during this 2-year study. Rye roots contributed nearly 12 kg N ha'1
to the succeeding corn crop and rye shoots contributed nearly 4 kg N ha'1 N to
succeeding com crops at Microplots. Rye roots plus shoots contributed slightly
more than 15 kg N ha'1 during the two-year study.

Soil aggregates from rye cover crop treatments were much more resistant
to erosive forces applied to the external and transitional concentric layers by the
SAE chambers than soil aggregates from no rye cover crop treatments. Erosion
rates of soil layers were reduced 2-10 fold by rye cover crop treatments. Smaller
soil (2.0-4.0 mm) aggregates were much more resistant to erosion than larger
(6.3—9.5 mm across) aggregates. Transitional layers of soil aggregates were
more resistant to erosion than exterior layers of soil aggregates.

In summary, short-tenn contributions of rye root and shoot N can be
identified more rapidly when soil aggregates are peeled into different concentric
layers and analyzed. Nitrogen flux rates and stability changes among the
concentric layers and internal regions were reported within soil aggregates

greater than 4 mm across. These gradients were identified in larger but not

smaller soil aggregates and suggest alternative models are needed for predicting
the formation of soil aggregates. Rye roots successfully reduced nitrate leaching
from a Kalamazoo loam soil. Mineralization of N from decomposing rye roots

supplied three times more nitrogen to succeeding corn plants.

ACKNOWLEDGMENTS

I want to thank to my major advisor, Dr. Alvin J. M. Smucker for giving me
the opportunity to work on this research, for constant assistance and for his
exciting ideas. I thank the members of my committee for their input and
guidance: Dr. Eldor A. Paul, Dr. Richard R. Harwood and Dr. Nathaniel E.
Oamm.

The assistance of John Ferguson, Brian Graft, Greg Parker, Mark
Halvorson, Tom Galecka, Brian Long, Jeff Smeenek, Sven Bohm and John Dahl,
is also greatly appreciated.

I thank my fellow graduate students, especially James Kinyangi, Jose
Sanchez and Drs. Daniel Rasse, Djail Santos, Ayman Suleiman and Ralph
DiCosty.

I thank Dr. Curtis Dell for his ideas and helps during the installing of
instruments to the field and taking soil samples. I thank all undergraduate
students for assisting me in the laboratory and at the field.

I particularly want to thank the Com Marketing Board of Michigan and the
LTER graduate student research grant for their financial support during my
doctoral studies at Michigan State University.

Finally great hug for my parents, my husband “Ismail” and my daughter

”Selin”.

vi

TABLE OF CONTENTS

 

 

 

 

 

 

 

ACKNOWLEDGMENTS -- -- VI
TABLE OF CONTENTS VII
LIST OF TABLES ...... - - . - -- - -- _ - - - .IX
LIST OF FIGURES - -- -- -- ........ XI
CHAPTER 1 _- - -- - l
DISTRIBUTION OF COVER CROP N RETAINED BY SOIL AGGREGATES IN RYE-CORN
AGROECOSYSTEM ........................................................................................................... I
INTRODUCTION ...................................................................................................... 1
REFERENCES .......................................................................................................... 9
CHAPTER 2 -- 14
SOIL N CONSERVATION BY ROOTS AND SHOOTS OF A RYE COVER CROP AND
RELEASED To A SUBSEQUENT CORN CROP ................................................................. 14
ABSTRACT .............................................................................................................. 14
INTRODUCTION .................................................................................................... 16
MA TERIALS AND METHODS .............................................................................. 21
Experimental design and treatments at Interactions site ....................................... 21
Instrumentation at Interactions site ....................................................................... 23
Experimental design and treatments at Microplots site ........................................ 25
Instrumentation at Microplots Site ......................................................................... 26
Plant measurements ............................................................................................... 28
Soil measurements ................................................................................................. 29
Statistical analysis ................................................................................. . ............... 32
RESULTS AND DISCUSSION ............................................................................... 33
Soil nitrogen .......................................................................................................... 33
Cover crop roots .................................................................................................... 54
Rye biomass and nitrogen ..................................................................................... 66
Corn yield and nitrogen ......................................................................................... 68
REFERENCES ........................................................................................................ 71
CHAPTER 3 - - 75
SOIL AGGREGATE SEQUESTRATION OF COVER CROP ROOT AND SHOOT RESIDUE
NITROGEN ...................................................................................................................... 75
ABSTRACT .............................................................................................................. 75
INTRODUCTION .................................................................................................... 77
MATERIALS AND METHODS .............................................................................. 83
Experimental design and treatments ..................................................................... 83
15 N experiment ...................................................................................................... 84

vii

Soil sampling ......................................................................................................... 85

 

Rye root and shoot sampling ................................................................................. 86
Soil and plant analyses .......................................................................................... 87
Aggregate erosion ................................................................................................. 88
Statistical analysis ................................................................................................. 89
RESULTS AND DISCUSSION ............................................................................... 90
Total soil nitrogen (TN) ........................................................................................ 90
Rye root and Shoot derived nitrogen ................................................................... 106
Aggregate erosion rate ........................................................................................ 118
REFERENCES ...................................................................................................... 121
CHAPTER 4 127
COVER CROP ROOT AND SHOOT NITROGEN CONTRIBUTIONS TO SUCCEEDING CORN
CROP IN SITU ................................................................................................................ 127
ABSTRACT ............................................................................................................ 12 7
INTRODUCTION .................................................................................................. [28
MATERIALS AND METHODS ............................................................................ 13]
Experiments with 15N .......................................................................................... 131
Soil sampling and analyses ................................................................................. 133
Rye and corn root and shoot sampling and analyses ........................................... 134
Calculations ......................................................................................................... 135
Statistical analyses ............................................................................................... 136
RESULTS AND DISCUSSIONS ........................................................................... 137
Corn N recovery from rye shoots ........................................................................ 137
Corn N recovery from rye roots .......................................................................... 137
Retention and loss of rye root and Shoot derived N from soil ............................. 146
REFERENCES ...................................................................................................... 15]
SUMMARY AND CONCLUSIONS--- - - _ - - - -- - - - 153

 

viii

LIST OF TABLES

Table 2.1. Distribution and loss by leaching of inorganic N in rye cover and soil
(0-150 cm) of a Kalamazoo loam at the Interactions sites in May 1998 ...... 47

Table 2.2. Distribution, loss or gain (in parentheses) of inorganic N in rye cover
and soil (0-150 cm) of a Kalamazoo loam at the Interactions sites in April
1999. ........................................................................................................... 47

Table 2. 3. Cover crop modifications of total N and C in whole plant corn shoot
responses to tillage and N fertilization of a Kalamazoo loam in July 1998,
n=4. ............................................................................................................. 50

Table 2. 4. Cover crop modifications of total N and C in whole plant com shoot
responses to tillage and N fertilization of a Kalamazoo loam in July 1999,
n=4. ............................................................................................................. 51

Table 2.5. Percentage of root lengths in individual root width classes in Ap, Btl,
Bt2, Cl and C2 horizons of conventionally tilled and fertilized (CT-F),
conventionally tilled and non-fertilized (CT-NF), no tilled and fertilized (NT-F)
and no tilled and non-fertilized, rye cover planted, treatments in a
Kalamazoo loam soil at KBS-lnteractions sites on May 20,
1998.(SE=standard errors, n=4) .................................................................. 56

Table 2.6. Percentage of root lengths in individual root width classes in Ap, Bt1,
Bt2, C1 and 02 horizons of N fertilized and rye cover planted treatment in a
Kalamazoo loam at KBS-Microplots sites on April 11, 1998. (SE=standard
errors, n=4) .................................................................................................. 57

Table 2.7. Root length density, volume density and surface area density in Ap,
Bt1, Bt2, C1 and Cg horizons of conventionally tilled and fertilized (CT-F),
conventionally tilled and non-fertilized (CT-NF), no tilled and fertilized (NT-F)
and no tilled and non-fertilized (NT-NF), rye cover planted, treatments in a
Kalamazoo loam soil at KBS Interactions sites on May 20, 1998. (SE:
standard errors, n=4) ................................................................................... 58

Table 2.8. Root length density, volume density and surface area density in Ap,
Bt1, Bt2, C1 and Cg horizons of N fertilized and rye cover planted treatment in
a Kalamazoo loam at KBS-Microplots sites on April 11, 1998. (SE=standard
errors, n=4) .................................................................................................. 59

Table 2.9. Correlation coefficients (r) between extractable inorganic N content
and rye root length, volume and surface area in Ap, Bt1, Bt2, C1 and C2
horizons of N fertilized (N) and non-fertilized (NF) treatments of a

Kalamazoo loam soil at KBS-lnteractions sites in May 20, 1998, n=8
(p=0.05). ...................................................................................................... 60

Table 2.10. Correlation coefficients (r) between extractable inorganic N content
and rye root length, volume and surface area in Ap, Bt1, Bt2, C1 and 02
horizons of N fertilized and rye cover planted treatment a Kalamazoo loam
soil at KBS-Microplots sites in April 11, 1998, n=8 (p=0.05) ........................ 61

Table 2.11. Dry biomass, N, C and C:N contents of rye in conventional tillage
(CT) and no tillage (NT) plots with nitrogen fertilization (F) and with no
fertilization applied (NF) plots sampled in May 15, 1998 at Interactions sites,
n=4. ............................................................................................................. 67

Table 2.12. Dry biomass, N, C and C:N contents of rye in conventional tillage
(CT) and no tillage(NT) plots with nitrogen fertilization (F) and with no
fertilization applied (NF) plots sampled in April 19,1999 at Interactions sites,
n=4. ............................................................................................................. 67

Table 3.1. Total nitrogen and carbon concentrations and C:N ratios of whole
aggregates, exterior layers, transitional layers and interior regions of
aggregates between 6.3 - 9.5 mm at 0-5 cm depths of a Kalamazoo loam
soil on July 7, 1999 ...................................................................................... 91

Table 3.2 Total nitrogen and carbon concentrations and C:N ratios of whole
aggregates, exterior layers, transitional layers and interior regions of
aggregates between 4.0 - 6.3 mm at 0-5 cm depths of a Kalamazoo loam
soil on July 7, 1999 ...................................................................................... 92

Table 3.3. Total nitrogen and carbon concentrations and C:N ratios of whole
aggregates, exterior layers, transitional layers and interior regions of
aggregates between 2.0 — 4.0 mm at 0-5 cm depths of a Kalamazoo loam
soil on July 7, 1999 ...................................................................................... 97

Table 3.4. 15N concentrations of whole aggregates, exterior layers and interior
regions of aggregates between 2.0—4.0 mm, 4.0 — 6.3 mm and 6.3 - 9.5 mm
at 0-5 cm depths of Kalamazoo loam soil on July 7, 1999. ....................... 116

Table 4.1. Thickness, pH and bulk density of Ap, 8t, and Bt2 horizons at LTER
Microplots at Kellogg Biological Station, Ml ............................................... 143

Table 4.2. Weather data for 1998 and 1999 of LTER Microplots at Kellogg
Biological Station, Ml. ................................................................................ 143

Table 4.3. Contributions of rye shoot, root and root+shoot to corn N contents in
1998 and 1999 at LTER Microplots of a Kalamazoo loam, n=4. ............... 144

Fl

LIST OF FIGURES

Figure 2. 1. Extractable soil mineral nitrogen within the profiles of
no-tilled and N fertilized (NT-F) with rye cover and no cover crop
plots of a Kalamazoo loam, at KBS Interactions sites, in May 20, 1998
and April 21, 1999, (n=4). Values of mm are precipitation for
September-April of each year ........................................................... 34

Figure 2. 2. Extractable soil mineral nitrogen within the
profiles of conventionally tilled and N fertilized (CT-F) with rye cover
and no cover crop plots of a Kalamazoo loam, at KBS Interactions
sites, in May 20, 1998 and April 21, 1999, (n=4). Values of mm are
precipitation for September-April of each year. (n=4). ........................... 35

Figure 2. 3. Extractable soil mineral nitrogen within the profiles of a
Kalamazoo loam in April 11, 1998 and April 29, 1999. Rye and no rye
cover crop treatments were applied to the Microplots site at KBS
(n=4). Values of mm are precipitation for September-April of each year....36

Figure 2.4. Precipitation and temperature recorded at the KBS
LTER weather station from December 1997 to December 1999 .............. 37

Figure 2. 5. N03 leaching from drainage waters of monolith lysimeters
containing conventionally tilled (CT) and no tilled (NT) Kalamazoo loam
without N fertilization since 1991, at the Interactions site, from May 1996
to August 1999. Statistical bars are standard deviations for n=2 .............. 42

Figure 2.6. Distribution of inorganic N in rye cover and soil
(0-150 cm) and leaching from a Kalamazoo loam soil in May 1998.
(A) was conventionally tilled and N fertilized with rye cover;
(B) was conventionally tilled and N fertilized with no rye cover;
(C) was no tilled and N fertilized with rye cover; and
(D) was no tilled and N fertilized with no rye cover at the Interaction
sites ............................................................................................ 43

Figure 2.7. Distribution of inorganic N in rye cover and soil
(0-150 cm) and leaching from a Kalamazoo loam soil in May 1998.
(A) was conventionally tilled and no N fertilized with rye cover;
(B) was conventionally tilled and no N fertilized with no rye cover;
(C) was no tilled and no N fertilized with rye cover; and
(D) was no tilled and no N fertilized with no rye cover at the Interaction
sites ............................................................................................ 44

Figure 2.8. Distribution of inorganic N in rye cover and soil
(0-150 cm) and retention or leaching from a Kalamazoo loam soil in April

xi

1999. (A) was conventionally tilled and N fertilized with rye cover;

(B) was conventionally tilled and N fertilized with no rye cover;

(C) was no tilled and N fertilized with rye cover; and

(D) was no tilled and N fertilized with no rye cover at the Interaction

sites ............................................................................................ 45

Figure 2.9. Distribution of inorganic N in rye cover and soil
(0-150 cm) and retention or leaching from a Kalamazoo loam soil in April
1999.(A) was conventionally tilled and no N fertilized with rye cover;
(B) was conventionally tilled and no N fertilized with no rye cover;
(C) was no tilled and no N fertilized with rye cover; and
(D) was no tilled and no N fertilized with no rye cover at the Interaction
sites ............................................................................................ 46

Figure 2.10. Corn grain yields at KBS Interactions sites in 1998 and 1999. CT
and NT refer to conventional and no tillage, F and NF refer to 150 and 0 kg
N fertilizer per ha. NR and R refer to no rye cover and rye cover crops ..... 53

Figure 2.11. Com grain yields at KBS-LTER in main areas of Microplots
in 1998 and 1999 ........................................................................... 54

Figure 2.12. Relationship between rye shoot biomass and nondestructive
minirhizotron evaluations of manually counted rye root lengths in 0-106 cm
depths of a Kalamazoo loam in Spring, 1999 at KBS Microplots, n=16......63

Figure 3.1. Total nitrogen (TN) concentrations of exterior layers
and interior regions of 6.3- to 9.5 mm soil aggregates from 0-5 cm
depth Of a Kalamazoo loam soil in 1999. Significant differences between
exterior layers and interior regions of aggregates within the same
treatment at the p< 0.05 (*) and p< 0.005 (**) probability levels ............... 93

Figure 3.2. Total nitrogen (TN) concentrations of exterior layers
and interior regions of 6.3- to 9.5 mm soil aggregates from 0-5 cm
depth of a Kalamazoo loam soil in 1998. Significant differences between
exterior layers and interior regions of aggregates within the same treatment
at the p< 0.05 (*) and p< 0.005 (**) probability levels ............................. 94

Figure 3.3. Total soil nitrogen (TN) concentrations in aggregate size fractions
from 0-5 cm depth of a Kalamazoo loam soil in July 1999. Values followed
by the same letter within a treatment and between aggregate size fractions
are not significantly different at p>0.05 according to Duncan's multiple range
test ............................................................................................. 98

Figure 3.4. Relationship between change in the ratio of N concentration of

exterior layer to interior region (Ne /Ni) of 6.3 - 9.5 mm soil aggregates from
July 1999 to September 1999 and corn biomass

xii

at harvest of no- rye cover crop ....................................................... 100

Figure 3.5. Relationship between change in the ratio of N
concentration of exterior layer to interior region (Ne /Ni) of the
6.3 - 9.5 mm soil aggregates from July 1999 to September 1999
and com biomass at harvest of rye cover crop treatments .................... 101

Figure 3.6. Total 15N contents of exterior layers and interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,
shoot, root and root+shoot treatments of the Kalamazoo loam
soil on July, 1999. Significant differences between exterior layers and
interior regions of aggregates within the same treatment at the p< 0.05 (*)
and p< 0.005 (**) probability levels .................................................. 104

Figure 3.7. Total 15N contents of exterior layers and interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,
shoot, root and root+shoot treatments of the Kalamazoo loam
soil on June, 1998. Significant differences between exterior layers and
interior regions of aggregates within the same treatment at the p< 0.05 (*)
and p< 0.005 (**) probability levels .................................................. 105

Figure 3.8 . Total 15N contents of exterior layers and interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,
shoot, root and root+shoot treatments of the Kalamazoo loam
soil on October, 1998. Significant differences between exterior layers and
interior regions of aggregates within the same treatment at the p< 0.05 (*)
and p< 0.005 (**) probability levels .................................................. 110

Figure 3.9. Total 15N contents of exterior layers and interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,
shoot, root and root+shoot treatments of the Kalamazoo loam
soil on September, 1999. Significant differences between exterior
layers and interior regions of aggregates within the same treatment at the
p< 0.05 (*) and p< 0.005 (**) probability levels ................................... 111

Figure 3.10. Concentrations of 15N in aggregate size fractions
sampled from 0-5 cm depths of a Kalamazoo loam soil in July 1999.
Values followed by the same letter within each treatment and
among aggregate size fractions are not significantly different at p>0.05
according to Duncan's multiple range test, n= .................................... 112

Figure 3.11. Percentage of N derived (A) from rye roots (%Ndfr)
and (B) shoots (%Ndfs) in the exterior layers and interior regions of
6.3-9.5 mm soil aggregates from 0-5 cm depth of a Kalamazoo loam
soil in July, August and September 1999. Bars represent standard
deviations for n=4 ........................................................................ 1 17

xiii

Figure 3.12 . Erosion rates of external and transitional layers of 2.0-4.0,
4.0-6.3 and 6.3-9.5 mm aggregates from no-rye (control) and rye
cover cropped treatments of the Kalamazoo loam soil on October 1998
Bars represent standard errors for n=4 ............................................. 120

Figure 4.1. Percentage of 15N recovered from rye shoots, roots and
roots+shoots by com ear, leaf, stem and root at harvest in 1998, n=4 ..... 138

Figure 4.2. Percentage of 15N recovered from rye shoots, roots and
roots+shoots by com ear, leaf, stem and root at harvest in 1999, n=4 ..... 139

Figure 4.3. Partitioning of total recovered N from rye shoots, roots and
roots+shoots by com ear, leaf, stem and root at harvest in 1998, n=4.....139

Figure 4.4. Partitioning of total recovered N from rye shoots, roots and
roots+shoots by com ear, leaf, stem and root at harvest in 1999, n=4 ..... 141

Figure 4.5. Percentage of 15N from 9 shoots, roots and
roots+shoots retained by soil, 1 N remained in residue and
lost from Iysimeter soil profile of a Kalamazoo loam soil at
harvest in 1998, n=4 ..................................................................... 149

Figure 4.6. Percentage of 15N from rye shoots, roots and roots+shoots

retained by soil, 15N remained in residue and lost from Iysimeter soil
profile Of a Kalamazoo loam soil at harvest in 1999, n=4 ...................... 150

xiv

lrr

Elf

CHAPTER 1

DISTRIBUTION OF COVER CROP N RETAINED BY SOIL AGGREGATES IN

RYE-CORN AGROECOSYSTEM

INTRODUCTION

Plants obtain nitrogen from sources of applied N fertilizers, mineralized
soil organic matter (SOM) N, inorganic soil N and, biologically fixed atmospheric
N2. Although mineralization of SOM provides some available N for uptake by
plants, the rate of N mineralization produces inadequate supply of N for
maximizing corn yields. Nitrogen fertilizers provide the majority of plant-available
N, varying from 17 to 80 % depending on rate and time of fertilizer applications
(Meisinger et al., 1985, Karlen et al., 1998). The amount of N fertilizer recovered
by plants at harvest was 25 to 56 °/o (Jokela and Randall, 1997). Therefore,
approximately 50% of applied N remained in the soil and becomes available for
leaching, volatilization and denitrification.

The amount of NOa-N found in the groundwater is often related to the
amount of N fertilizer applied to crops (Hallberg, 1986). Nitrate leaching depends
on soil texture, amount and frequency of precipitation, fertilizer management,
irrigation and N transformations in soils (Smith and Cassel, 1991). Therefore soil
N is best managed using systems that maximize nitrogen (N) fertilizer absorption

efficiency and minimize leaching of NOa-N into groundwater.

Cover crops can be used to improve management systems, which reduce
leaching and build SOM through continuous additions of cover crop residues in
the early spring. Effectiveness of cover crops to reduce leaching depends on its
ability to uptake soil N when the temperature is low. Rye, ryegrass, winter wheat
and rape can uptake residual N during the cold season (Martinez and Guirarud,
1990). Rye, ryegrass and other non-leguminous cover crops remove N03 from
the soil more efficiently than some of the leguminous cover crop (Meisinger et al.,
1991, Shipley et al., 1992, Groffman et al., 1986). Ranels and Wagger (1997)
reported that a rye-crimson clover mixture was capable of recovering greater
residual soil N than only crimson clover monoculture, but less than a rye
monoculture. However, Rasse et al., 1999, reported that alfalfa removed soil
inorganic N efficiently and alfalfa crown and roots contained an average of 115
kg N ha".

Decomposition of cover crop residues (Wagger et al., 1998), residue N
release (Ranells and Wagger, 1992) and uptake of N released by cover crops by
a succeeding crop (Clark et al. 1994, Hargrove, 1986) have been investigated,
however, little attention has been given to root system contributions to these N
mineralizations. These effects and the importance of roots must be considered in
agricultural systems and in nutrient cycles. In general, the root biomass of annual
cover crops are a relatively small portion of the total biomass, but their total
contributions across longer periods of time are very influential and important
forms of C which enhance soil physical, chemical and biological properties and

processes. Rye established early in fall and had greater root development

compared to hairy vetch from November to April (Upendra et al., 1998). These
rye roots can plug macropores in soil profile and reduces N03 leaching during
the rainy season. Ditsch et al., 1993, reported that on a silt loam soil winter rye
was effective cover crop for accumulating large quantities of residual N, NOa-N
and NH4-N derived from fertilizers and native mineralization of SOM.

Rye grows and matures rapidly in spring. Consequently the timing of
spray-killing the rye is a very important management factor. If rye is killed too
early, soil water contents can be conserved, yet intense spring rains can
dramatically leach excess N03 into the groundwater. Spray-killing a rye cover
crop several weeks before no-till corn planting resulted in more N availability to
succeeding com plants (Vaughan and Evanylo, 1998). Late spring applications of
herbicides to a rye cover crop can reduce available soil moisture, produce
excessive biomass which limits seed contact with soil, reducing plant populations
and corn yields. Termination of rye cover crop at the time of soybean planting
resulted in soil water depletion by the rye, which delayed emergence of soybean
(Campbell et al. 1984). Wagger (1989) reported that average rye biomass
increases 39% for every 2 weeks delay. Late desiccation of rye just before no-till
corn planting increased soil, fertilizer and rye N immobilization (Vaughan and
Evanylo, 1998). After termination of cover crop, much of this cover crop N may
be partly returned via plant decomposition and mineralization of plant residues
for subsequent crop uptake (Wagger et al., 1985). Cereal cover crops can serve
as a sink-source for soil N. Decomposition of rye root and shoot residues, their

by- products and living rye roots effect on soil nutrient cycle and aggregate

stabilization and formation. Kladivko (1994), reported that, microbial
decomposition of fresh organic material is one of the main contributors to
aggregate stabilization in the soil. Microorganisms decompose organic residues,
producing polysaccharides and other compounds that bind soil particles together
into aggregates. Recently the availability of Round-up ready corn has provided
another Opportunity for expanding the use of rye cover crop management to
retain more soil N. If only a small boarder of the rye cover along the corn row is
killed at the time of corn planting and rest of the rye remains between the com
rows, living rye roots plug macropores and reduce nitrate leaching from the soil.
After the 2nd or 3rd leaf stage of corn, when com roots start to develop, broadcast
applications of Round-up across the entire plot, develops a strategic timing for
release of cover crop N for the direct utilization by the succeeding corn crop.
Numerous studies on the formation, stabilization, and effect of different
soil and crop management systems on soil aggregation have been reported
(Wood et al., 1991, Roberson et al., 1995). However, there is little information
about the location of recently decomposed plant residues within soil aggregates
(Angers et al., 1997). Growth of plant roots and development of soil aggregates
conversely affect each other. Soil contains cracks and planes of weakness
between aggregates. Roots preferentially grow in these cracks and on the
aggregate surfaces rather than within the aggregates. (Whiteley and Dexter,
1983). The root systems of grasses are extensive and their position is generally
inter-aggregate (Allison, 1973). Living roots influence the chemical and biological

properties of rhizosphere soil (Fisher et al., 1989). Since roots preferentially grow

around the aggregate surfaces, rhizosphere effects are greater on the surfaces
of the aggregates. Roots control the concentrations and fluxes of soil N by
absorbing soil water and soluble N compounds (Harper et al., 1995 and Frensch
et al., 1996). Released N in situ, from decomposing plant roots and shoots
contribute to stabilizing soil aggregation processes (Oades, 1993). Dead roots
act as a readily decomposable SOM and cause increased oxygen consumption
in rhizosphere (Fisher et al., 1989). Root exudates modify the solubility, sorption
and transport of ions to the root surfaces, affect the microbial activity. N is
deposited in the rhizosphere as NH4, N03, and root debris. The amount of N
deposited in the rhizosphere of wheat was up to 20% of total plant N (Janzen,
1990 and Janzen and Bruinsma, 1993). Excreted of plant available N forms and
mineralized N from rhizodeposits may be reabsorbed by the plant.

Clay illuviation, preferential movement of water, weathering of clay and
preferential growth of roots, can change biogeochemical properties of aggregate
surfaces (Smucker et al., 1997, Whiteley and Dexter, 1983, Wilcke and
Kaupenjohann, 1998). Center of aggregates contain less oxygen than the
aggregate surfaces (Sierra and Renault, 1996, Sextone et al., 1985, Hojberg et
al., 1994 ). Therefore nitrogen transformations can be varied between aggregate
interiors and surfaces (Seech and Beauchhamp, 1988)

Recent studies showed that aggregates develop by adding concentric
layers of cations, carbon (Santos et al., 1997, Horn 1990, Smucker et al. 1997,
Dell et al., 1999) and heavy metals (Wilcke and Amelung, 1996). These short-

terrn effects of cropping on SOM can be determined when concentric layers are

removed from soil aggregates. Angers and Mehuys (1989) found that 2 yrs of
alfalfa and barley resulted in 15-25% larger carbohydrate contents compared to
fallow or corn and 46 — 83% more carbohydrates compared to fallow treatment
(Angers and Mehuys, 1990). Six weeks after planting ryegrass seeds, exterior
layers of soil aggregates had 20% and interior region of soil aggregates had 8%
of new C3-C (Santos, 1998). Therefore, under cover crop rotation, recently
derived rye cover crop shoot and root nitrogen should be deposited at greater
concentrations on the surfaces of soil aggregates. To understand cover crop N
contributions to succeeding plant and soil aggregation, N sources and location in
the soil must be identified. In this research, the contributions of roots and shoots
on soil N pool were measured separately.

Labeled rye shoot and root residues can be used in order to distinguish
between soil N and residue derived N. Using stable 15N can be a very effective
tool to determine the location of rye root and shoot derived N in soil and
succeeding plant. 15N stable isotopes have been used in soil-plant systems to
investigate nitrogen transformations (Davidson et al., 1991 ), biological N fixation,
natural abundance (Yoneyama et al. 1990), fertilizer utilization (Angle et al.
1993), mineralization and immobilization (Davidson et al., 1991 , Shen et al.
1984, Stephen and Myrold, 1996), denitrification (Blackmer and Bremner, 1977),
plant uptake (Thomsen, 1997), leaching (Hallberg, 1986 ). Isotopic tracers used
in plant and soil studies should have similar reaction rates as the ion to be
studied (Menzel and Smith, 1983). The 15N stable isotope is ideal for tracing N

through the plant- soil-microbial pathways associated with soil aggregation and

soil nitrogen processes. Field and Iysimeter experiments give more realistic

estimations of N transformations and their direct measurement of N recovery

than laboratory experiments (Lazzari, 1982). Most studies on 15N labeled
residue-decomposition and associated nitrogen transferred to the following crop
have used dried plant shoots, stems and sometimes roots. Some researchers
determined N uptake from root residues by succeeding plants however they first
extracted roots from pots or microplots and then incorporated into the soil (Harris

and Hesterrnan, 1990, and Norman et al., 1990), Hubbard and Jordan, 1996

reported 15N recovery of corn from labeled soil plus wheat root mix and they

could not identify the direct recovery from roots only. Stevenson, (1998) used on
indirect approach to estimate legume root derived N to the succeeding plan. Few
studies used in—situ labeling of plant material such as foliar N-fertilization in the

field (Zebarth et al., 1991, Jordan et al., 1996).

The objectives of this dissertation research were;

1. To develop a two stage herbicide spray killing control of the rye cover crop
and determine N retention within the soil profiles of corn grown in
conventional tillage (CT) and no tillage (NT) management systems of a
Kalamazoo loam.

2. To determine relationships between cover crop root systems and soil mineral
N contents within the profile of a Kalamazoo loam soil.

3. To identify the contributions of rye root and shoot N to different regions of
concentric layers within aggregates ranging from 2.0 to 9.5 mm across in the

AD horizon of a Kalamazoo loam soil.

4. To determine recovery of N from rye roots and shoots by a succeeding corn

crop.

REFERENCES

Allison, F. E. 1973. A factor in soil aggregation and root development. In. Soil
organic matter and its role in crop production. p:314-345.

Angers D. A. and G. R. Mehuys. 1989. Effects of cropping on carbohydrate
content and water-stable aggregation of a clay soil. Can. J. Soil Sci. 69:373-380.

Angers, D. A. and GR. Mehuys. 1990. Barley and alfalfa cropping effects on
carbohydrate contents of a clay soil and its size fractions. Soil Biol. Biochem. 22:
285-288.

Angers, D. A., S. Recous and C. Aita. 1997. Fate of carbon and nitrogen in
water-stable aggregates during decomposition of 13C‘E‘N labeled wheat straw in
situ. European Journal of Soil Sci. 48:295-300.

Angle, J.S., C.M. Gross, R.L. Hill and MS. McIntosh. 1993. Soil nitrate
concentrations under com as affected by tillage, manure, and fertilizer
applications. J. Environ. Qual. 22: 141-147.

Blackmer, AM, and J.M. Bremner. 1977. Nitrogen isotope discrimination in
denitrification of nitrate in soils. Soil Biol. Biochem. 9:73-76.

Campbell, R. B., R. E. Sojka and D. L. Karlen. 1984. Conservation tillage for
soybean production in the US. Southeastern Coastal Plain. Soil Tillage Res.4:
531-541.

Clark, A.J., A. M. Decker and J.J. Meisinger. 1994. Seeding rate and kill date
effects on hairy vetch-cereal rye cover crop mixtures for corn production. Agron.
J. 86:1065-1070.

Davidson, E. A., R.W. Eckert, S.C. Hart and M. K. Firestone. 1991. Direct
extraction of microbial biomass nitrogen from forest and grassland soils of
California. Soil Biol. Biochem. 21 :773

Dell, C. J., Y. Kavdir and A. J. M. Smucker. 1999. Carbon gradients within soil
aggregates of agroecosystems. 4th Eastem Canadian Soil Structure Workshop
Proceedings (in press), Ontario, Canada .

Ditsch, D. C., M. A. Alley, K.R. Kelley and Y2. Lei.1993. Effectiveness of winter
rye for accumulating residual fertilizer N following com. J. Soil Water Consev.
48: 125-132.

IrtCV

Fisher R.W., H. Flessa and G. Schaller. 1989. pH values and redox potentials in
microsites of the rhizosphere. Z. Pflanzenemahr. Bodenk., 152: 191-195.

Frensch, J., T.C. Hsiao and E. Steudle. 1996. Water and solute transport along
developing maize roots. Planta. 198:348-355.

Grofman, PM, RE. Hendrix and DA. Crossley, Jr. 1986. Nitrogen dynamics in
conventional and no-tillage agroecosystems with inorganic fertilizer or legume
nitrogen inputs. Plant Soil. 97:315-332.

Halberg, GR. 1986. From hoes to herbicides: Agriculture and groundwater
quality. J. Soil Water Conserv. 41:357-364.

Hargrove, W.L. 1986. Winter legumes as a nitrogen source for nO-till grain
sorghum. Agron. J. 78:70-74.

Harper, L.A., P. F. Hendrix, G.W. Langdale and D. C. Coleman. 1995. Clover
management to provide optimum nitrogen and soil water conservation. Crop
Science. 35: 1 76-1 82.

Harris, G. H., and O. B. Hesterrnan. 1990. Quantifying the nitrogen contribution
from alfalfa to soil and two succeeding crops using nitrogen-15. Agron. J. 82:129-
134.

Hojberg, O, N.P Revsbech and J.M. Tiedje. 1994. Denitrification in soil
aggregates analyzed with microsensors for nitrous oxide and oxygen. Soil Sci.
Soc. Am. J. 58:1691-1698.

Horn, R. 1990. Aggregate characterization as compared to bulk soil properties.
Soil Till. Res. 17:265-289.

Hubbard, V. C. and D. Jordan. 1996. Nitrogen recovery by com from 15N labeled
wheat residues and intact roots and soil. Soil Sci. Soc. Am. J. 60:1405-1410.

Janzen H. H. and Y. Bruinsma 1993. Rhizosphere N deposition by wheat under
varied water stress. Soil Biol. Biochem. 25:631-632.

Janzen, H. H. 1990. Deposition of nitrogen into the rhizosphere by wheat roots.
Soil Biol. Biochem. 22:1155-1 160.

Jokela, W. E. and G. W. Randall. 1997. Fate of fertilizer nitrogen as affected by
time and rate of application on com. Soil Sci. Soc. Am. J. 61 :1695-1703.

Karlen, D. L., L. A. Kramer and S. D. Logsdon. 1998. Field-scale nitrogen

balances associated with long term continuous corn production. Agron. J.
90:644-650.

10

Kladivko, E. J. 1994. Residue effects on soil physical properties. In: Managing
agricultural residues. Unger P.W. Ed., Lewis Publishers. p:123-141.

Lazzari, M. A. 1982. Distribution of 15N fertilizer in field lysimeters sown with
garlic (Allium sativum) and foxtail millet (Setaria italica). Plant and Soil. 67:187-
191.

Martinez, J. and G. Guiraud. 1990. A Iysimeter study of the effects of a ryegrass
catch crop during a winter wheat/maize rotation, on nitrate leaching and on the
following crop. J. of Soil Sci. 41 :5-16.

Meisinger, J.J., W.L. Hargrove, R.B Mikkelsen, J.R. Williams and V. W. Benson.
1991. Effects of cover crops on groundwater quality. p:57-68. In W. L. Hargrove
(ed). Cover crops for clean water. Proc. Int. Conf. Jackson TN. 9-11 April 1991.
Soil and Water Conserv. Soc. Am., Ankeny, IA.

Meisinger, J.J, V.A. Bandel, G. Stanford and JD. Legg. 1985. Nitrogen utilization
of corn under minimal tillage and moldboard plow tillage. l. Four-year results
using labeled N fertilizer on an Atlantic coastal plain soil. Agron. J. 77:602-611.

Menzel, R.G. and SJ. Smith. 1983. Soil fertility and plant nutrition. Inzlsotopes
and radiation in agricultural sciences vol (1). M. F. L’Annunziata and J. O. Legg
(Ed.). Academic Press, p: 1—35.

Norman, R.J., J.T. Gilmour and ER Wells. 1990. Mineralization of nitrogen from
nitrogen 15 labeled crop residues and utilization by rice. Soil Sci. Soc. Am. J.
54:1351-1356.

Oades J.M. and AG. Waters. 1991. Aggregate hierarchy in soils. Aust. J. Soil
Res. 29:815-828.

Ranells, NM. and MG. Wagger. 1992. Nitrogen release from crimson clover in
relation to plant growth stage and composition. Agron. J. 84:424-430.

Ranells, MN. and MG. Wagger. 1997. Nitrogen 15 recovery and release by rye
and crimson clover cover crops. Soil Sci. Soc. Am J. 61 :943948.

Rasse, D.P., A.J.M. Smucker and O. Schabenberger. 1999.Modifications of soil
nitrogen pools in response to alfalfa root systems and shoot mulch. Agron. J.
91 :471-477.

Roberson, E., S. Sarig, C. Sherman and MK. Firestone. 1995. Nutritional

management of microbial polysaccharide production and aggregation in an
agricultural soil. Soil Sci. Soc. Am. J. 59:1587-1594.

11

Santos, D., S.L.S. Murphy, H. Taubner, A. J. M. Smucker and R. Hom.1997.
Uniform separation of concentric surface layers from soil aggregates. Soil Sci.
Soc. Am. J. 61:720-724.

Santos, D. 1998. Contributions of roots and organic matter to soil aggregate
development and stabilization. Thesis (Ph.D.) Michigan State University. Dept. of
Crop and Soil Sciences. 149 p.

Seech, AG and E.G. Beauchamp. 1988. Denitrification in soil aggregates of
different sizes. Soil Sci. Soc. Am. J. 52:1616-1621.

Sextone, A.J., N.P. Revsbech, T.B. Parkin and J.M. Tiedje. 1985. Direct
measuremens of oxygen profiles and denitrification rates in soil aggregates. Soil
Sci.Soc. Am. J. 49:646-651.

Shen, S.M., G. Pruden and D. S. Jenkinson. 1984. Mineralization and
immobilization of nitrogen in fumigated soil and the measurement of microbial
biomass nitrogen. Soil Biol. Biochem. 16:437-444.

Shipley P. R., J.J. Meisinger and A. M. Decker. 1992. Conserving residual com
fertilizer nitrogen with winter cover crops. Agron. J. 84:869-876.

Sierra, J. and P. Renault. 1996. Respiratory activity and oxygen distribution in
natural aggregates in relation to anaerobiosis. Soil Sci.Soc. Am. J. 60: 1428-
1438.

Smith, S. J. and D.K. Cassel. 1991. Estimating nitrate leaching in soil materials.
p:165-188. In. managing N for ground water quality and farm profitability. R.F.
Follett et al. (ed.). SSSA, Madison, WI.

Smucker, A.J.M., D. Santos and Y. Kavdir. 1997. Concentric layering of carbon,
nitrogen, and clay within soil aggregates from tilled and nontilled ecosystems. D.
Anger (ed.)., Third Eastern Canada Soil Structure Symposium Proceedings.
p:129-141.

Stephen, C and DD. Myrold. 1996. 15N Tracer studied of soil nitrogen
transformations. In: mass spectrometry of soils. (T.W. Boutton and S. Yamasaki,
eds.) Marcel Dekker, lnc.pp: 225-247.

Stevenson F.C., F.L. Walley and C. van Kessel. 1998. Direct vs. indirect
nitrogen-15 approaches to estimate nitrogen contributions from crop residues.
Soil Sci. Soc. Am. J. 62:1327-1334.

Thomsen, l.K, V. Kjellerup. and B. Jensen. 1997. Crop uptake and leaching of N-

15 applied in ruminant slurry with selectively labeled faeces and urine fractions.
Plant and Soil. 197:233-239.

12

Upendra M.S., B.P. Singh and W. F. Whitehead. 1998. Cover crop root
distribution and its effect on soil nitrogen cycling. Agron. J. 90:511-518.

Vasilas, B.L., J. O. Legg, and D. C. Wolf. 1980. Foliar fertilization of soybeans:
absorption and translocation of 15N labeled urea. Agron J. 72:271-275.

Vaughan, JD. and GK. Evanylo. 1998. Corn response to cover crop species,
spring desiccation time, and residue management. Agron. J. 90:536-544.

Wagger, M.G, D.E. Kissel and SJ. Smith. 1985. Mineralization of nitrogen from
nitrogen15 labeled crop residues under field conditions. Soil Sci. Soc. Am. J.
49:1220-1226.

Wagger, M.L., N. Cabrera and N. Ranells. 1998. Nitrogen and carbon cycling in
relation to cover crop residue quality. Jour. of Soil and Water Cons. 53:214-218.

Whiteley, G. M. and AR. Dexter. 1983. Behavior of roots in cracks between soil
peds. Plant and Soil. 74:153-162.

Wilcke, W. and W.Amelung. 1996. Small-scale heterogeneity of aluminum and
heavy metals in aggregates along a climatic transect. Soil Sci. Soc. Am. J. 60:
1490-1495.

Wilcke, W. and M. Kaupenjohann. 1998. Heavy metal distribution between soil
aggregate core and surface fractions along gradients of deposition from the
atmosphere. Geoderma. 83: 55-66.

Wood, C. W., D. G. Westfall and G. A. Peterson. 1991. Soil carbon and nitrogen
changes on initiation of no-till cropping systems. Soil Sci. Soc. Am. J. 55 : 470-
476.

Yoneyama, T, T. Muraoka, N. Boonkerd, P. Wadisirisuk, S. Siripin and K. Kouno.
1990. Natural 15N abundance in shrub and tree legumes, Casuarina, and non-
fixing plants in Thailand. Plant Soil. 128:287-294.

Zebarth, B.J., V. Alder and R.W. Sheard. 1991. In situ labeling of legume

residues with a foliar application of a 15N enriched urea solution. Commun. In Soil
Sci. Plant Anal. 22 2437-447.

13

CHAPTER 2

SOIL N CONSERVATION BY ROOTS AND SHOOTS OF A RYE COVER

CROP AND RELEASED TO A SUBSEQUENT CORN CROP

ABSTRACT

Effects of rye root and shoots and their time of desiccation on soil
inorganic N were monitored in a Kalamazoo loam soil. Experiments were
conducted at Microplot and Interaction Sites of Kellogg Biological Station in
Southwest Michigan from November 1997 to November 1999. Four treatments
were used at Interactions sites: Conventionally tilled and N fertilized (CT-F),
conventionally tilled and non fertilized (CT-NF), no tilled and N fertilized (NT-F)
and no tilled and non fertilized (NT-NF). Microplots consisted of four treatments:
bare fallow (C), bare fallow with rye (Secale cereale L.) shoots applied as surface
mulch (RS), in situ rye roots (RR), and in situ rye roots plus shoots applied as a
surface mulch (RRS). Soil samples were collected to depths of 150 cm for
extractable inorganic N contents. Volumetric soil water contents were measured
by time domain reflectometry (TDR). Non destructive developments of roots were
monitored by minirhizotron technology. Soil solution samples were collected by
suction lysimeters.

Rye root and shoot uptake of residual soil N reduced inorganic N leaching

from the soil profile. During the 2-year study, cover crop and tillage influenced the

14

amounts of inorganic N lost below the rooting of the Kalamazoo loam soil. Dual
spray applications of herbicide to Roundup-ready com, planted directly into
decomposing rye cover crops, resulted the cover crop contributions of at least 28
kg N per ha (NT-NF) to the successive corn crop reducing nitrogen losses to
groundwater below the root zone by 27 to 73 kg N per ha during the entire year.
N leaching from CT treatments was 60 kg ha'1 y’1 greater than those from NT
treatments when they were fertilized with N. Leaching losses from non-fertilized
CT-NF treatments were 27 kg N ha'1 y’1 greater than those from NT-NF
treatments. Using rye cover crops between two successive com crops had no

significant effects on corn yields.

15

INTRODUCTION

Plants obtain nitrogen from sources of applied N fertilizers, mineralized
soil organic matter (SOM) N, inorganic soil N and, biologically fixed atmospheric
N2. Although mineralization of SOM provides some available N for uptake by
plants, the rate of N mineralization produces an inadequate supply of N for
maximimum corn yields. Nitrogen fertilizers provide the majority of plant-
available N, varying from 17 to 80 % depending on rate and time of fertilizer
applications (Meisinger et al., 1985, Karlen et al., 1998). The amount of N
fertilizer recovered by plants at han/est was 25 to 56 % (Jokela and Randall,
1997). Therefore, approximately 50% of applied N remained in the soil and
becomes available for leaching, volatilization and denitrification.

Increasing quantities of N03 have been accumulating in the groundwaters
of Michigan during the past 3 or 4 decades, causing groundwater wells in
Michigan to exceed 10 mg N03 L", the maximum concentration permitted for
potable water by the EPA. In Kalamazoo County, Michigan water sampled from 6
of 46 wells contained NOa-N levels above the EPA limit of 10 mg L'1 (Rheaume,
1990). In Van Buren County, 22% of the wells tested had nitrate concentrations
that exceeded the EPA limit. Nitrogen inputs in Van Buren County were deemed
to be 72.7% from fertilizer, 21.3% from precipitation, 4.5% from animal wastes,
and 1.5% from septic tanks (Cummings et. al., 1990). Water containing nitrate

concentrations greater than 10 mg L'1 is considered unsafe for infants, including

16

wt

for use in mixing of baby formulas. Nitrate causes methemoglobinemia, the
cause of bluish coloring in babies and may be fatal unless properly treated.

Quantities of NOa-N in the groundwater are often directly related to the
rates and timing of N fertilizer applications, soil texture and tillage of nearby soils
(Hallberg, 1986; Meisinger et al., 1985), the amounts and frequencies of
precipitation, crop rotation (Weed and Kanwar., 1996), as well as irrigation
scheduling and N transformations in the soil (Smith and Cassel., 1991).
Reductions in the application of N fertilizers are often viewed as a major cause
for lower crop yields. Therefore, the best management of soil N is when crop
production systems efficiently utilize added N with minimal leaching of NOa-N into
groundwater.

Rapid growth of cold-tolerant cover crops improves the retention of soil N
in cash cropping systems. Roots of actively growing rye or wheat cover crops
reduce leaching of nutrients and build soil organic matter through the addition of
residues (Kuo et al., 1997). They also reduce surface crusting, increase
infiltration thereby decreasing erosion of surface soils, especially in the early
portion of the Spring (Kessavalou and Walters, 1999). Rye, ryegrass, winter
wheat and hairy vetch uptake residual N during the rainy cold seasons between
cash crops in Europe (Martinez and Guirarud, 1990), the Atlantic coast of the
US. (Shipley et al., 1992), in Eastern Canada (Raimbault et al., 1991), and many
midwestem regions of the US (Vaughan and Evanylo, 1998). Ditsch et al., 1993,
reported that on a silt loam soil, winter rye (Secale cereale L.) was an effective

cover crop for accumulating both NOa-N and NH4-N which were N residues of N

17

 

IE

TC

bl

tir

SE

r e

fertilizers. Therefore, the utilization of cereal cover crops between cash crops are
of great interest to farmers in Michigan. Rye appears to be the most suitable
cover crop for Michigan soils because it can be easily established late in the Fall
and it is resistance to the long harsh winter climate.

Legume and non-leguminous crops have been reported to successfully
uptake residual soil N between cash crops. Rasse et al. (1999) demonstrated,
that once established, alfalfa removed nearly all nitrates from 0-60 cm profiles of
Kalamazoo loam soils in southern Michigan. Cereal grasses, which can be
established more easily than legumes, have been reported to be more efficient in
the uptake of residual soil N than legumes when both have been compared as
winter cover crops (Meisinger et al., 1991, Shipley et al., 1992, Groffman et al.,
1986). McCracken et al., (1994) reported that rye reduced N03 leaching by 94%,
compared with 48% for hairy vetch. Nitrogen contents of small grain cover crop
residues varied ranging from 25 to 50 kg N ha'1 (Reeves, 1994).

Rye quickly established in the fall and had greater root development
compared to hairy vetch from November to April (Upendra et al., 1998). These
root systems can plug some of the macropores throughout the soil profile and
both uptake and plugging reduces water flow and N03 leaching.

Rye growth and biomass production is rapid in early spring. Strategically
timing the killing of rye is a very important management factor. If rye is killed
early, soil moisture can be conserved. However, if rye is killed too early
excessive spring rains will enhance N03 leaching. Untimely late killing of rye

results in soil moisture losses for succeeding cash crops. Excessive production

18

on

the

kllle
lea:
Dlac
inma
Provi

SOII s

of cover crop shoot biomass will interfere with seed to soil contact at planting,
reducing seed germination (Mehdi et al., 1999). In sub-humid and humid regions
cover crops should be permitted to grow until they produce sufficient above
ground biomass to cover the soil surface and maximize root proliferation, yet
terminated early enough to maintain adequate soil profile water storage prior to
planting the next crop (Unger and Vigil, 1998). In Michigan, cover crop residues
on the surface must be managed so that they do not reduce soil temperature but
provide increased conservation of surface soil well into the growing season of the
next crop.

Spring suppression of rye by applications of small quantities of glyphosate
herbicides resulted in higher corn dry matter (44%) at harvest than when the rye
biomass was not chemically killed (Morris et al., 1998). Recently the availability
of Round-up ready corn has provided another Opportunity for expanding the use
of rye cover crop management to retain more soil and cover crop biomass N. If
only the rye cover on the com row is killed at the time of corn planting and rest of
the rye remains viable between the corn rows until the entire rye cover crop is
killed by Roundup. These living rye roots between rows, reduce soil nitrate
leaching by plugging soil macropores and absorbing soil nitrates. Strategic band
placement of herbicide provides continued protection of soil between the rows,
initiates slow N release of biomass N from the rye cover crop at the row and
provides continuous N absorption by the rye cover across more than 65% of the

soil surface between rows.

19

These concepts led us to develop a two-year field experiment with the
following objectives:1)To develop a strategic two-stage herbicide spray-killing
control of the rye cover crop in N-fertilized conventional tillage (CT) and no tillage
(NT) management systems of a Kalamazoo loam.

2)To identify dependent relationships between rye cover crop root

systems and soil mineral N contents within the profile of a Kalamazoo loam soil.

20

Exp

Tree
Tree
2) r
lenil
llllag
nalu

Spill

cere
ol 8‘
rate
blom
incor

bloat

(Spat

May

MATERIALS AND METHODS

Experimental design and treatments at Interactions site

Field treatments were incorporated into 16 plots, 40 x 27 m, of the
Interactions site on a Kalamazoo loam soil (coarse-loamy, mixed, mesic Typic
Hapludalf), established in 1986 at the KBS/LTER site near Kalamazoo, Michigan.
Treatments were replicated four times in a randomized complete block design.
Treatments consisted of: 1) conventional tillage and nitrogen fertilization (CT-F),
2) no tillage and nitrogen fertilization (NT-F), 3) conventional tillage and no
fertilization (CT-NF), and 4) no tillage and no fertilization (NT-NF). Conventional
tillage (CT) plots were moldboard plowed, and two of the associated four large
natural lysimeters (Rasse, 1997) were manually tilled to 20 cm by a shovel in the
Spring of 1998 and 1999.

Each field plot (27 x 40m) was split in half. Cereal grain rye (Secale
cereale, L.) was drilled in the west one-half (13.5 x 40 m) of each plot at the rate
of 81 kg ha’1 in the Spring of 1998 and broadcast applied at the same seeding
rate in the Fall of 1998. Following deep sampling of the soil profile and rye
biomass measurements, conventional tillage plots were plowed on May 5, 1999
incorporating the cover crop into the soil. Urea nitrogen fertilizer (46-0-0) was
broadcast applied at the rates of 150 kg N ha‘1 to the 8 fertilized (F) plots on May
7, 1999. Then CT plots were disced and cultivated following fertilizer application.

Roundup-ready corn, Dekalb 493 (Zea mays, L.), was planted in rows
(spaced at 70 cm) at the rate of 71,136 seeds per hectare on June 6, 1998 and

May 9, 1999. Immediately following corn planting, narrow strips (25 cm wide)

21

lir
w
W.”
86

CC

ar
lee
US

dl‘

By

an
ha

were sprayed in both the CT and NT treatments and rye plants in the corn rows
were killed with these band spray applications of Roundup (glyphosate) Ultra,
without ammonium sulphate (4.5 L ha"). Roundup (4.5 L ha") and Atrazine (4.5
L ha") were broadcast sprayed, using 187 L ha’1 of water, to kill the remaining
rye cover crop on each plot. Herbicide applications were split in two different
times in 1998. Half of the rye planted plots (9 com rows in an area 6.75 x 40 m)
were broadcast sprayed at the 2nd leaf stage of corn growth and the other half
was sprayed at the 4th leaf stage of com. Rye was taller than the young com
seedlings as it had begun heading at the 4th leaf stage of com. Soil inorganic N
contents at the 2nd and 4th leaf stages did not change significantly. In order to not
risk corn yield reductions during the second year, band strips, 25 cm wide, of rye
were killed at com planting with applications of Roundup Ultra, without
ammonium sulphate (4.5 L ha") and all plots were broadcast sprayed at the 2nd
leaf stage of corn growth with Roundup (4.5 L ha") and Atrazine (4.5 L ha"),
using 187 L ha'1 of water. Rainfall during the months of May was 119 mm in 1998
and 153 mm in 1999. Summer rainfall in June and July of 1998 were 178 mm,
well below the 30-year average (193.04 mm). In 1999, June and July totals were
even lower, 157 mm at KBS. Consequently, 38 mm of irrigation water were
applied to all plots by an overhead traveling gun on July 1999. Com crops were

harvested on October 28, 1998 and October 27, 1999.

22

in:
60

TD

Instrumentation at Interactions site

Two suction lysimeters (Model 1900, Soil Moisture, Santa Barbara, CA)
equipped with porous ceramic cups with a 1 bar air-entry value, 4.8 cm diameter
and 91 cm long, were installed into the middle of each Bt2 horizon, averaging 60
cm deep, at 45° angles to the soil surface. Suction lysimeters were located in the
7th or 8th row, at 2 meter spacings, of each fertilized NT-F plot in 1998 and in
each fertilized NT-F and CT-F plot in 1999. Vacuum (~ 20 inches of mercury)
was applied to each suction Iysimeter 24 to 30 hours before soil solutions were
sampled, by using either manual or electrical vacuum pumps. Solution samples,
ranging from 10 to 20ml, were extracted and stored in plastic scintillation vials at
4°C. N03 and NH4 contents were analyzed by the cadmium reduction method
using a OuickChem Automated Ion Analyzer (Lachat Instruments, West Mill
Road, Milwaukee, WI).

TDR probes were inserted horizontally into each Btz horizon (at the same
depth as the suction lysimeters) in rye and no rye plots of all 16 plots. Horizontal
installations of 20 cm TDR probes were facilitated by digging small trenchs, 50 to
60 cm in width and between the corn rows, to the middle of each Btz horizon. The
TDR probes were inserted horizontally, directly below the com row by pushing
probe into the soil immediately following planting. The cable of each TDR probe
was brought to the soil surface and soil profiles were recompacted, horizon by
horizon, to approximately their original densities. The TDR probes consisted of
three stainless steel wave guides (0.5 cm diameter x 20 cm length) and were

constructed at the Soil Biophysics Laboratory, Michigan State University, MI,

23

USA, as described by Huang (1995). Cable tester meter readings were
collected from the TDR cables at the soil surface using a Tektronix cable tester
(Model 15020, Tektronix Inc, Beaverton, Oregon, USA). Volumetric soil water
contents were calculated using the equation by Topp et al. (1980) as described
below:
Qv= [-5.3 x 10'2 +(2.92 x 10'?- *Ka)-(5.5 x 10‘4x K32)+(4.3x10'6 x K83)] x 100 [1]
Where:
Ka is the apparent dielectric constant and Ka=ct/L2,

t is the signal travel time in nanoseconds,

t = (B-A)/(Vp x c),

c = Propagation velocity (Vp) of an electromagnetic wave in free space

and c: 30 cm/nsec,

Vp = 0.99.

L = Length of the transmission line or waveguide probe (20 cm)

A = Distance in feet, of the TDR cable from the TDR wave guide probes

to the pulse generator,

B = Distance in feet that the reflected pulse is from the pulse generator.

Root growth, demographics and dynamics were monitored in situ by
minirhizotrons (Upchurch and Ritchie, 1983; Smucker et al., 1987). Two
minirhizotron tubes (0.05 x 2.4 m) were inserted in the soil, directly under the
corn row, to depths of 106 cm at 45° angles in both rye and no rye cover crops of

the 8 fertilized plots. The objectives of this study did not require the installation of

24

Eacl

prev,

MR tubes into non-fertilized (NF) plots. Micro-video color camera (Bartz
Technology 00., Santa Barbara, CA), equipped with an index handle and color
monitor, were used to video record root images intersecting the upper 1.8 x 1.35
cm region at each stop along the MR tube. Root images were digitized and
processed by the computer algorithm MR-RIPL, version 3.0, using a Sun Ultra
2.0 in the Root Image Processing Laboratory (RIPL), at Michigan State

University. lntemet address of the site is http://www.rootdig.css.msu.edu.

Experimental design and treatments at Microplots site
A two-year field experiment (1997-1999) was conducted on 16 microplots
(6 by 10m) established in August 1994 (Rasse, 1997) on a Kalamazoo loam soil
(coarse-loamy, mixed, mesic Typic Hapludalf) at the KBS/LTER site in
southwestern Michigan. There were four treatments:
1) Bare soil control (C);
2) Bare soil fallow, where rye shoots were applied as soil surface mulch,
before corn planting (RS);
3) Rye cover crop roots, where shoots were cut and removed and roots
remained in situ in the soil (RR);
4) Rye cover crop roots and shoots, where rye shoots were cut and
returned to the soil surface as a mulch and the roots remained in situ in the
soil (RRS).
Each treatment was replicated four times in a randomized block design. The

previous crop from August 94 to April 97 was alfalfa (Rasse, 1997). The alfalfa

25

was spray-killed with a Roundup Ultra application in April 1997 and plots were
maintained plant free, by two additional applications of Roundup Ultra during the
successive seven months. Rye was seeded at the rate of 81 kg ha'1 by
broadcasting seed onto the soil surface in late October 1997 and at the rate of
162 kg ha'1 in mid September 1998. Following a severe open winter, an
additional seeding of rye was broadcast applied on April 7, 1998 to better
establish a complete rye cover crop on each plot, at the rate of 81 kg ha". Finely
ground limestone was applied to all plots at the rate of 2 tons/ha on April 4, 1998.
No limestone was applied in the Spring of 1999.

All plots were broadcast sprayed with Roundup (glyphosate) Ultra without
ammonium sulphate (4.5 L ha") ,mixed with water using 186 L ha'1 to kill cover
crops and weeds approximately two weeks before com planting.

Corn was planted at the rate of 64,220 seeds ha", at 70 cm row spacing,
on June 6, 1998 and May 17, 1999. When corn growth was at the fifth leaf stage,
the 16 main plots, except the two 15N lysimeters in each plot, were side dressed
with of 150 kg N ha’1 of NH4N03. In July 1999, 38 mm of irrigation water were
applied to all plots using a traveling gun sprinkler irrigation system. Corn was

harvested in September of 1998 and 1999.

Instrumentation at Microplots site
Non destructive sampling of soil solutions, using in situ instruments of 3
suction lysimeters, soil water by 3 TDR probes and root dynamics by video

recording 2 MR, were completed on a regular basis using these instruments which

26

had I
Poly
inste
qua:
Des
Cor
plol
Ge;

IUI

had been previously installed in NE quarter of each plot by Rasse (1997).
Polyvinyl chloride (PVC) cylinders (30 cm in diameter and 60 cm in depth)
installed to soil depths into the middle each Btz horizon (~60 cm) in the NW
quarter of each plot, served as lysimeters for containing the 15N labeling study.
Destructive deep profile soil sampling was limited to the SW quarter of each plot.
Corn yields samples were taken from the SE quarter of each plot. Each of the 16
plots were isolated by a black garden boarder surface plastic barrier, installed to
depths of 10 cm with 5 cm remaining above the soil surface to prevent run-off and

run-on between plots. A one-meter border was reserved around each plot.

Each field plot was equipped with three suction lysimeters were installed
to soil depths of 15, 35 and 60 cm at 45° to the soil surface. Water samples were
collected from suction lysimeters 24 to 30 hrs following application of vacuum as
described above. Samples were stored and analyzed for N03 and NH4 as
described above.

Soil water contents of each horizon were estimated by TDR wave-guides
installed at the same depths as the three suction lysimeters, as described by
Rasse, (1997). Root dynamics were monitored nondestructively and image
processed by the RIPL, as described above. Since MR tubes were installed
before (Rasse, 1997) each rye planted plots had 3 MR tubes and each no-rye
planted plots had 1 MR tube. In this experiment an additional MR tube was
installed in the bare plots to obtain duplicate root images in the C and RS plots.

Root images were taken from the 2 MR tubes from each field. Root surface areas

27

W;
ar

19

and volumes were calculated as a sum of all areas and volumes for all width

classes. Formula [1] was used to calculate root surface area:

A: 21r[(r1x l1) + (r2 x l2) +...(r5 x l5)] [1]
Where:
A is the root area (cm2),
Rn = are the respective root radii (cm) for the five root width classes,
Ln = total length of root segments for each width class (cm).
The formula used to calculate root volume was:
v = 1r[(r12x l1) + (r22 x 12)+ ..... (r52 x15)] [2]
Where:
V is the volume (cma),
r = root radius (cm) for each width class,

I = total root length of root segments for each width class (cm).

Plant measurements

Interactions site Rye subsamples (1 m2) were harvested prior to corn
planting, oven dried at 70° C and rye biomass was calculated per ha. Corn plants
were sub-sampled to determine total dry biomass of each of the 32 plots from an
area of 2.1 m2 in July 1998 and 1999, dried and weighed and dry corn biomass
was calculated per ha. Total corn biomass (stalk + ear) for subsamples (2.1 m2)
and grain yield for each plot were determined for in early September and at the

1998 harvest. Total corn biomass (stalk + ear) for subsamples (10.64 m2), and

28

grair
199$
take
ma‘
0.5
we
ca

dr

grain yield for each total plot were also determined in early September and at the
1999 harvest. To determine water content of the grain, 500 g subsamples were
taken from each of the 32 plots and oven dried at 70°C. All rye and corn plant
materials were oven dried at 70°C. Plants were finely ground to pass through a
0.5 mm screen, mixed thoroughly and subsamples ranging from 5 to 7 mg were
weighed to 5 decimal places and recorded before transferring into small tin
capsules and placed into the autosampler. Total C and N were determined by the
dry combustion method (Kirsten, 1983) using a C/N/H analyzer (Model NA 1500,

series 2, Carlo Erba Stumentazione, Milano, Italy).

Microplots site: Rye subsamples (1 m2) were harvested prior to corn
planting and rye biomass per he was calculated. Total corn biomass (stalk + ear)
and grain yields were subsampled from the middle 2.1 m2 of each plot
immediately before each of the 1998 and 1999 harvests. Aboveground biomass,
C and N contents of the com grain and stalks plus leaves were determined as

described above.

Soil measurements

Interaction and Microplot m Soil samples were taken from the main
field plots of both experimental sites to depths of 150 cm in the early spring, mid
summer and fall, following corn harvests in 1998 and 1999, using a hydraulic
probe which forced a metal core (8.9 cm diameter) vertically into the soil

(Giddings Machines Co., Ft. Collins, CO). Two core samples were taken from

29

ea

lal

each plot. One sample was taken from the com row and another sample was
taken adjacent to first sampling row, but midway between the rows. Soil cores
were divided into Ap (average thickness of this horizon was 31 cm), Bt1 (~18 cm),
Btg (~13 cm), C1 (~52 cm) and 02 (~37+) horizons. Each core was vertically
divided into two halves. One half of each sampled horizon was stored at 4°C until
roots were washed free of soil and other half was used for analyses of inorganic
N extraction. Rye and com roots were extracted from the soil matrix by
hydropneumatic elutriation (Smucker et al., 1982). No effort was made to neither
identify nor separate roots from rye or com. Subsamples of extracted roots (~30
mg) were taken for total N and C analysis. Subsamples were oven dried at 70°C
for 24 hrs. Roots were finely ground to pass through a 0.5 mm sieve, mixed
thoroughly and sub samples (5 to 7 mg) were weighed into small tin capsules
and placed into the autosampler. Total C and N was determined by the dry
combustion method of Kirsten (1983) using a C/N/S analyzer (Model NA 1500,
series 2, Carlo Erba Stumentazione, Milano, Italy). The remaining portion of the
washed root samples were stored in labeled Whirlpack plastic bags at 49C
containing 20% (v/v) methanol solution. To quantify their morphological
parameters, roots were uniformly distributed on a clear plastic square (9.4 x 9.4
cm) Petri dish. To avoid overlapping roots, large root samples were split into 2 or
more dishes before scanning. Surfaces of roots were covered with a thin water
film, distributed evenly across the Petri dish for image recording on a flatbed
digital scanner (Model 6300 C, Hewlett Packard). Images were digitized at 200

dpi resolutions. Scanned root images were processed by WR-RIPL, version 2.0

30

rc
SIl

80

Au
WI
Wer

Calc

using the Sun Ultra computer in the Michigan State University RIPL
(http://www.rootgi_g.css.msrflfl). Root length was calculated using an image
resolution of 78.74 pixels per cm (for 200 dpi resolution only) by the image
processing system. Surface area and volume were calculated using equations [1]
and[2j

Soil subsamples were weighed at their field water contents and mineral N
was extracted from field moist samples. Approximately twenty grams of moist soil
were sampled from the one-half each of the two hydraulic core samples from
each horizon sampled in each field plot, placed in a 250 ml Erlenmayer flask
containing 50 ml of 1 N KCI extraction solution and shaken to equilibrium on a
rotary shaker for one hour. Clear solutions were expressed by filtering the soil
slurry through Whatman No.1 filter papers, folded and placed in funnels. Clear
soil sample solutions were stored in 20 ml scintillation vials at 4°C until analyzed
for N03 and NH4 by the cadmium reduction method using a OuickChem
Automated Ion Analyzer (Lachat Instruments, 6645 West Mill Road, Milwaukee,
WI). Average concentration values of NOa-N plus NH4-N (mg g'1 oven dry soil)
were combined with soil bulk density for each horizon, adjusted to air dry soil to

calculate kg N per ha using the following equation.

Kg N ha": (mg N kg") x (b.d x1000) x (10000) x (thickness / 100) [3]
1000000

 

b.d = Bulk density of soil horizon (t m'°),

Thickness = Thickness of soil horizon (cm)

31

Statistical analysis

Plant and soil parameters were analyzed by a PROC-GLM (General
Linear Models) procedure using Statistical Analysis System (SAS Institute, 1999,
Cary, NC). SAS codes for ANOVA were generated by using code generator
program (fig/lwwwcasvtedu/schabean ). Average of two subsamples of soil N
in each plot was determined and standard deviations of means were calculated
for four field replications. Fishers LSD test was used to separate means of
measurements. Correlation analyses was used to determine relationships
between root, plant and soil parameters. Analyses were conducted separately for

1998 and 1999. All significant tests were set at Probability levels of at least 0.05.

32

a:

CC

du

RESULTS AND DISCUSSION

Soil nitrogen

Soil nitrogen contents in 1998 were higher in the AD horizons when NT-F
treatments had a rye cover crop. This trend was reversed in 1999 when fall
through early spring precipitation was 38% less (Figure 2.1). Cover crops
appeared to have a smaller affect on soil N contents in the AF, horizons of CT-F in
1998 (Figure 2.2). Lower fall through early spring precipitation and a better winter
and early spring rye cover crop in 1999 lowered soil N contents in the Ap horizon
from 50 kg N per he to 25 to 30 kg N per ha regardless of soil tillage (Figures 2.1
and 2.2). Rye cover crops modified the distributions of N within the soil profiles Of
conventional tillage treatments (Figure 2.2) as more N accumulated in the Btg
and C1 horizons when rye cover treatments were present during the wetter year
in 1998, but these two deeper horizons as well as the shallower horizons
contained lower N during the drier year in 1999.

Less N leaching from the Ap horizons of no rye treatments resulted in 1999
due to nearly 40% less precipitation during the fall through early spring compared

to the same period of time in 1998 (Figures 2.1- 2.4).

33

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1998
Ap ~ ........... O
"""" NT-F
811 a
2
O Bt2 a
.5
t-
2 C1 -
.= + No e
o ry 505 mm
0 5 10 1 5 20 25 30 35 4O 45 50 55 60
1999
Ap -
W
= - NT-F
O 811
.5
‘6
.C 812 r
'5
ca 01 -
----o Rye 312mm
02 -
0 5 1 0 1 5 20 25 30 35 40 45 50 55 60
Soil extractable NH,,+NO3 (kg ha")

Figure 2. 1. Extractable soil mineral nitrogen within the profiles of
no-tllled and N fertlllzed (NT-F) with rye cover and no cover crop
plots of a Kalamazoo loam, at KBS Interactions sites, in May 20, 1998

and April 21, 1999, (n=4). Values of mm are precipitation for
September-April of each year.

34

I. h sic c d

Bri-

Soll horizons

Soil horlzons

 

Ap-

BE!-

1

Cd

C2:-

 

1 998

CT-F

 

505 mm

 

 

 

 

 

 

5 10 15 20 25 30 35 40 45 50 55 60

 

Ap'~

BU -

 

1 999

CT-F

 

312 mm

 

 

 

 

 

 

5 10 15 20 25 30 35 40 45 50 55 60

Soil extractable NH 4+NO3 (kg ha'1)

Figure 2. 2. Extractable soil mineral nitrogen within the profiles of

conventionally tilled and N fertilized (CT-F) with rye cover
and no cover crop plots of a Kalamazoo loam, at KBS Interactions

sites, in May 20, 1998 and April 21, 1999, (n=4). Values of mm are
precipitation for September-April of each year. (n=4).

35

Soil horizons

Soll horizons

 

Ap-

811 ~

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

BIZ ‘
C1 5
C2 *
0 10 20 30 40 50 60 70 80 90 100 110 120
1999
AP ‘ + No Rye
Bt2 - "0..
CI a
C2 -* O"
0 10 20 30 40 50 60 70 80 90 1 00 1 I 0

Soil extractable NH4+NO3 (kg ha‘1)

Figure 2. 3. Extractable soil mineral nitrogen within the profiles of

a Kalamazoo loam in April 11, 1998 and April 29, 1999. Rye and no
rye cover crop treatments were applied to the Microplots site at KBS
(n=4). Values of mm are precipitation for September-April

of each year.

36

 

120

30

25

20

Gov OLBEOQES cues.

 

 

 

- Precipitation

— Temperature

 

 

23%..
......mswmimmmwwwyfi .. ...,
. 535133.333

3.3.5.... @3333 ..

 

.mx.._w,u.w.._:..§ , ..\ns&www\w§a , , . ”maxim...

.. . t. . h/ U... ... . A . . 34V... ...\ . 5.}...
.. , ...3........ , goxmnmwgfi .

 

1.x.< : tax . .
. x 43....3&3 . $.9ch. v

. \ \ .4 .
33%.. 33,. ,§§ ... .u,

u... ... .. .xis . . \XI(XV~ .\ .. .
. 13...... .
..A...x....n. at, gxfifg

.... ANA».

A; . .
r}.
3:: -.
92R .3.- - ._
\‘ ’, "s , ‘ '
::~:- «:61 >3»:
'. '. 3. \ .
’ -\-.
‘;\
,.
1":
‘.'. '1' l'.‘. 3‘. .
’ '- s?- ,- '
. .:~ .‘ >23. \‘ ’

>.32>§.. 3.33s? 33m.
|
.... ....H“MES.fifiggmfiwfimg.«H “...”...

../.. 3 . 9.? ....x. .3 .. \/ (\Iv / \v
..........i.. . x. ...C..\.\. x...

.........

:.‘-2\'
“.1 52'.
,3. :i x:
. V
3 <5:
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180

160 '-

140 -

120 ~

- u u

.m. m .0.
ES 3.5.3.85

Dec-97 Mar-98 Jul-98 Nov-98 Mar-99 Jul-99 Nov-99

Figure 2.4. Precipitation and temperature recorded at the KBS
LTER weather station from December 1997 to December 1999.

37

of

ro«

wit
fer

pro

ram

the l

dem

Nin

Absorption of soil N by the rye cover crop (Figures 2.6 and 2.7) together
with plugging of some of the soil macropores by rye roots (Rasse, et al., 1999)
appeared to reduce the adverse effects of precipitation on N leaching through
soil profiles protected by cover crops. More soil inorganic N was retained within
the soil profiles seeded to a fall rye cover in 1999 compared to spring-seeded rye
cover in 1998. Spring seeding of rye in wet year resulted in leaching of inorganic
N below the rooting zone of CT-F plots during the Fall of 1997 and Spring
of1998. However in the absence of rye, leaching of inorganic N beyond the
rooting zone of corn was much greater in 1999 (Figure 2.2).

Rye planted, fertilized and no-tilled plots (NT-F) retained 57% more N
within the soil profile in April 1999 than in May 1998 (Figure 2.1). Rye planted,
fertilized and conventionally tilled plots (CT-F) retained 28% more N within soil
profile in April 1999 than in May 1998 (Figure 2.2).

Fall seeded rye cover removed 24 kg N/ha from the AD horizon in 1999
(Figure 2.2). Less rainfall and infiltration retained larger gradients of soil N from
Ap to Cg horizons in the no rye cover treatments in 1999. However, more N was
removed from the AD horizon by cover crop and/or retained in the Bt horizons by
the mechanical plugging of soil pores by living rye roots which invaded these soil
depths.

By subtracting plant N plus soil inorganic N in April from total soil inorganic

N in November, distributions of N within plant and soil could be calculated. The

38

unaccounted N was presumed lost by leaching. If mineralization of soil N is
greater than the leaching (dry year), then there is retention of N in the soil.

The roots and shoots of rye cover crop, removed 25% of N (Figure 2.6A)
from the soil profile and retained from 36.4 to 53.1 kg soil N ha'1 in
conventionally tilled and from 28.0 to 40.7 kg soil N ha’1 in no-tilled plots in 1998
(Table 2.1).

Cover crop reduced soil loss of N by absorbing 53.1 kg soil N ha’1 from
CT-F plots and 40.7 kg soil N ha‘1 from NT-F plots in 1998 (Table 2.1). Residual
N from Fall to next Spring was distributed as 11, 14, 41% among the rye root,
shoot and soil respectively for conventionally tilled and fertilized plots (Figure 2.6
A). Consequently 34% of the soil N was lost from CT-F below the root zone by
leaching. In the absence of cover crop leaching below the root zone was 66%
(Figure 2.6 B) which was 138.6 kg soil N ha".

Residual N of no-tilled and fertilized (NT-F) plots in Spring was distributed
as 14, 22, 59% among the rye root, shoot and soil respectively (Figure 2.6 C).
Nitrogen loss from these plots was 5% that was very low. In the absence of
cover crop, loss of N by leaching below the rooting zone was 69% (Figure 2.6 D)
equivalent of 78.7 kg soil N ha"(T able 2.1).

Nitrogen distribution was 11, 29, 54% among the rye root, shoot and soil
respectively and the loss was 6% for conventionally tilled and non- fertilized
plots (Figure 2.7 A). Plots with no cover crop lost 59% (Figure 2.7 B).of soil N

equivalent of 53.9 kg soil N ha'1 (Table 2.1).

39

Residual N distribution was 9, 30 and 53% among the rye root, shoot and
soil respectively and loss was 8% for no-tilled and non- fertilized plots (Figure
2.7 C). In the absence of rye cover crop leaching loss was 45% (Figure 2.7 D).of
soil N which was 32.3 kg soil N ha'1 (Table 2.1).
N retention was greater than the N loss in a dry year (Figures 2.8 and 2.9)
in both rye and no-rye cover cropped plots in April 1999 compared to May 1998.
Gain and the retention of soil N was greater under conventionally tilled
plots in 1999. Rye planted and conventionally tilled plots retained more N than no
cover cropped and conventionally tilled plots in 1999 (Figure 2.8 A and B).
However no-tilled and rye planted plots with N fertilizers lost 10.9 kg residual N
ha‘1 in 1999 compared to no cover cropped plots (Table 2.2). The differences
between retained soil N of cover cropped and no cover cropped plots of NT were
not statistically significant in the absence of N fertilization (Figures 2.9 C and D).

Greater gain of N under CT plots in a dry year (1999) and greater N loss
from CT plots in a wet year (1998) suggest that there is more N leaching
potential for these plots if the weather conditions are suitable. Residual soil N in
1997 fall was twice as much greater than that in fall 1998. Previous alfalfa crop
was spray killed in Spring of 1996 and N derived from mineralized alfalfa roots
increased soil N pool from February 96 to January 98 (Figure 2.5). Due to greater
residual N in the soil profile combined with wet fall, winter and spring, more N
leached from the soil profile in 1998 than in 1999.

Fertilized, conventionally tilled and cover crop planted plots lost 29% more

residual inorganic N than no tilled soil (Figures 2.6 A and C). Similar results

40

indicating greater N03 leaching from CT plots compared to NT plots have been
reported. (Goss et al, 1993). Ploughing increased the loss by 21% compared to
no till. In this research, tillage differences on N03 leaching were greater with
fertilizer application than no fertilized treatments. Angle et al, (1993), reported
that average N03 concentration below a depth of 30 cm for all treatments and all
years under CT plot was greater than those under the no till plots. Consequently,
despite the greater cumulative water drainage from non fertilized NT plots
(Rasse, 1997, Weed and Kanwar, 1996), total amount of inorganic N leaching
was lower than CT plots due to higher NOa-N concentrations in CT leachates

(Figure 2.5 ).

41

.NH: .2 25.532. Eaucfim 0.5 when ioamzfim .33 6:93 2
mam. >22 So...— .o=m 22522:. 2: «a .53 3:3 coronary— z 505.3 :82 8qu23. PE 3.... on can
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42

Ear“!

 

  

Rye shoot N
22%

      

Rye Cover NO Cover

Figure 2.6. Distribution of inorganic N in rye cover and soil
(0-150 cm) and leaching from a Kalamazoo loam soil in May 1998.
(A) was conventionally tilled and N fertilized with rye cover;

(B) was conventionally tilled and N fertilized with no rye cover;
(C) was no tilled and N fertilized with rye cover; and

(D) was no tilled and N fertilized with no rye cover at the
Interaction sites.

43

  

Rye |

root N Rye shoot N
9° 30%

 

Rye Cover No Cover

Figure 2.7. Distribution of inorganic N in rye cover and soil

(0—150 cm) and leaching from a Kalamazoo loam soil in May 1998.
(A) was conventionally tilled and no N fertilized with rye cover;

(B) was conventionally tilled and no N fertilized with no rye cover;
(C) was no tilled and no N fertilized with rye cover; and

(D) was no tilled and no N fertilized with no rye cover at the
Interaction sites.

  

Rye Cover No Cover

Figure 2.8. Distribution of inorganic N in rye cover and soil
(0-150 cm) and retention or leaching from a Kalamazoo

loam soil in April 1999.

(A) was conventionally tilled and N fertilized with rye cover;

(B) was conventionally tilled and N fertilized with no rye cover;
(C) was no tilled and N fertilized with rye cover; and

(D) was no tilled and N fertilized with no rye cover at the
Interaction sites.

45

    

N retained
4%

    

NT-NF

   

Rye shoot N
1 %

      
 

Rye Cover No Cover

Figure 2.9. Distribution of inorganic N in rye cover and soil

(0-150 cm) and retention or leaching from a Kalamazoo

loam soil in April 1999.

(A) was conventionally tilled and no N fertilized with rye cover;

(B) was conventionally tilled and no N fertilized with no rye cover;
(C) was no tilled and no N fertilized with rye cover; and

(D) was no tilled and no N fertilized with no rye cover at the
interaction sites.

46

Table 2.1. Distribution and loss by leaching of inorganic N in rye cover and soil
(0-150 cm) of a Kalamazoo loam at the Interactions sites in May 1998.

 

 

 

Rye root Rye shoot Soil Loss
Cover N SEt N SE N SE SE
kg ha'1

CT-F + 23.2 0.8 29.9 2.5 84.5 28.8 71.4 32.0
- - - - - 70.3 15.1 138.6 18.5

CT-NF + 9.9 3.0 26.5 3.4 49.6 10.3 5.9 8.3
- - - - - 38.0 21.0 53.9 39.3

NT-F + 15.4 0.2 25.3 8.8 66.6 18.9 6.0 18.5
- - - - - 34.6 4.0 78.7 7.2

NT-NF + 6.2 1.7 21.8 4.0 37.7 9.0 5.8 .8.0
- - - - - 39.2 12.0 32.3 27.5

 

t SE is the standard errors of the means, n=4

47

Table 2.2. Distribution, loss or gain (in parentheses) of inorganic N in rye cover
and soil (0-150 cm) of a Kalamazoo loam at the Interactions sites in April 1999.

 

 

 

Rye
Rye root shoot Soil Loss or
Cover N SEt N SE N SE gain SE
kgha"
CT-F + 14.0 3.2 8.9 2.4 123.0 0.5 (48.6) 5.1
- - - - - 133.5 46.5 (18.2) 40.2
CT-NF + 13.0 0.2 3.5 0.2 110.1 38.9 (57.8) 32.5
- - - - - 70.9 2.1 (2.6) 4.5
NT-F + 6.0 0.9 4.1 1.0 83.4 10.5 10.9 15.1
- - - - - 117.7 11.3 (31.7) 0.1
NT-NF + 7.0 2.3 1.6 0.3 75.5 20.2 (84.3) 13.9
- - - - - 104.2 7.3 (54.2) 12.2

 

t SE is the standard errors of the means, n=4.

48

Regardless of cover crop treatment, grain yield, stalk biomass and
utilization of N by com were significantly greater under CT treatment than NT
treatment (Figure 2.10 and Tables 2.3 and 2.4) in 1998 and 1999. Therefore,
residue return to the CT plots after corn harvest and associated N mineralization
are higher than those in NT soil (McCarthy et al., 1995). Greater N gain in spring
of a dry year may be due to more mineralization of residue and soil organic
material from CT plots than NT plots. An additional explanation to why soil
inorganic N leaching was lower under NT plots may be that denitrification and
microbial immobilization of N is often higher under NT treatments (Angle et al.,
1993, Meisinger et al., 1985, Rice and Smith, 1982). More water infiltration
through NT plots (Rasse, 1997), may have kept soil profile wetter and created
more anaerobic sites which enhances denitrification (Weed and Kanwar, 1996).
However, we do not have the measurements of denitrification rates from CT and
NT plots to support this explanation.

During the entire 2 yrs study, cover crop had greater influence on the
amount of inorganic N lost below rooting zone of Kalamazoo loam soil. Low
residual soil inorganic N following rye in a wet year was due mainly to nitrogen
uptakes by rye roots and shoots which were 53.1 and 40.7 kg residual soil N ha'1
from CT-F and NT-F plots in 1998 respectively. Shipley et al., (1992) reported
that cereal rye retained 48 kg N ha“1 of residual soil N when it was killed in mid
April. Kessavalou and Walters (1999), observed similar uptakes by rye cover
crop (42-48 kg N ha"). However in the absence of cover crop leaching losses

from CT-F and NT-F plots were 138.6 and 78.7 kg residual soil N ha". Therefore,

49

85.5 and 38 kg N ha'1 retained in the soil profile of CT-F and NT-F plots as a
result of occupation and mechanical plugging of soil pores by rye roots.
Reduction of inorganic soil N leaching under rye cover crop was observed by
several investigators (Kuo et al., 1997, McCracken et al., 1994, Ditsch et al.,
1993, Shipley et al., 1992). Dual spray applications of herbicide to Roundup-
ready com, planted directly into decomposing rye cover crops, resulted in the
contribution of at least 28 kg N per ha (NT-NF) to the 1998 corn crop and
reduced nitrogen losses to groundwater below the root zone by 26.5 to 72.7 kg N

per ha during the year.

50

Table 2. 3. Cover crop modifications of total N and C in whole plant com shoot
responses to tillage and N fertilization of a Kalamazoo loam in July 1998, n=4.

 

 

 

 

 

 

 

 

Treatment N C C:N Total corn
biomass

% kg ha'1

Tillage

CT 3.6at 43.6a 12.5b 3766a

NT 2.8b 42.7a 16.3a 1677b

Fertilization

F 3.5a 44.7a 12.5b 3220a

NF 2.8b 42.7b 16.3a 2223b

Cover

Rye Cover 3.2a 43.2a 13.1 b 2545b

No Rye cover 3.2a 44.2a 15.7a 2899a

No Rye cover

CT-F 4.1a 43.9a 10.8d 5014a

CT-NF 3.3b 43.5a 1320 2786b

NT-F 3.0b 46.4a 15.4b 2357b

NT-NF 2.4c 42.9a 17.9a 1436b

Rye Cover

CT-F 4.0a 44.7a 11.2b 3621 a

CT-NF 3.0ab 42.5a 14.9ab 3643a

NT-F 3.0ab 43.8a 1 5.0ab 1 886b

NT-NF 2.6b 41 .8a 16.8a 1029b

 

t Columns (for each treatment), labeled with the different letters are

significantly different at P<0.05 according to Fisher’s LSD mean separation

test.

51

Table 2. 4. Cover crop modifications of total; N and C in whole plant corn shoot
responses to tillage and N fertilization of a Kalamazoo loam in July 1999, n=4.

 

 

 

 

 

 

 

 

Treatment N C C:N Total corn
biomass

0/0 kg ha"

Tillage

CT 1 .3at 43.6a 33.7b 5031 a

NT 1 .1 b 42.7a 38.6a 3000b

Fertilization

F 1 .4a 43.4a 32.75b 4874a

NF 1.1 b 42.8a 39.53a 3156b

Cover

Rye Cover 1.2a 43.5a 36.33 3703b

No Rye cover 1.2a 42.7a 36.0a 4328a

No Rye cover

CT-F 1 .7a 42.2a 25.4b 6582a

CT-NF 1.1 b 42.4a 38.6a 3815b

NT-F 1 .2b 43.8a 379a 4166b

NT-NF 1 .0b 42.5a 42.0a 2750b

Rye Cover

CT-F 1 .4a 43.5ab 31 .5a 5166a

CT-NF 1 .1 b 42.6b 39.0a 4562ab

NT-F 1 .2ab 44.2a 36.2a 3583b

NT-NF 1 .2b 43.8ab 38.5a 15000

 

f Columns (for each treatment), labeled with the different letters are
significantly different at P<0.05 according to Fisher’s LSD mean separation
test.

52

Corn yield (kg ha")

10000

...L
o
8
C

 

90005

7000 -
6000 -
5000 ~
4000 -
3000 ~
2000 4
1000 .

 

O-i

 

CT-F

 

AB

CT-NF

 

 

 

-NR
:R

1 998

 

 

 

 

9000 d
8000 ~
7000 -
6000 a
5000 -
4000 ~
3000 -
2000 -
1000 -‘

 

0..

CT-F

 

 

 

CT-NF

 

 

1 999

 

 

NT-NF

Figure 2.10. Corn grain yields at KBS Interactions sites in 1998 and
1999. CT and NT refer to conventional and no tillage, F and NF refer

to 150 and 0 kg N fertilizer per ha. NR and R refer to no rye cover and

rye cover crops.

Mean values followed by the different uppercase letter wlthln rye cover crop
treatment and different lowercase letter within no rye cover crop treatments
are significantly different at p<0.05 according to Fisher's LSD mean separation
test. Star (*) indicates the significant difference between the corn yield of rye vs.
no rye cover crop plots (P<0.05).

53

Corn yield (kg ha")

10000

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[:3 Control
9000 1 m Shoot
- Root
3000 - - Root+shoot
a
7000 -
ab
6000 ~ a
b
5000 ~ b
4000 « .
......
1000 —
0
1998 1999

Figure 2. 11. Corn grain yields at KBS-LTER In main areas of
microplots in 1998 and 1999. Mean values followed by the
different letter within a same year and between treatments are

slgniflcantly different at p< 0.05 according to Fisher's LSD mean
separation test.

54

Cover crop roots

Most of the rye root widths (SO-100%) were classified as Class 2 sized
roots having 0.05 cm. Roots in Ap and Bt1 horizons of the CT-F and NT-F
treatments had more roots with larger diameters than those in below soil
horizons (Table 2.5) and resulted with greater volume and surface area (Table
2.7). All of the rye roots in Microplots sites had diameter between 0.02 and 0.05
cm and were classified as Class 2 (Table 2.6).

Root surface area density was in the range of 1.15-5.88 m2 m'3 for Ap
horizons of Interactions and Microplots sites (Tables 2.7 and 2.8). This range
was slightly lower than the one for cotton roots (approximately 7) reported by
Brouder and Cassman (1990).

Negative correlations were observed between extractable soil inorganic N
content (Table 2.9) and root length, volume and surface area in Ap horizons of all
fertilized and non-fertilized treatments of interactions sites (p<0.05). Similar
negative relationships were also observed in Microplots sites (Table 2.10).
Supporting results were reported by Upendra et al.,(1998). They found negative
correlations between cover crop minirhizotron root counts and soil nitrate
concentrations in Fall (r=-0.55) and in Winter (r=-0.51) at 0-30 cm soil depths.
This negative correlation appeared to be due to uptake of N from the Ap horizon
by the rye roots. Since inorganic soil N and roots were extracted from the same
soil core sample, larger root numbers could be correlated with depletion of soil N

in primarily in the Ap horizon.

55

 

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56

 

 

 

 

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59

Table 2.9. Correlation coefficients (r) between extractable inorganic N content
and rye root length, volume and surface area in Ap, Bt1, Bt2, C1 and C2 horizons
of N fertilized (N) and non-fertilized (NF) treatments of a Kalamazoo loam soil at
KBS-Interactions sites in May 20, 1998, n=8 (p=0.05).

 

 

 

 

 

 

 

N Fertilized Len th Volume Surface area

Soil horizons m ha' m3 ha'1 m2 ha'1

Ap -O.63 -0.67 -0.70

Bt1 0.95 0.87 0.93

Btz 0.84 0.62 0.61

C1 0.21 0.1 8 0.20

02 NA NA NA
Non- fertilized Length Volume Surface area
Soil horizons m ha'1 :113 ha'1 :112 ha'1

Ap -0.27 -O.78 -0.48

Bt1 0.74 0.31 0.18

Btz 0.09 -0.42 -O.19

C1 0.28 0.37 0.31

C2 NA NA NA

 

60

Table 2.10. Correlation coefficients (r) between extractable inorganic N content
and rye root length, volume and surface area in Ap, Bt1, Bt2, C1 and C2 horizons
of N fertilized and rye cover planted treatment a Kalamazoo loam soil at KBS-
Microplots sites in April 11, 1998, n=8 (p=0.05).

 

 

 

N Fertilized Length Volume Surface area
m ha'1 m3 ha“1 m2 ha'1
AP 076 -0.63 -0.63
Bt1 0.95 0.73 0.98
312 1.00 1.00 1.00
C1 NA NA NA
C2 NA NA NA

 

61

In contrast, positive relationships were observed between extractable soil
inorganic N content and root length, volume and surface area in Bt1 and Bt2
horizons of fertilized treatments at both of the Interaction and Microplots sites
(Tables 2.9 and 2.10). The reason for a negative correlations in Ap horizon and
positive correlations in Bt, could be the different residence times of rye roots in
these horizons. For example, root residence time in Ap horizon was much longer
than those in deeper horizons resulting in more N uptake by roots from this
horizon. Upendra et al.,(1998) reported that rye had much more greater root
density in early growing season than in April in surface soil horizon. Therefore
root length density could be longer in Ap horizon before April and May samplings

and uptake of N by this roots reduced inorganic N in Ap horizon. Nitrogen uptake

by rye roots in the Ap horizon of CT-F treatments was 27 (:5) mg N/m2root/day
and from Bt, horizon was 9 (:6) mg N/mzroot/day. Nitrogen uptake by rye root
from Ap horizon of NT-F treatment was 8 (:1.4) mgN/mzroot/day and from Bt,

horizon was 3 (:1.6) mgN/mzroot/day. Therefore, the negative correlations

between inorganic N and surface areas of roots in Ap horizons were due to the
more efficient N uptake by rye roots in this horizon. The reasons for positive
correlations in Bt horizons (Tables 2.9 and 2.10) appears to be due to the
accumulations of N that leached from primarily the Ap horizon and occupation of
this N rich horizon by rye roots. Similar root surface areas between the Ap and

Bt1 horizons for the N fertilized tillage treatments at the Interactions site (Table

62

(I)

2.9) and microplot site (Table 2.10) further suggest N-stimulation of root growth
and greater plant root uptake by the higher N treatments. Plugging of soil pores
by these roots can be another reason for greater root and greater N content
relation in this horizon. We previously reported that in the absence of cover
crops, leaching losses from CT-F and NT-F treatments were 138.6 and 78.7 kg
ha'1 (Table 2.1). Therefore 85.5 and 38 kg N ha'1 appeared to have been
retained by soil due to plugging of soil pores by rye roots.

Rye roots absorbed 18.5 kg N ha'1 from 0-150 cm soil depth of Microplots
in 1998. Absorption of soil N by rye roots was 23.2 kg N ha'1 (CT-F) where soil N
content was 252 kg N ha". Absorption of soil N by roots were 14.0 kg ha'1 in
1999 and soil N content was 115 kg N ha'1from 0-150 cm soil depth. Rye roots
and shoots reduced N leaching by 67.2 kg N ha'1 from the CT-F plots compared
to no cover cropped plots in 1998 (Table 2.1). Residual soil N uptake by rye roots
were similar to rye shoots however root contribution of N to the succeeding corn
plant and soil N pool was greater than those by shoots (Kavdir, 2000-Chapter 4).
Alfalfa root contributions to soil N pool was reported by Rasse et al.,(1999).
Alfalfa shoot mulch contributed little to increases in soil N pools, while crowns
and roots contributed larger quantities to the soil N pool. The positive correlation
(R2=0.55) was observed between total MR root length and rye shoot biomass in
Spring 1999 (Figure 2.12). These results also suggest that increased root surface
area and associated uptake of soil N by rye resulted in greater production of rye

shoot biomass.

63

   

y = 1.9801x + 142.57

Rye shoot biomass (kg ha")
a)
O
O

 

 

R2 = 0.55
400 * p<0.05
200 1
0 l l l l 1
0 100 200 300 400 500

MR rye root length (mm)

Figure 2.12. Relationship between rye shoot biomass and
nondestructive minirhizotron evaluations of manually counted
rye root lengths in 0-106 cm depths of a Kalamazoo loam in
Spring, 1999 at KBS Microplots, n=16.

64

in a summary, rye cover crop roots absorbed N mostly from Ap horizon
and reduced N leaching by immobilizing N in their shoots and roots. Root length
density in Ap horizons of CT-F and CT—NF and those of NT-F and NT-NF were
not significantly different. However they were significantly different in Bt1
horizons of CT-F and NT-F treatments. Root surface area density was greater in
Ap horizon of NT-NF treatment than those of NT-F treatment. Nitrogen source of
non fertilized treatments are limited with plant root and shoot residues and SOM
which are greater in Ap horizon of soil profile. Therefore N limitation in deeper
soil horizons were resulted with greater root length and surface area in Ap
horizons of NT-NF treatments than Bt horizons (Table 2.7). Root surface area
density was not significantly different in Ap horizon of CT-NF than those of CT—F
treatments (Table 2.7). Root surface area density was greater in Bt1 horizons of
CT-F treatment than those of CT-NF treatment and greater in NT-F treatment
than those of NT-NF treatment. Greater root length and surface area density
observed in Bt1 horizons of fertilized plots than those in non fertilized plots due
to more inorganic N content of this horizon. Greater N content in Bt1 horizon of N
fertilized treatments compared to non-fertilized treatments resulted in greater
root penetration and positive linear correlation in this horizon. Therefore growing
cover crop and keeping active roots in the soil between two successive cropping
can reduce N leaching due to N utilization by cover crop roots and mechanical

plugging of pores by these roots in upper soil horizons. Retained N in soil profile

65

together with N released from cover crop residues increases N utilization by the

succeeding cash crop.

Rye biomass and nitrogen

Nitrogen fertilization increased rye biomass in 1999 (P<0.05). Rye shoot N
uptake averaged 22 to 30 kg ha'1 in 1998 and 10 to 16 kg ha‘1 in 1999. The C:N
ratio of the rye shoot residue was found to be mostly dependent on the time of
killing and decomposition rate. In the spring of 1998 (Table 2.11), relatively late
rye killing and corn planting date resulted in more rye cover crop growth than
1999. Early killing of rye resulted in a narrower C:N ratio. C:N ranged from 10 to
14 in 1999 (Table 2.12). However, total biomass was smaller than that of later
killed rye in 1998 (Table 2.11). It was suggested that small grain cereals should
be killed when their C:N ratios are smaller than 30:1 (Reeves, 1994). However, in
practice they are killed when their C:N ratio are over 30:1. This ratio results in an
immobilization of N during the cropping season (Doran and Smith, 1991). In this
study C:N ratios of rye were smaller than 30:1 at the time of desiccation in both
years and resulted in early N availability for succeeding corn plant (Chapter 4).
Other researchers also reported that lower C:N ratio of rye at the time of
herbicide desiccation increased N mineralization (Kessavalou and Walters, 1999,

and Kuo et al., 1997).

66

Table 2.11. Dry biomass, N, C and C:N contents of rye in conventional tillage
(CT) and no tillage (NT) plots with nitrogen fertilization (F) and with no fertilization
applied (NF) plots sampled in May 15, 1998 at Interactions sites, n=4.

 

05/1 5/1 998

Treatments Biomass SD N SD C SD C:N SD

 

kg ha'1 % %
CT-F 1221 473 2.6 0.5 42.7 0.8 17.1 4.0
CT-NF 985 222 2.7 0.4 43.2 0.6 16.3 2.5
NT-F 1271 875 2.0 0.1 42.1 0.5 21.1 1.2
NT-NF 1314 346 1.7 0.3 41.9 0.3 26.1 5.2

 

Table 2.12. Dry biomass, N, C and C:N contents of rye in conventional tillage
(CT) and no tillage(NT) plots with nitrogen fertilization (F) and with no fertilization
applied (NF) plots sampled in April 19,1999 at Interactions sites, n=4.

 

04/1 9/1 999

Treatments Biomass SD N SD C SD C:N SD

 

kg ha" % °/.
CT-F 339 163 2.7 0.5 26.4 2.4 10.0 1.6
CT-NF 130 32 2.0 0.2 26.3 1.9 13.4 1.2
NT-F 133 34 2.2 0.4 27.7 1.4 13.0 1.6
NT-NF 79 33 2.0 0.1 23.2 0.6 13.9 0.3

 

67

Corn yield and nitrogen

Corn biomass and associated N concentrations increased on the CT-F
treatments by 58% (no rye cover) and 44% (rye cover) compared to NT-F
treatments in July 1999 (Table 2. 4). The reasons for the lower N uptake by com
in NT treatments can be the immobilization of fertilizer N by microbial biomass
during decomposition of cover crop residue in NT plots and/or more N03 leaching
from NT plots. Lower corn uptake of N should not have resulted from N03
leaching from the NT plots as field suction Iysimeter data showed greater N03
leaching from CT plots than NT plots. N03 concentrations of leachate samples
moving into the Btz horizon averaged 37 mg L‘1 in CT-F plots and 26 mg L'1 in
NT-F plots in July, 1999. Large monolith Iysimeter data also confirmed that CT
plots lost more N03 by leaching from the soil profile (Rasse, 1997 and Figure
2.5). Another reasons for the greater N uptake by com in CT treatments can be
the aggregate breakdown by tillage. Nitrogen in the center of the aggregates
becomes more available for plant uptake when aggregates are broken (Kavdir,
2000-Chapter 3).

It has been shown that microbial biomass C was greater when residues
incorporated into the soil. However, microbial biomass N tended to be greater in
soil with surface residues (Hubbard and Jordan, 1996). N immobilization by
microbial biomass in NT treatments where residues placed on the soil surface

may have caused less soil N uptake by com plants in July compared to those in

68

CT treatments. Accumulations of corn biomass early in the growing season
resulted in higher grain yields.

Grain yields of CT-F plots were 28% greater than the grain yields of NT-F
plots in the absence of cover crop. In the presence of cover crop, com grain
yields of CT-F plots were 35% greater than the grain yields of NT-F plots in 1999
(Figure 2.10).

Grain yields of corn increased with N fertilization. Com yields in rye planted
plots increased by 52% in CT and 224% in NT plots due to the application of N
fertilizer. in the absence of rye cover crop, grain yields increased by 76% and
132% in CT-F and NT-F plots respectively due to N fertilizer in 1998 (Figure 2.
10). N fertilization resulted in 103% and 584% increments in yield in 1999 within
no cover crop planted and 145% and 524% yield increments in rye planted plots
of CT-F and NT-F respectively.

Presence of rye in all plots did not significantly increased or decreased
grain yield in both years. However, a direct linear correlation (P<0.05) between
rye N content and grain yield (R2=0.75 in 1993 and 32:0.75 in 1999) was
observed for all non-fertilized and rye planted treatments.

Grain yields in Microplots were significantly different between treatments in
1998 (Figure 2. 11). However, grain yields of rye planted plots of Microplots were
98% greater than those in Interactions plots in 1998 and 49% greater than those
in Interactions plots in 1999. In the absence of cover crop treatment, grain yield
of Microplots were 70% and 100% greater than the grain yields of rye planted

NT-F plots of Interactions sites in 1998 and 1999 respectively. Killling time of rye

69

was different between these two sites. Rye was killed before corn planting in
Microplots and rye was killed at the same time with corn planting in Interactions
sites. Therefore, one of the reasons for yield differences can be different time of
killing rye cover crops in Microplots and Interactions sites. However, grain yields
of no rye planted treatments of Microplots were also different than those in
Interactions sites . Also, other factors, such as slightly different seedling rate,
differences in depths of soil horizons and the length of no tillage (12 yrs. ln
Interactions sites) treatment could have been effective on the yield.

Tillage and fertilization significantly increased grain yield in both years.
Long term NT treatments of Interactions sites had lower grain yield than CT
treatments. NT treatments had also lower corn yield than CT treatments in
previous years (Rasse .1997) at Interactions sites. Overall cover crop treatment
did not significantly change corn yield except NT-NF treatment in 1999 which had

lower yield than no cover cropped NT-NF treatment.

70

REFERENCES

Angle, J.S., C.M. Gross, Hill, R.L. and M. S. McIntosh. 1993. Soil nitrate
concentrations under com as affected by tillage, manure, and fertilizer
applications. J. Environ. Qual. 22:141-147.

Brouder, SM. and K.G. Cassman. 1990. Root development of two cotton
cultivars in relation to potassium uptake and plant growth in a vermiculitic soil.
Field Crops Research. 23:187-203.

Cummings, T.R., F.R. Twenter and DJ. Holtschlag. 1984. Hydrology and land
use in Van Buren County, Michigan. USGS Water-Resources Investigation
Report 84-4112, 124 p.

Ditsch, D.C., M.A. Alley, K.R. Kelley and Y.Z. Lei.1993. Effectiveness of winter
rye for accumulating residual fertilizer N following com. J. Soil Water Conserv.
48:125-132.

Goss, M.J., K.R. Howse, P. W. Lane, D.G. Christian and G L. Harris. 1993.
Losses of nitrate nitrogen in water draining from under autumn-sown crops
established by direct drilling or mouldboard ploughing. Jour. of. Soil Sci. 44:35-
48.

Grofman, P.M., P.E. Hendrix and DA. Crossley, Jr. 1986. Nitrogen dynamics in
conventional and no-tillage agroecosystems with inorganic fertilizer or legume
nitrogen inputs. Plant Soil 97:315-332.

Halberg, G. R. 1986. From hoes to herbicides: Agriculture and groundwater
quality. J. Soil Water Conserv. 41 :357-364.

Hubbard, V. C. and D. Jordan. 1996. Nitrogen recovery by com from 15N labeled
wheat residues and intact roots and soil. Soil Sci. Soc. Am. J. 60:1405-1410.

Huang, B. 1995. Tillage modifications of root and shoot growth responses to soil
water content and nitrogen concentration altered by seasons. Thesis
(Ph.D.).Michigan State University. Dept. of Crop and Soil Sciences.

Jokela, W. E. and G.W. Randall. 1997. Fate of fertilizer nitrogen as affected by
time and rate of application on corn. Soil Sci. Soc. Am. J. 61 :1695-1703.

Karlen, D.L., L.A. Kramer and SD. Logsdon. 1998. Field-scale nitrogen balances
associated with long term continuous corn production. Agron. J. 90:644-650.

71

Kessavalou A. and D. Walters. 1999. Winter rye cover crop following soybean
under conservation tillage:Residual soil nitrate. Agron. J. 91 :643-649.

Kirsten, W.J. 1983. Organic Elemental Analysis. Academic Press, New York, NY.

Kuo, S., U. M. Sanju and E.J. Jellum. 1997. Winter cover cropping influence on
nitrogen in Soil. Soil Sci. Soc. Am. J. 61 :1392-1399.

Martinez, J. and G. Guiraud. 1990. A Iysimeter study of the effects of a ryegrass
catch crop during a winter wheat/maize rotation, on nitrate leaching and on the
following crop. J. of Soil Sci. 41 :5-16.

McCarthy,G.W., J.J. Meisinger and F.M.M. Jenniskens. 1995. Relationship
between total-N, biomass-N and active-N under different tillage and N fertilizer
treatments. Soil Biol. Biochem. 27:1245-1250.

McCracken, D.V., Ms. Smith, J. H. Grove, C.T. MacKown and R.L. Blevins.
1994. Nitrate leaching as influenced by cover cropping and nitrogen source. Soil
Sci. Soc. Am. J. 58:1476-1483.

Mehdi, 3.8., CA. Madramootoo and G. R. Mehuys. 1999. Yield and nitrogen
content under different tillage practices. Agron. J. 91 :631-636.

Meisinger, J.J., V.A. Bandel, G. Stanford and JD. Legg. 1985. Nitrogen
utilization of corn under minimal tillage and moldboard plow tillage. l. Four-year
results using labeled N fertilizer on an Atlantic coastal plain soil. Agron. J.
77:602-611.

Meisinger, J.J, W.L. Hargrove, R.B. Mikkelsen, J.R. Williams and V. W. Benson.
1991. Effects of cover crops on groundwater quality. p:57-68. In W. L. Hargrove
(ed). Cover crops for clean water. Proc. Int. Conf. Jackson TN. 9-11 April 1991.

Soil and Water Conserv. Soc. Am., Ankeny, IA.

Morris J.L., V.G. Allen, D.H. Vaughan, J.M. Luna and MA. Cochran. 1998.
Establishment of corn in rotation with alfalfa and ryezlnfluence of grazing, tillage,
and herbicides. Agron. J. 90:837-844.

Raimbault, B.A., T.J. Vyn and M. Tollenaar. 1991 .Com response to rye cover
crop, tillage methods, and planter options. Agron J. 83:287-290.

Rasse, D. P., A. J. M. Smucker and O. Schabenberger. 1999. Modifications of
soil nitrogen pools in response to alfalfa root systems and shoot mulch. Agron. J.
91 :471-477.

72

Rasse, D. P. 1997. Alfalfa and corn root modifications of soil nitrogen flux and
retention. Thesis (Ph.D.).Michigan State University. Dept. of Crop and Soil
Sciences.149 p.

Reeves. D. W. 1994. Cover crops and rotation. In Crops Residue Management.
(ed.) J.L. Hatfield and BA. Stewart. P:125-171. CRC Press. Inc.

Rheaume, SJ. 1990. Geohydrology and water quality of Kalamazoo County,
Michigan, 1986-88. USGS Water-Resources Investigations Report 904028,.
102p. http://mi.water.usqs.qov/reports/R_heaume2.html

Rice, CV. and MS. Smith. 1982. Denitrification in no-till and plowed soils. Soil
Sci. Soc. Am. J. 46:1168-1173.

Shipley P.R., J.J. Meisinger and A M. Decker. 1992. Conserving residual corn
fertilizer nitrogen with winter cover crops. Agron. J. 84:869-876.

Smith, SJ. and D.K. Cassel. 1991. Estimating nitrate leaching in soil materials.
p:165-188. In. managing N for ground water quality and farm profitability. R.F.
Follett et al. (ed.). SSSA, Madison, WI.

Smucker, A.J.M., J.C Ferguson, W.P. Debruyn., R.K. Belfordand and J.T.
Ritchie. 1987. Image analysis of video recorded plant root systems, in
minirhizotron observation tubes: methods and application for measuring
rhizosphere dynamics. ASA Special publication #50, p:67-80.

Smucker, A.J.M., S.L. McBumey and AK. Srivastava. 1982. Quantitative
separation of roots from compacted soil profiles by the hydropneumatic
elutriation system. Agron. J. 74:500-503.

Topp, G.C., J.L. Davis and AP. Arman. 1980. Electromagnetic determination of
soil water content: Measurements in coaxial transmission lines. Water Resour.
Res. 16:574-582.

Unger P.W and M.F. Vigil. 1998. Cover crop effects on soil water relationships. J.
Soil and Water Conserv. 53:200-206.

Upchurch, DR. and J.T. Ritchie. 1983. Root observation using video recording
systems in minirhizotrons. Agron. J. 75:1009-1015.

Upendra M.S., B.P. Singh and W. F. Whitehead. 1998. Cover crop root
distribution and its effect on soil nitrogen cycling. Agron. J. 90:511-518.

Vaughan, JD. and GK. Evanylo. 1998. Com response to cover crop species,
spring desiccation time, and residue management. Agron. J. 90:536-544.

73

Weed D. A. J. and R. S. Kanwar. 1996. Nitrate and water present in and flowing
from root-zone soil. J. Environ. Qual. 25:709-719.

74

CHAPTER 3

SOIL AGGREGATE SEQUESTRATION OF COVER CROP ROOT AND
SHOOT RESIDUE NITROGEN

ABSTRACT

Rye (Secale cereale L.) roots and shoots release C and N compounds in
situ during their decomposition. Plant deposition of N by rye roots and shoots
onto soil aggregates was determined by labeling rye shoots with stable N isotope
during rye cover cropping of com agroecosystems. Rye plants were labeled with
foliar applications of solutions containing 99% atom (‘5NH4)2SO4, Isotopic
enrichment of soil aggregates ranging from 2.0 - 4.0, 4.0 - 6.3 and 6.3 - 9.5 mm
across was determined following plant residue applications. Concentric layers of
aggregates were removed from each aggregate by newly designed meso soil
aggregate erosion (SAE) chambers.

Significant correlations were observed between changes in ratios of N
concentrations in external layer/N concentrations in internal regions of
aggregates 6.3-9.5 mm across and corn biomass production in 1999 (r2=0.88 for
no cover crop and r2=0.71 for rye cover crop treatments). Some of the N in the
external layers of soil aggregates were utilized by com roots and/or diffused to
the interior regions of aggregates.

Non-unifonn distributions of total N and recently derived rye N in soil
macroaggregates, across time, suggested that the formations and functions of

macroaggregates are very dynamics processes. Rye roots contributed as much

75

N as rye shoots to the soil N pool. Therefore maintaining active plant root in the
soil and keeping N on the surfaces of macroaggregates are the best

management systems for maximizing soil N availability and reducing N leaching.

76

INTRODUCTION

Cover crops used to reduce leaching of N03 (Ditsch et al., 1993,
McCracken et al., 1994) also contribute to the improvement of soil organic matter
through the addition of residues in the early spring and throughout the summer.
Microbes rapidly deplete decomposing plant residues of most sugars, amino
sugars, organic acids, starches, and simple proteins (Paul and Clark, 1996). As
decomposition continues, hemicelluloses, fats, waxes, and Iignin are broken
down into more consumable compounds (Killham,1994). As rye matures, the
ratio of resistant to non-resistant materials increases, as does the C : N ratio in
the plant. These changes in plant composition affect the residence times of C
and N compounds adhering to the soil matrix.

Living rye roots, decomposition and by-products associated with the rye
root and shoot residues are effective contributors to soil nutrient cycling and
aggregate formation and stabilization. Kladivko (1994) reported that microbial
decomposition of fresh organic material produced polysaccharides and other
compounds that became the main contributors to the soil aggregate stabilization.
Numerous studies have been reported on the formation, stabilization, and effect
of different soil and crop management systems on soil aggregation (Wood et al.,
1991, Roberson et al., 1995). However, there is little information on the location
of recently decomposed plant residues within soil aggregates (Angers et al.,
1997). Soil aggregation appears to improve when cover crops are added to cash

crop rotations (Dorrnaar and Lindwall, 1989, and Angers et al.,1992). Plant roots

77

and shoot residues, wetting-drying cycles, soil organisms and soil texture control
formation and stability of soil aggregates (Oades, 1993).

Growth of plant roots and development of soil aggregates conversely
affect each other. Roots preferentially grow in the cracks and planes of weakness
between aggregates rather than through aggregate interiors. Soil structure
stability and associated mechanical impedance also impact root elongation rates,
degree of contact at the soil-root interface as well as uptake of immobile nutrients
(Whiteley and Dexter, 1983). The root systems of grasses are extensive and their
position is generally inter-aggregate (Allison, 1973). Clay illuviation, preferential
movement of water, weathering of clay and preferential growth of roots can
change the compositions of aggregate surfaces (Smucker et al., 1997, Whiteley
and Dexter, 1983, Wilcke and Kaupenjohann, 1998). Whiteley and Dexter (1983)
reported that even when the soil was near saturation with a low penetrometer
resistance, ped surfaces became a barrier to root penetration.

Sierra and Renault, (1996) reported that centers of aggregates contained
less oxygen than the aggregate surfaces. Oxygen concentrations in interior
decreased in large aggregates (Sextone et al., 1985, Hojberg et al., 1994).
Aerobic respiration potential was reported to be greater near the surfaces of
aggregates (Sierra and Renault, 1996 ). Therefore, it was assumed that oxygen
gradients most likely control microbial activities, associated SOM decomposition,
C and N accumulation in the soil aggregates. Contrasting C concentrations within
interior and surface regions of aggregates were reported by Santos, (1998) and

Smucker et al., (1997).

78

Living roots influence the chemical and biological properties of rhizosphere soil
(Fisher et al., 1989). Rhizosphere effects are greater on the surfaces of the
aggregates since roots are preferentially growing around the aggregate surfaces.
Living roots can change pH, redox potential, water and nutrient content of the
rhizosphere. They may create rapid wetting-drying cycles that enhance SOM
degradation (Bottner, 1985). They may induce microbial activity and increase
SOM decomposition (Cheng and Coleman, 1990). Roots control the
concentrations and fluxes of soil N by absorbing soil water and soluble N
compounds (Harper, 1995 and Frensch, 1996). Released N in situ, from
decomposing plant roots and shoots contribute to stabilizing soil aggregation
processes (Oades, 1993). Dead roots act as a readily decomposable SOM and
cause increased oxygen consumption in rhizosphere (Fisher et al., 1989).
Rhizodeposition, loss of organic materials from the roots, modifies rhizosphere
soil (Mary et al., 1993, Ehrenfeld et al., 1997, Qian et al., 1997, Texier and Billes
1990, Jensen, 1996). Root exudates modify the solubility, sorption and transport
of ions to the root surface, affecting the microbial activity. Rhizodeposition
materials are water soluble exudates: sugars, aminoacids, organic acids,
hormones, vitamins, water insoluble materials, cell walls, dead roots and
mucilage (Cheng et al., 1993, Jensen, 1996). Nitrogen is deposited in the
rhizosphere as NH4, N03, and root debris. Janzen (1990) and Janzen and
Bruinsma (1993) reported up to 20% of total plant N could be deposited to the
rhizosphere of wheat plants. The amount of N deposited from pea residue was

48% of belowground N and from barley was 71% of total belowground N at

79

maturity (Jensen, 1996). It is assumed that, some of the extracted plant available
N forms and mineralized N from rhizodeposits are reabsorbed by the plant.
Janzen (1990) reported that the N rhizodeposits were usually labile but they
become more recalcintrant with increased plant age.

Recent studies showed that soil aggregates develop by adding concentric
layers of cations, carbon (Santos et al., 1997, Horn 1990, Smucker et al. 1997)
and heavy metals (Wilcke and Amelung,1996). Short term effects of cropping on
soil organic matter and associated rhizodeposition can be determined more
quickly when concentric layers are removed from soil aggregates. Kay et al.,
(1988) did not find any changes in C and N contents of soil in short term.
However, Angers and Mehuys (1989) found that 2 yrs of alfalfa and barley
resulted in 15-25% higher carbohydrate contents compared to fallow or corn and
46—83% more carbohydrates compared to fallow treatment (Angers and Mehuys,
1990). Six weeks after planting ryegrass in a greenhouse potted study (Santos,
1998) showed exterior layers of soil aggregates contained 20% newly deposited
C while interior regions contained only 8% new Ca-C. Therefore, under field cover
crop conditions, it is suggested that recently derived rye cover crop shoot and
root nitrogen could be deposited at greater concentrations on the surfaces of soil
aggregates than interiors. To understand cover crop contributions of N to
successive plant uptake and soil aggregation processes, sources and specific
locations of N must be identified within soil aggregates. In this research, the
contributions of roots and shoots by rye cover crop plants on soil N pool were

measured separately. 15N stable isotopes have been used in soil-plant systems

80

to investigate nitrogen transformations (Davidson et al., 1991), biological N
fixation, natural abundance (Yoneyama et al., 1990), fertilizer utilization (Angle et
al., 1993), mineralization and immobilization (Davidson et al., 1991 , Shen et al.,
1984, Stephen et al., 1996), denitrification (Blackmer and Bremner, 1977), plant
uptake (Thomsen et al., 1997) and leaching (Hallberg, 1986 ). The 15N stable
isotope is ideal for tracing N through the plant-soil-microbial pathways associated
with soil aggregation, soil nitrogen, and plant uptake processes.

Field and Iysimeter experiments give more realistic estimations of N
transformations and their direct measurement of N recovery than laboratory
experiments (Lazzari, 1982). Most studies on 15N labeled residue-decomposition
and associated nitrogen transferred to the following crop have used dried leaves,
stems, but very few have used roots. Few studies have used in-situ labeling of
plant material to determine plant uptake of N from foliar fertilizers in the field
(Zebarth et al. 1991, Jordan et al., 1996). Zebarth et al.,(1991), reported that
foliar-applied urea was absorbed very rapid and 44 to 67% of foliar-applied 15N
urea was recovered in tops and roots of soybean (Vasilas et al, 1980).

In this study, we determined whether plant-derived organic N, sequestered
at different locations in soil aggregates, affected N absorption by subsequent
corn crops. Seasonal and spatial distributions of plant-derived organic N were
determined in aggregates and sieved to different size classes within concentric
regions of aggregates within each size class. Nitrogen released from root and
shoot portions of rye cover crops were determined by applying 15N labeled

fertilizer to the shoot before cutting or spray killing the rye cover crop.

81

The objective of this study was to identify the contributions of rye root and shoot
N to different regions within aggregates ranging from 2.0 to 9.5 mm across in the
AD horizon of a Kalamazoo loam soil. This objective was evaluated during a two-
year field study at the Kellogg Biological Station (KBS), MSU located near

Kalamazoo, Michigan.

82

MATERIALS AND METHODS

Experimental design and treatments

A two- year field experiment (1997-1999) was conducted on 16 Microplots
(6 by 10 m) established in August 1994 (Rasse, 1997) on a Kalamazoo loam soil
(coarse-loamy, mixed, mesic Typic Hapludalf) at the KBS/LTER (long term
ecological research) site in southwestern Michigan. There were four treatments;
1) Bare soil control (C) where all plant life was eliminated by frequent
application of Roundup ultra.
2) Bare soil where rye shoots were applied before corn planting (RS).
3) Rye cover crop where shoots were cut and removed and roots in the
soil remained in situ (RR).
4) Rye cover crop roots and shoots (RRS) where rye shoots were cut and
placed on soil surface.

Each treatment was replicated four times in a randomized complete block
design. The previous crop from August 94 to April 97 was alfalfa. The alfalfa was
spray-killed with a Roundup Ultra application in April 1997 and plots were
maintained plant free by adding two applications of Roundup Ultra during the
successive seven months. Finely ground limestone was applied to the all plots at
the rate of 2 tons/ha on April 4, 1998. N0 limestone was applied in the Spring of

1999.

83

15N experiment

Two open-ended PVC cylinders, 30 cm in diameter and 60 cm in depth,
were driven through the Ap horizon and into the center of the 812 horizon in each
plot by a front fork loader of a tractor. To maintain the balance and prevent
damage to the PVC container an iron plate with a cylinder adaptor that fitted
outside each PVC chamber facilitated these insertions of the PVC cylinders into
soil. The distance between the centers of two cylinders was approximately 75
cm. During the Fall of 1998, 2 additional cylinders were inserted into each plot
approximately 75 cm apart from cylinders installed in Spring 1998.

In spring and fall of 1998, approximately 45 rye seeds were planted into
each PVC cylinder of the rye treatment plots. In an effort to avoid soil
contamination by 15N, the soil surface in each cylinder was covered with plastic
sealed around the walls of the PVC cylinder and each row of rye using nontoxic
clay sealant. Pine wood shavings were placed on the plastic to absorb any mist
or droplets of the 15N labeled spray materials preventing them from contacting
the to soil surface. Cylinders with no cover crops also covered plastic cover and
pine shavings to receive the same treatments.

Rye plants were labeled with 15N by foliar applications of solutions
containing 6.39 g (‘5NH4)ZSO4 containing 99 atom% 15N dissolved in 9 L of
distilled water in May. This solution was applied in 3 to 4 separate applications to
prevent run off. Each time equal amounts of 15N solution (approximately 125 ml)
were manually applied rye shoots using graduated spray bottles. All PVC

chambers were covered by a plastic webbed-sided clothes-basket securely

84

anchored to the soil to prevent foliar losses during rain, threatening rain and at
night. These covers were removed during sunny days.

Following a two-week translocation period, the rye plants were spray-killed
with Roundup Ultra without ammonium sulphate that (4.5 L ha") mixed with 186
L ha'1 water in early May of 1998 and 1999. Pine wood shavings were cleaned
using vacuum cleaner and the plastic cover and clay sealants were removed.
Above ground plant parts of rye were manually cut, weighed and subsamples
were taken for analyses. Rye shoots were removed or placed on the soil surface
inside PVC cylinders with appropriate treatments.

Com seeds (6-8) were hand planted into each PVC cylinder. Metal
screens with 1 cm openings were placed on the top of the soil and secured with
nails to prevent residue losses by wind or animal consumption of corn seeds or
rye shoots. Each chamber received 500 ml water from the soil surface. Following
germination PVC 15N lysimeters were thinned to two corn plants, 2 days after
emergence. Thinned corn plants were left on the soil surface of the chambers to

retain 100% of 15N within the PVC lysimeters.

Soil sampling

Background soil samples (0-5 and 5-15 cm) were taken from each 15N
lysimeters using a small (2.5 cm in diameter) PVC pipe before 15N application.
After spray killing of rye, approximately 1 cm soil crust was subsampled from

each 15N lysimeters to determine if any 15N soil contamination had occurred

85

during labeling. There were no significant differences in the soil concentration
sampled from labeled and non labeled PVC chambers.

Soil samples for aggregate analyses were extracted from 0-5 and 5-20 cm
depths from the soil surfaces by periodically pushing PVC pipe into the soil and
removing it using a small garden shovel. Soil samples were air-dried and
manually sieved with the 9.5 mm sieve. Aggregates between 2.0-4.0mm, 4.0-
6.3mm and 6.3-9.5mm separated into their exterior and transitional layers and
interior regions using meso soil aggregate erosion (SAE) chambers. Samples
>95 mm and < 2 mm were stored in the laboratory for further analyses. In this
research only soils from 0-5 cm depth were analyzed. In addition, samples that
were sampled in July 1999 sieved into 9 size classes: >9.5, 9.5-6.3, 6.3-4.0, 4.0-
2.0, 2.0-1.0, 1.0-0.5, 0.5-0.25, 0.25-0,106, and <0.106 mm and aggregates were

analyzed for total N, 15N and SOC.

Rye root and shoot sampling

Rye root and shoot sub samples were taken before and after the 15N
labeling to determine initial and final 15N contents of plant shoots and roots.

Rye root samples were extracted from the top 15 cm depth of soil surface
by pressing PVC cores (117 cm3) into the soil to sample rye roots before and
after 15N application. Roots were removed from this sample by developing a
slurry of distilled water which was poured through a 53 um screen and the

retained roots were washed under water. Fine and white roots were picked from

86

the sand and residue remaining on the screen by tweezers. Both roots and

shoots were oven dried at 70°C for 24 h.

Soil and plant analyses

Following the separation of aggregates into 3 equal concentric layers,
samples were further processed by grinding in mortar and pestle. Analyses of
transitional layers of soil was only performed aggregates sampled in July 1999
(Tables 3.1, 3.2 and 3.3). Transitional layer was in between the exterior layer and
the interior region and the concentrations of N and 15N of this layer was similar to
one of exterior layers or interior regions depended on aggregate size. Therefore,
we limited our aggregate layer analyses only with external layers and internal
regions.

Sand was removed by sieving each peeled and ground sample through a
53 um screen to increase the concentration of the 15N and N in the small sample
size associated with each concentric layer of each aggregate. Resultant clay and
silt samples were weighed into small tin capsules, approximately to 10 mg and 5
decimal accuracy and placed into an autosampler. Total C and N (organic N plus
inorganic NH, and some N03) of plant and soil materials were determined by the
dry combustion method (Kirsten, 1983) using a C/N/H analyzer NA 1500 series 2
(Carlo Erba Stumentazione, Milano, Italy) and %‘5N by using Isotope Ratio Mass
Spectrometer Model 2020 (Europa Scientific, Crewe, UK). Total C content of a
Kalamazoo loam soil from 0-5 cm depth was assumed to be equal to soil organic

carbon as reported by Santos (1998).

87

15N signatures in soil aggregate concentric layers were measured and
EN and atom %‘5N was calculated as below (Yoneyama, 1996):

%‘5N = [(‘5N/‘4N) .p. - (15N/“M ..., / (15N/”N) std1 x 100 [1]
where;

15N = Atom % 15N which gives the absolute number of atoms of a N-15

isotope in 100 atoms of total N element.

Atom % 15N = [‘5N / (15N +‘4N)] x 100 [2]

14N = Atom % 14N

Atom % 14N =[14N/(‘5N +‘4N)] x 100 [3]

spl = sample

std=standard (atmospheric N2)

Aggregate erosion

Concentric layers of aggregates were removed from each aggregate by
the meso soil aggregate erosion (SAE) chamber technique reported by Smucker
et al. (1999). Briefly, stainless steel chambers having 2.5 cm (ID) diameters and
lengths of 3.0 cm, were rotated between 200-250 rpm on a rotary shaker. Peeled
soil fell through a 352 pm screen into the base. Aggregates were selected
according to their uniform shapes. Priority was given to the most spherical
aggregates with no visible roots to minimize errors originating from peeling plant
root residues. Three aggregates were selected and peeled from the 6.3-9.5 mm
size fraction, 3-4 aggregates from the 4.0-6.3 mm size fraction and 5 aggregates

from the 2-4 mm size fraction of each plot in order to obtain adequate sample for

88

analyses. A single aggregate was weighed, placed in each SAE chamber and
covered with aluminum foil. Each SAE chamber was placed in a glass beaker
(100 ml) and secured with sponge packing. SAE chambers were placed on rotary
platform shaker (lnnova, model 2300, New Brunswick Scientific Co. Inc, New
Jersey, USA), having a rotational diameter of 2.5 cm. External layers were
removed by peeling for about 6 to 90 min at 180-250 rpm. Aggregates that broke
were discarded and replaced. This process was repeated, with frequent
weighing, until the exterior one third (g/g) was removed. An Excel spreadsheet
was used to calculate and predict 33 :1.5 % (g/g) of each soil aggregate was

removed.

Statistical analysis

Treatment effects on measured parameters were estimated by a PROC-
GLM procedure using Statistical Analysis System (SAS Institute, 1999).
Duncan’s multiple range test was used to separate means of measurements.
Carbon, nitrogen and 15N contents of exterior and interior layers of soil
aggregates were compared by paired t-test using Statistical Analysis System
(SAS). Correlation analysis was used to determine relationship between plant

and soil parameters. All significant tests were set at the 0.05 level.

89

RESULTS AND DISCUSSION

Total soil nitrogen (TN)

Soil aggregates, 6.3-9.5 mm across, accumulated the most total N (TN) in
their external and transitional layers in the 0-5 cm depth samples in July 1999
(Figure 3.1 and Table 3.1). Total N was not significantly between aggregate
layers in June 1998 (Figure 3.2). Greater N concentrations resulted in the highest
ratios of external (No) to internal (Ni) in July, which diminished through
September of 1999 (Figure 3.1). These fluctuations in N concentrations on
external regions of larger aggregates reflect greater flux rate of N movement
through larger soil pores associated with aggregates 6.3-9.5 mm across. Greater
soil Ne in July reflects greater soil and residue N mineralization and agrees with
Mendes et al. (1999), who reported that readily mineralizable N content of soil
aggregates was greater in June under cereal cover crop treatmenst than those
measured in September. They found significantly less amount of mineralizable N
under bare fallow treatments than the cover crop treatment.

Exterior layers and transitional (ie., one-third of the soil mass between
exterior and interior regions) layers of soil aggregate size fractions 4.0 - 6.3 and
6.3 - 9.5 mm across, retained more soil N than bare (Control) soils when either or
both root or/and shoot residues of the rye cover crop were present (Tables 3.1

and 3.2)

90

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92

Total nitrogen (g kg'1)

 

 
 

Exterior

 

  

 

 

     

 

 

 

 

 

 

 

 

 

 

 

1-5‘ It _ May1999
1.0 q n error
0.5 ~
0.0 -
Root Root+shoot

2.0

* Jul 1999
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1.0 ~
0.5 . : f
0.0 .

Control Shoo Roo Root+shoot

2.0

Aug 1999
2 0 Control Shoot Root Root+shoot

. Sep 1999

 

Control Shoot Root

 

 

Root+shoot

Figure 3.1. Total nitrogen (TN) concentrations of exterior layers
and interior regions of 6.3- to 9.5 mm soil aggregates from 0-5 cm
depth of a Kalamazoo loam soil in 1999. Significant difl‘erences
between exterior layers and interior regions of aggregates within

the same treatment at the p< 0.05 (*) and p< 0.005 (**) probability levels.

93

Total nitrogen (g kg'1)

 

2.0
May 1998

0.5 ‘

 

 

 

 

 

 

 

 

 

 

 

 

 

0.0 . m .
Control Shoot Root Root+shoot

 

2.0 .
- Exterior June 1998

15 . Interior

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0.5 -

 

 

 

 

 

 

 

    

 

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Control Shoot Root Root+shoot

 

2.0
Oct 1998
1.5 r
1.0 r

0.5 -

 

 

       

 

I _ .

 

0.0 ** .
Control Shoot Root Root+shoot

Figure 3.2. Total nitrogen (TN) concentrations of exterior layers
and interior regions of 6.3- to 9.5 mm soil aggregates from 0-5 cm
depth of a Kalamazoo loam soil in 1998. Significant differences
between exterior layers and interior regions of aggregates within
the same treatment at the p< 0.05 (*) and p< 0.005 (**) probability levels.

94

Total nitrogen concentrations were greater in microaggregates in most
treatments except root+shoot treatment, where there were no significant
differences in TN observed among different aggregate sizes (Figure 3.3).
Transition layers of smaller aggregates, 2.0 - 4.0 mm across, retained
significantly higher TN compared to other layers of aggregates (Table 3.3)
suggesting more transient flow of N across the entire regions of these smaller
soil aggregates. Smaller aggregates also have better oxygen supply in their
internal regions (Sextone et al., 1985). Consequently, the turnover rates of N and
possibly SOM appear to be greater in the interior regions of smaller than larger
aggregates. If there is a homogenous distribution of SOM during the hierarchial
processes of soil aggregate formation (Oades and Waters, 1991), then N
concentrations should be similar in all exterior layers and interior regions of all
sizes of soil aggregates. These reported N gradients suggest alternative
formation processes involving concentric deposits of organo-mineral fractions to
exteriors of expanding size fractions of soil aggregates.

Whole soil aggregates generally had N concentrations that were lower or
equal to N in exterior and transitional layers and interior regions of the same size
soil aggregates (Tables 3.1, 3.2 and 3.3). Similar results were observed for some
of the treatments by Santos (1998). Total N contents within whole soil
aggregates are expected to be in the same range as maximum and minimum N
concentrations obtained for exterior layers and interior regions of soil aggregates.
One reason for the different (lower) N concentrations for whole aggregates may

be the relative heterogeneity of different soil aggregates as analyses of whole

95

and peeled aggregates were on different aggregates randomly selected from the
same whole soil sample.

Sixty soil aggregates having a size fraction between 4.0 - 6.3 mm across
and sampled from a 0-5 cm soil depth in the same 15N Iysimeter of the rye root
(RR) treatment, were analyzed for TN, SOC and 15N. The average TN value for
60 aggregates was 1.3 g kg" having a CV of 15%. The average SOC value was
12.2 g kg'1 with a CV of 19% for the same 60 aggregates. Therefore, there
appears to be considerable variation, of at least these two parameters, among
individual soil aggregates from the same volume of soil. Therefore, soil
heterogeneity is our best explanation for this discrepancy. Further studies are

needed, however, to verify this conclusion.

96

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98

Root Root+shoot

Shoot
Figure 3.3. Total soil nitrogen (TN) concentrations in aggregate size fractions

Control
the same letter within a treatment and between aggregate size fractions are not

from 0-5 cm depth of a Kalamazoo loam soil in July 1999. Values followed by
significantly different at p>0.05 according to Duncan's multiple range test.

Gradients in N concentrations within soil aggregates, as indicated by the
ratio of Ne to Ni (Ne/Ni), for soil aggregates 6.3-9.5 mm across became smaller
and the difference of the ratios became greater as the growing season of corn
progressed (ie., aboveground com biomass increased). Although nitrogen
concentrations in exterior layers increased in July as soil aggregate size
increased, Tables 3.1 — 3.3, significant correlations between changes in ratios of
Ne/Ni and corn biomass were observed for only largest aggregates measured
(6.3-9.5 mm) which were sampled from rye cover crop plots (3:071) and no rye
cover cropped plots (r2=0.88), Figures 3.4 and 3.5.

No significant correlations were observed between changes in Ne/Ni ratios
and the biomass of corn, during early growth nor at the end of the corn growing
season, for soil aggregates 2.0-4.0 and 4.0-6.3 mm across (data not shown). The
lack of correlation between corn biomass and TN in aggregates sizes smaller
than 6.3 mm may be due to the shorter distances between exterior layers and
interior regions within smaller aggregates. Mineralized N, as soil N03 form, can
rapidly diffuse into and out of centers of smaller soil aggregates quickly and
become less available for plant uptake. Rye treatments appeared to have no
influence on TN concentrations in of exterior layers nor and interior regions of 2-4
mm aggregates in July, 1999 (Table 3.3). When whole aggregates were crushed
and analyzed for TN, rye root+shoot treatments contributed the most N to these

small aggregates (p<0.05).

99

Corn blomass (g)

 

 

r l T U T T l

-0.15 -O.1 -0.05 0 0.05 0.1 0.15
Change In ratlo of Ne I Nl

Figure 3.4. Relationship between change in the ratio of N
concentration of exterior layer to interior region (Ne INI) of
6.3 - 9.5 mm soil aggregates from July 1999 to September
1999 and corn biomass at harvest of no-rye cover crop.

100

Corn biomass (g)

180 ~
160+
140 ‘1
12m
100 -
so —
60 it
«Hi; ’
21) ~

R2 = 0.71 ’

 

<3

i l I T l

0 0.1 0.2 0.3 0.4 0.5

Change in ratio of Nel Ni

Figure 3.5. Relationship between change in the ratio of N
concentration of exterior layer to interior region (Ne INI)
of the 6.3 - 9.5 mm soil aggregates from July 1999 to
September 1999 and corn biomass at harvest of rye cover
crop treatments.

101

Rye shoot mulching appeared to contribute more than in situ rye roots, to
whole aggregate N concentrations (Table 3.3, Figure 3.6). Therefore, it is
concluded that deposition of N, originating from decomposing rye roots and
shoots, onto soil aggregates caused larger gradients to develop within the
concentric layers of larger aggregates during the growth of a subsequent corn
crop. Rye root + shoot treatments contributed the most TN and 15N to soil
aggregates than the separate contributions of either in situ rye roots or shoot
mulches.

Nitrogen gradients between the external layers and internal regions of soil
aggregates 6.3-9.5 mm across were greatest in July, when compared to August
or September of 1999 (Figure 3.1). Nitrogen isotope transfer from rye to
concentric layers within soil aggregates (Figures 3.6 and 3.7) suggest rapid
transfer of 15N from dying roots and shoots to soil aggregate surfaces soon after
the spray-killing of rye. Decomposing roots and shoots contribute large quantities
of C and N to soils (Huntjens, 1971, Cheng and Coleman, 1990, Mary et al.,
1993, Ehrenfeld et al., 1997). Although, mineralization of N compounds can be
inhibited by living and dying plant components, much of the mineralized N,
derived from dead roots or root exudates is immobilized by the microorganisms
due to addition of C to the medium. Microorganism utilization of N from the rye
cover crop is a highly probable explanation since C:N ratios of rye roots were
greater than fifty at the sampling date. If C:N ratio of plant residue is greater than

25:1, N will be taken from the inorganic N pool and decomposition continue

102

slowly until the death of microbial population (Paul and Clark, 1996). Therefore
measured total N included recently decomposed residue N, soil N and microbial
biomass N in July 1999. N mineralization from rye root and shoot residue
increased in July and resulted in N gradients of 6.3-9.5 mm aggregates (Figure
3.1) as it was also reported by Mendes et al., (1999). Roots can influence
chemical composition of soil they contact by absorbing water and nutrients and
by releasing C rich root exudates. Root uptake of water creates greater and more
frequent wetting and drying cycles, which increases SOM degradation. This
mineralized N is utilized by com and/or lost from the soil during com growth.

Total N concentrations decreased from spring to harvest (Figures 3.1 and 3.2).

103

15N content (atom %)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.39
0.39 1 Exterior 2'0'4'0 mm
033 . Interior
0.38 ‘
0.37 -
0.37 1 %—_l
0.36 :18
0.00 z 4 /A //, l
0 39 Control Shoot Root Root+shoot
039 _ Exterior 4.0-6.3 mm
0.38 - Interior *
0.38 -
0.37 -
0.37 - %—l
0.36:1: 1 a:
0.00 -
Control Shoot Root Roch-shoot
0.39
0.39 - 5mm" .. 6.3-9.5 mm
0.38 . Interior ..
0.38 - "
0.37 - **
0.37 ~ %—I
0.36 a: I a:
0.00
Control Shoot Root Root+shoot

Figure 3.6. Total 15N contents of exterior layers and interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,

shoot, root and root+shoot treatments of the Kalamazoo loam

soil on July 1999 . Significant differences between exterior layers
and interior regions of aggregates within the same treatment at the
p< 0.05 (*) and p< 0.005 (**) probability levels.

104

15N content (atom °/o)

 

0.60
0.55 a
0.50 .
0.45 -
0.40 .

 

0.60

0.35:}: W %— Z1 :1?
0.00 -M—_%_%

Exterior 2-0'4-0 mm
[:1 Interior

 

 

 

 

 

Control Shoots Roots Roots-i-shoots

 

0.55 -
0.50 1
0.45 1
0.40 -

 

0.00 -

0.60

0.35% W%% '%l

Exterior 4.0-6.3 mm

[:1 Interior

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Control Shoots Roots Roots+shoots

 

 

 

0.55 -
0.50 q
0.45 -
0.40 .
0.359

Exterior 6'3'9'5 mm
E Interior

*

R W %— Z‘ a?

 

0.00 -

 

 

 

 

me

Control Shoots Roots Roots+shoots

 

Figure 3.7. Total “N contents of exterior layers and interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,

shoot, root and root+shoot treatments of the Kalamazoo loam

soil on June, 1998 . Significant differences between exterior

layers and interior regions of aggregates within the same
treatment at the p< 0.05 (*) and p< 0.005 (**) probability levels.

105

 

Rye root and shoot derived nitrogen

Distribution of 15N among the full range of aggregate size fractions
extracted from bulk soils of the three rye cover crop treatments showed trends of
greater 15N contents in smaller aggregates and lesser 15N contents in larger
aggregates (Figure 3.10). Microaggregates (<0.25 mm) retained the second
highest rye-derived 15N. This suggests more uniform distributions of 15N within
the smaller aggregates and possible 15N gradients established within larger
aggregates. Nitrogen from rye roots and shoots could be detected on the exterior
layers of soil aggregates of 4.0 - 6.3 and 6.3 - 9.5 mm as early as 17 days after
rye shoot applications to the soil surface in 1998 (Figure 3.7). Rye root
contributions of N were greater than that of rye shoot N, presumably due to the
more rapid decomposition and direct contact of rye roots to soil aggregates.
Contrasting gradients of 15N, derived from rye increased with increasing
aggregate size (Figure 3.7). Similar increases of 15N gradients with aggregate
sizes were also observed in July 1999, during the second year of these
experiments (Figure 3.6). These results support that most of roots grow
preferentially around surfaces of soil aggregates rather than through aggregates.
Although concentrations of 15N on surface layers and interior regions of
aggregates 2.0-4.0 mm across were the same as the surface layers of larger
aggregates, no gradients of 15N from rye cover crops were observed for
aggregates 2.0-4.0 mm across (Figures 3.6 - 3.9). Organic materials derived
from rye roots and shoots appeared to be uniformly distributed throughout

aggregates 2.0 - 4.0 mm across with minimum 15N gradients at the beginning of

106

the corn growing season (Figures 3.6 and 3.7). Contents of 15N within aggregates
2.0 - 4.0 mm across decreased with no gradients were observed at harvest
(Figure 3.8). The 15N gradients developed within larger soil aggregates, 6.3-9.5
mm across, decreased in October 1998, 116 days after rye shoot application
(Figure 3.8). Nitrogen isotope gradients between external layers and internal
regions of soil aggregates 4.0 — 6.3mm across developed early in the summer
(Figure 3.7) diminished as the season progressed (Figure 3.8). Exterior layer of
soil aggregates contained similar concentrations of 15N as interior regions at com
harvest (Figures 3.8 and 3.9). In summary, there seemed to be a migration of 15N
materials from rye roots and shoots into soil aggregates at a constant rate. Early
in the season, more 15N migrated to the interior regions of the smallest
aggregates, 2 - 4mm across, but was limited to only surfaces and transitional
regions of the larger aggregates, 6.3 — 9.3 mm across. At harvest, more of the
15N located within interior regions of the smallest sized aggregates was
withdrawn by com growth while more 15N remained within the interior regions of
the medium sized aggregates, 4 - 6.3 mm across (Figures 3.7-3.8 and 3.6 - 3.9.
Mineralization of SOM may be more rapid on the surfaces of soil
aggregates and may be stimulated by growing corn roots (Sanchez, et al., 2000).
Living roots provide large quantities of C compounds to the surfaces of soil
aggregates (Santos, 1998) promoting microbial biomass activities and greater N
mineralization (Texier and Billes, 1990). Therefore, corn roots appear to be
important C sources for stimulating microorganisms in the soil. Their specific

locations on soil aggregates of different sized fractions need further investigation.

107

When roots preferentially grow on the surfaces of soil aggregates, as discussed
above, these roots should increase N mineralization in the external regions of
aggregates. Frequent wetting-drying cycles will diffuse more N into interior
regions of soil aggregates of all sizes. Mean-free pathways, however, limit the
diffusive distance or depths within aggregates of different size fractions.
However, when roots are present or when soil water contents are high, highly
mobile mineral N, located on surface layers of larger aggregates and throughout
smaller aggregates can either be absorbed or leached from these respective
areas of multiple sized soil aggregates with subsequent diffusion from their
interior regions towards their exterior regions. The good correlation (r2=0.68)
between 15N ratio of exterior layers to interior regions of soil aggregates and 15N
uptake by com plant support these conclusions. Similar correlations were found
between changes in the ratios of total N of external layers and internal regions of
soil aggregates at the beginning and end of the corn growing season and corn
biomass (Figures 3.4 and 3.5) in 1999. Increases in the ratios of Ne/Ni (in July) -
Ne/Ni (in September) demonstrate root uptake of N during the corn growing
season. As these ratios increased, greater com biomass was observed (Figures
3.4 and 3.5). Thus, it is clear that uptake of N is more efficient from the surface of
aggregates from a Kalamazoo loam soils larger than 4 mm across.

Most of the 15N presented in the interior regions of soil aggregates greater
than 4 mm across was preserved at the time of corn harvest. Especially since
many of the roots appear to grow around exterior regions of soil aggregates

(Allison, 1973, Whiteley and Dexter, 1983). It was also observed that

108

approximately 20% of the aggregates contained some of the finer roots which
had penetrated and passed through soil aggregates. These soil aggregates
containing roots were not selected for analyses. Any fine rye root fragment
located within aggregates of any size would result in the deposition of
mineralized 15N which could be sequestered within larger aggregates and
become unavailable to corn roots unless they penetrated the same larger soil
aggregate (Rasse and Smucker, 1999).

More rye root-derived N accumulated on exterior layers of soil aggregates
6.3 - 9.5 mm across than rye shoot-derived N (Figure 3.7). In the first year of
experiment soil aggregate samples were taken 17 days after application of
labeled rye shoots on the PVC chambers. During the labeling period some 15N
was transferred from rye shoots to roots and was released to soil by rye roots as
root exudates. During applications of Round Up and cutting rye shoots, dead
roots continued to release N compounds to the soil. Therefore, more rye root-
derived N was deposited on the exterior layers of aggregates in 1998 (Figure
3.7). In the second year of the experiment, first samples were taken 51 days after
application of labeled rye shoots. During that time root derived N was already
utilized by com and shoot derived N concentration was greater than root derived

N (Figure 3.6).

109

15N content (atom °/o)

 

0.60
0.55 3
0.50 -
0.45 ‘
0.40 -

0.35:? 55555551 I 1333331 I E32222;

Exterior
[:1 Interior

2.0-4.0 mm

%

 

 

W

 

 

=1

 

_%

 

 

 

 

 

 

0.00 -
Control Shoots Roots Roots+shoots
0.60
0.55 . Exterior 4.0-6.3 mm
0.50 . [2 Interior
0.45 ~
0.40 - @f
0.35 a: m ' W % I {=6
0.00 + /A 4 //A J.
Control Shoots Roots Roots+shoots
0.60
0.55 - Exterior 6.3-9.5 mm
0.50 4 1:] Interior
0.45 -
0.0, _W%

 

 

 

Control Shoots Roots

 

 

Roots+shoots

Figure 3.8 . Total 15N contents of exterior layers and Interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,

shoot, root and root+shoot treatments of the Kalamazoo loam
soil on October 1998 . Significant differences between exterior
layers and interior regions of aggregates within the same treatment
at the p< 0.05 (*) and p< 0.005 (**) probablllty levels.

110

15N content (atom %)

0.39

0.39 ' Exterior 2.04.0 mm
0.38 - [:1 Interior

0.38 —
0.37 -
0.36:: a
0.00 W M LW_1__

Control Shoots Roots Roots+shoots

 

J)

 

 

 

0.39

0.39 - Exterior
0.38 _ [:1 Interior

0.38 -

323:: %—‘ Z7
3233M M 1%!

Control Shoots Roots Roots+shoots

 

4.0-6.3 mm

)1
D

 

 

 

 

 

 

 

0.39

Em: ......
0.38 .

0.38 - , *

0.37 -

- a7 a1 a1

0.36 an a:
0.00 -MW_1__

Control Shoots Roots Roots+shoots

Figure 3.9. Total 15N contents of exterior layers and interior regions
of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from control,

shoot, root and root+shoot treatments of the Kalamazoo loam

soil on September, 1999 . Significant differences between

exterior layers and interior regions of aggregates within the

same treatment at the p< 0.05 (*) and p< 0.005 (**) probability levels.

111

 

 

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112

Flgure 3.10. Concentrations of 15N in aggregate size fractions sampled from 0-5 cm depths of a
Kalamazoo loam soil in July 1999. Values followed by the same letter within each treatment

and among aggregate size fractions are not significantly different at p>0.05 according

to Duncan's multiple range test, n=4.

Nitrogen derived from rye shoots, roots and root plus shoots were not
significantly different for aggregates 2.0 - 4.0 and 6.3 - 9.5 mm across when
whole soil aggregates were analyzed (Table 3.4). However, separation of
external layers of individual aggregates demonstrated the contributions of short-
terrn rye shoot, root and shoot plus root to soil N pools.

Nitrogen derived from root (Ndfr) and shoot (Nuts) located in the exterior
layers diminished from planting to harvest (Figure 3.11). The percentage of total
N derived from rye shoot and rhizodeposition from rye roots was calculated using
equation [1], described in Chapter 4 (Kavdir,. 2000) Exterior layers of aggregates
6.0 - 9.5 mm across retained 1.6% of the Ndfr in July 1999, three times more than
their interior regions (Figure 3.11). This was slightly greater than the o/ONdfs. One
month later, during the com growing season 7onr and o/ONdfs were nearly equal
in exterior layers and interior regions of soil aggregates, possibly due to diffusion
within larger aggregates and uptake by com. At harvest, there were greater or
equal quantities of rye-N located in interior regions compared to exterior layers of
aggregates.

In the case of N fertilization, diffusion rate of N from exterior layers to
interior regions of aggregates and even leaching could be faster limiting
availability of N to the plant. Kinyangi (2000), reported that if there is more P in
the exterior layers of soil aggregates, 4.0-8.0 mm across, than interior regions at
the beginning of corn season, corn roots can easily utilize this P resulting in
increased corn yield. Aggregate sizes used in this study covered only 34% of the

total soil by weight. Additional research on the best management practices for

113

maintaining more N on surfaces of larger soil aggregates during crop growth as
well as sequestering mobile soil N within larger soil aggregates during wet soil
periods between cash crops needs to be completed

Aggregates greater than 2 mm contain many longer roots and hyphae
than microaggregates (Jastrow and Miller, 1998 and Degens et al., 1994).
Aggregates greater than 2 mm had 150 times longer hyphal lengths per
aggregate than aggregates smaller than 0.5 mm. Aggregates greater than 2 mm
also had 7 times longer root lengths within compared to soil aggregates 1.0-2.0
mm across (Degens et al., 1994). Therefore, main stabilizing factors for
macroaggregates are roots, root derived materials and hyphae. While rye and
corn roots develop between the planes of weakness and along surfaces of
aggregates, root derived materials also help to stabilize macroaggregate
surfaces. Continuous addition of SOM through dead and leaving roots and
uptake of nutrients and waters by roots contributes development and stabilization
of soil aggregates.

If we assume the aggregate hierarchy model, proposed by Oades and
Waters (1991) is the only model for the soil structure formation of the Kalamazoo
loam soil, then macroaggregates should consist of mostly microaggregates and
properties of the macroaggregates should be similar all the way across the
aggregate. The formation and function of soil macroaggregates are very dynamic
processes utilizing many biogeochemical factors. These factors include:
continuous additions of C and N compounds by roots (Santos et al., 1998,

Kavdir, 2000); additions of N and P by fertilizers (Kinyangi, 2000); weathering of

114

clay minerals by water, microbes and roots (Santos et al., 1997); dessication of
aggregates by the uptake of water by plant roots (Sissoko, 1997); nutrient
extraction by plant roots and leaching; frequent wetting and drying cycles and
countless microbial activities (Guggenberger et al., 1999) all appear to develop
concentric layers of various properties into the interior regions of soil aggregates.
In a summary, it was found that concentric gradients of rye residue-derived N
increased with aggregate size. The location of the N within soil aggregate played
an important role on com N uptake. Rye root and shoot derived N in exterior
layers of larger aggregates decreased by time. Therefore these studies suggest
increasing soil aggregate size and maintaining active plant root systems are the
best strategies for maximizing soil N availability to cash cropping systems and

reducing N leaching.

115

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116

N derived from rye roots (°/o)

N derived from rye shoots (°/o)

 

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.0 ..
+ Exterior layer
15 _ Interior region
1.0 .
0.5 A
0.0 I r 1
July August September
2.0
Shoot (B) .
+ Extenor layer
1'5 d --O-- Interior region
1.0 -
0.5 -
0.0 l T I

 

 

 

July August September

Figure 3.11 . Percentage of N derived (A) from rye roots (%Ndfr)
and (B) shoots (%Ndfs) in the exterior layers and interior regions of
6.3-9.5 mm soil aggregates from 0-5 cm depth of a Kalamazoo loam
soil in July, August and September 1999. Bars represent standard
deviations for n=4.

117

Aggregate erosion rate

Soil eroded from aggregate layers was reduced from 2-10 fold by rye
cover crop. Exterior soil layers peeled from 2.0-4.0 mm aggregates were 4.8
times more resistant in rye treatments than those from no rye treatments. (Figure
3.12). Transitional layers peeled from 2.0-4.0 mm soil aggregates from rye
treatments were 10.6 times more resistant for rye treatments than those from no
rye cover treatments.

Exterior soil layers peeled from 4.0-6.3 mm aggregates were 1.7 times
more resistant in rye treatments than those from no rye treatments. (Figure 3.12).
Transitional layers peeled from 4.0-6.3 mm soil aggregates from rye treatments
were 2.0 times more resistant for rye treatments than those from no rye cover
treatments.

Exterior soil layers peeled from 6.3-9.5 mm aggregates were 2.1 times
more resistant in rye treatments than those from no rye treatments. (Figure 3.12).
Transitional layers peeled from 6.3-9.5 mm soil aggregates from rye treatments
were 1.4 times more resistant for rye treatments than those from no rye cover
treatments.

Smaller (2.0-4.0) mm aggregates were much more resistant to erosion
than larger aggregates and rye contribution to aggregate stability of 2.0-4.0 mm
aggregates were much more distinctive than those of aggregates greater than 4
mm. Eroded soil weight per minute increased with increasing aggregate sizes.

Erosion rates (mg min") of soil materials from external layers were significantly

118

greater than those extracted from transitional layers for all three soil aggregate
fractions from control (no rye) treatments (Figure 3.12).

Dapaah and Vyn, (1998), reported that aggregate stability was higher
following cover crops (ryegrass, red clover and oilseed radish) than where no
cover crops were used. Raimbault and Vyn, (1991) also reported that under
cover crop treatments wet soil aggregate stability was greater than those of no
cover treatments. Results from Meso SAE chambers also confirmed that
aggregates from rye cover crop treatments were much more resistant to erode
than those from no rye cover crop treatments. Meso SAE chambers can be used
to compare short term cropping systems affects on soil aggregate stability for

different locations of aggregates.

119

Erosion rate (mg soil min'1)

 

 

2.0 - 4.0 mm
External

1:! Transitional

 

 

 

 

 

 

 

 

m...

3- T 4.0-6.3mm
2.

1. T

o .

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6.3 - 9.5 mm

 

 

 

 

 

 

 

 

 

 

 

I

Control Rye

Flgure 3.12 . Erosion rates of external and transitional layers

of 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm aggregates from no-rye

(control) and rye cover cropped treatments of the Kalamazoo

loam soil on October 1998 . Bars represent standard errors for n=4.

120

REFERENCES

Allison, PE. 1973. A factor in soil aggregation and root development. In. Soil
organic matter and its role in crop production. p:314-345.

Angers D.A. and GR. Mehuys. 1989. Effects of cropping on carbohydrate
content and water-stable aggregation of a clay soil. Can. J. Soil Sci. 69:373-380.

Angers, D.A. and GR Mehuys. 1990. Barley and alfalfa cropping effects on
carbohydrate contents of a clay soil and its size fractions. Soil Biol. Biochem.
22:285-288.

Angers, D.A., A. Pesant and J. Vigneux.1992. Early cropping-induced changes in
soil aggregation, organic matter and microbial biomass. Soil Sci. Soc. Am. J.
56:115-119.

Angers, D.A., S. Recous and C. Aita. 1997. Fate of carbon and nitrogen in water-
stable aggregates during decomposition of 13C‘SN labeled wheat straw in situ.
European Journal of Soil Sci. 48:295-300.

Angle, J.S., C.M. Gross, R. L. Hill and M. S. McIntosh. 1993. Soil nitrate
concentrations under corn as affected by tillage, manure, and fertilizer
applications. J. Environ. Qual. 22:141-147.

Blackmer, A. M. and J.M. Bremner. 1977. Nitrogen isotope discrimination in
denitrification of nitrate in soils. Soil Biol. Biochem. 9:73-77.

Bottner, P. 1985. Response of microbial biomass to alternate moist and dry
conditions in a soil incubated with 14C and 15N labeled plant material. Soil Biol.
Biochem. 17: 329-337.

Cheng, W., D. Coleman, C.R. Carroll and CA. Hoffman. 1993. In situ
measurement of root respiration and soluble C concentrations in the rhizosphere.
Soil Biol. Biochem., 25:1 189-1196.

Cheng, W. and 0.0. Coleman. 1990. Effect of living roots on soil organic matter
decomposition. Soil Biol. Biochem. 22:781-788.

Dapaah, H. K. and T. J. Vyn. 1998. Nitrogen fertilization and cover crop effects

on soil structural stability and com performance. Com. Soil Sci. Plant Anal. 29 :
2557-2569.

121

 

Davidson, E.A., R.W. Eckert, S.C. Hart and MK. Firestone. 1991. Direct
extraction of microbial biomass nitrogen from forest and grassland soils of
California. Soil Biol. Biochem. 21 :773-778.

Degens, B.P., G.P. Sparling and L.K. Abbott. 1994. The contribution from
hyphae, roots and organic C constituents to the aggregation of a sandy loam
under long-term clover based and grass pastures. European J. of Soil Sci.
45:459-468.

Ditsch, 0.0., M. A. Alley, K.R. Kelley and Y2. Lei.1993. Effectiveness of winter
rye for accumulating residual fertilizer N following com. J. Soil Water Conserv.
48:125-132.

Dormaar, J.F. and C. W. Lindwall. 1989. Chemical differences in dark brown
chemozemic Ap horizons under various conservation tillage systems. Can. J.
Soil Sci. 69:481-488.

Ehrenfeld J. G., W. F.J. Parsons, X. Han, R. W. Parrnelee and W. Zhu. 1997.
Live and dead roots in forest soil horizons: Contrasting effects on nitrogen
dynamics. Ecology. 78:348-362.

Fisher R.W., H. Flessa and G. Schaller. 1989. pH values and redox potentials in
microsites of the rhizosphere. Z. Pflanzenemahr. Bodenk., 152:191-195.

Frensch, J., T.C. Hsiao and E. Steudle. 1996. Water and solute transport along
developing maize roots. Planta. 198:348-355.

Guggenberger, G., E.T. Elliott, S. D. Frey, J. Six and K. Paustian. 1999. Microbial
contributions to the aggregation of a cultivated grassland soil amended with
starch. Soil Biol. Biochem. 31:407-419.

Halberg, G. R.1986. From hoes to herbicides: Agriculture and groundwater
quality. J. Soil Water Conserv. 41: 357-364.

Harper, L.A., P. F. Hendrix, G.W. Langdale and D. C. Coleman. 1995. Clover
management to provide optimum nitrogen and soil water conservation. Crop
Science. 35:176-182.

Hom,R. 1990. Aggregate characterization as compared to bulk soil properties.
Soil Till. Res. 172265-289.

Hojberg, 0., NF. Revsbech and J.M. Tiedje. 1994. Denitrification in soil

aggregates analyzed with microsensors for nitrous oxide and oxygen. Soil Sci.
Soc. Am. J. 58:1691-1698.

122

Huntjens, J. L. M. 1971. The influence of living plants on mineralization and
immobilization of nitrogen. Plant Soil. 35:77-94.

Janzen H. H. 1990. Deposition of nitrogen into the rhizosphere by wheat roots.
Soil Biol. Biochem. 22:1 155-1160.

Janzen H. H. and Y. Bruinsma.1993. Rhizosphere N deposition by wheat under
varied water stress. Soil Biol. Biochem. 25:631-632.

Jastrow, J. D. and RM Miller. 1998. Soil aggregate stabilization and carbon
sequestration: feedbacks through organomineral associations. ln:Soil processes
and the carbon cycle. R. Lal., J. M. Kimble, R. F. Follet and B. A. Stewart (Ed.).
CRC Press, p: 207-223.

Jensen, ES. 1996. Rhizodeposition of N by pea and barley and its effect on soil
N dynamics. Soil Biol. Biochem. 28:65-71.

Jordan, D., R.R. Bruce and DC. Coleman. 1996. Nitrogen availibility to grain
sorghum from organic and inorganic sources on sandy and clayey soil surfaces
in a greenhouse pot study. Biol. Fertil. Soils. 21 :271-276.

Kavdir, Y. 2000. Distribution of cover crop N retained by soil aggregates in rye-
com agroecosystem. Thesis (PhD). Michigan State University. Dept. of Crop and
Soil Sciences. 157 p.

Kay B. D., D.A. Angers, P.H. Groenevelt and J.A. Baldock. 1988. Quantifying the
influence of cropping history on soil structure. Can. J. Soil Sci. 68:359-368.

Killham, K. 1994. Soil Ecology. Cambridge University Press, Cambridge, UK, 242
PP-

Kinyangi J. M. 2000. Carbon, nitrogen and phosphorus sequestratiOn within soil
aggregates of maize and tree based agroforestry systems. MS. Thesis. Michigan
State University. Dept. of Crop and Soil Sciences.

Kladivko, E. J. 1994. Residue effects on soil physical properties. in: Managing
Agricultural Residues. Unger P.W. ed., Lewis Publishers, Boca Raton, FL. p:123-
141.

Kirsten, W.J. 1983. Organic elemental analysis. Academic Press, New York, NY.

Lazzari, MA. 1982. Distribution of 15N fertilizer in field lysimeters sown with garlic
(Allium sativum) and foxtail millet (Setan’a italica). Plant and Soil. 67:187-191.

123

Mary, B., C. Fresneau, J.J. Morel and A. Mariotti. 1993. C and N cycling during
decomposition of root mucilage, roots and glucose in soil. Soil Biol. Biochem. 25:
1005-1014.

McCracken, D.V., M.S. Smith, J. H. Grove, C.T. MacKown and R.L. Blevins.
1994. Nitrate leaching as influenced by cover cropping and nitrogen source. Soil
Sci. Soc. Am. J. 58:1476—1483.

Mendes, J.C., A. K. Bandick, R. P. Dick and P.J. Bottomly. 1999. Microbial
biomass and activities in soil aggregates affected by winter cover crops. Soil Sci.
Soc. Am. J. 63:873-881.

Oades J. M. and A. G. Waters. 1991. Aggregate hierarchy in soils. Aust. J. Soil
Res. 29:815-828.

Paul, EA. and FE. Clark. 1996. Soil microbiology and biochemistry, 2nd ed.,
Academic Press, San Diego, California. 273 p.

Qian, J.H., J.W. Doran and D.T. Walters. 1997. Maize plant contributions to root
zone available carbon and microbial transformations of nitrogen. Soil Biol.
Biochem. 29:1451-1462

Raimbault, BA. and T.J. Vyn. 1991. Crop rotation and tillage effects on corn
growth and soil structural stability. Agron J. 83:979-985..

Rasse, D. P. 1997. Alfalfa and corn root modifications of soil nitrogen flux and
retention. Thesis (Ph.D.).Michigan State University. Dept. of Crop and Soil
Sciences.149 p.

Rasse, DP. and A. J. M.Smucker. 1998. Root recolonization of previous root
channels in com and alfalfa rotations. Plant and Soil. 204:203-212.

Roberson, E., S. Sarig, C. Sherman and MK. Firestone. 1995. Nutritional
management of microbial polysaccharide production and aggregation in an
agricultural soil. Soil Sci. Soc. Am. J. 59:1587-1594.

Sanchez, J. E., E. A. Paul, T. C. Wilson, J. P. Smeenek and R. R. Harwood.
2000. Cropping effects on nitrogen mineralization. Submitted to Agronomy J.

Santos, D., S.L.S. Murphy, H. Taubner, A.J.M. Smucker and R. Horn. 1997.
Uniform separation of concentric surface layers from soil aggregates. Soil Sci.
Soc. Am. J. 61 :720-724.

Santos, D. 1998. Contributions of roots and organic matter to soil aggregate

development and stabilization. Thesis (Ph.D.) Michigan State University. Dept. of
Crop and Soil Sciences. 149 p.

124

Sextone, A.J., N.P. Revsbech, T.B. Parkin and J.M. Tiedje. 1985. Direct
measurements of oxygen profiles and denitrification rates in soil aggregates. Soil
Sci.Soc. Am. J. 49:646-651.

Shen, S.M., G. Pruden and D. S. Jenkinson. 1984. Mineralization and
immobilization of nitrogen in fumigated soil and the measurement of microbial
biomass nitrogen. Soil Biol. Biochem. 16:437-444.

Sierra, J. and P. Renault. 1996. Respiratory activity and oxygen distribution in
natural aggregates in relation to anaerobiosis. Soil Sci.Soc. Am. J. 60:1428-
1438.

Sissoko, F. 1997. Enhancement of soil aggregation by the combined influences
of soil wetting and drying and root-microbial associations Thesis (M.Sc.)
Michigan State University. Dept. of Crop and Soil Sciences. 109 p.

Smucker, A.J.M., D. Santos and Y. Kavdir. 1997. Concentric layering of carbon,
nitrogen, and clay within soil aggregates from tilled and nontilled ecosystems. D.
Anger (ed.)., Third Eastern Canada Soil Structure Symposium Proceedings. p.
129-141.

Smucker A.J.M., C.J. Dell. J.M. Kinyangi, Y. Kavdir and J. D. Jastrow. 1999.
Quantitative removal of concentric layers from soil aggregates by soil aggregate
erosion chambers. Abstracts for ASA-CSSA-SSSA, 91$t Annual Meeting, Salt
Lake City, Utah, October 1999.

Stephen, C. and D. D. Myrold. 1996. 15N Tracer studied of soil nitrogen
transformations. ln: mass spectrometry of soils. (T.W. Boutton and S. Yamasaki,
eds.) Marcel Dekker, lnc.pp: 225-247.

Texier, M. and G. Billes. 1990. The role of rhizosphere on C and N cycles in a
plant-soil-system. Symbiosis. 9:1 17-123.

Thomsen, l. K, V. Kjellerup and B. Jensen. 1997. Crop uptake and leaching of
N-15 applied in ruminant slurry with selectively labeled faeces and urine
fractions. Plant and Soil. 197:233-239.

Vasilas, B.L., J.O. Legg and DC. Wolf. 1980. Foliar fertilization of soybeans:
absorption and translocation of 15N labeled urea. Agron J. 72:271-275.

Whiteley, G. M. and A. R. Dexter.1983. Behavior of roots in cracks between soil
peds. Plant and Soil. 74:153-162.

125

Wilcke, W. and W. Amelung. 1996. Small-scale heterogeneity of aluminum and
heavy metals in aggregates along a climatic transect. Soil Sci. Soc. Am. J.
60:1490-1495.

Wilcke, W. and M. Kaupenjohann. 1998. Heavy metal distribution between soil
aggregate core and surface fractions along gradients of deposition from the
atmosphere. Geoderma. 83: 55-66.

Wood, C.W., D.G. Westfall and GA. Peterson. 1991. Soil carbon and nitrogen
changes on initiation of no-till cropping systems. Soil Sci. Soc. Am. J. 55:470-
476.

Yoneyama, T., T. Muraoka, N. Boonkerd, P. Wadisirisuk, S. Siripin and K.
Kouno. 1990. Natural 15N abundance in shrub and tree legumes, Casuarina, and
non-fixing plants in Thailand. Plant Soil. 128:287-294.

Zebarth, B.J., V. Alder and R.W. Sheard. 1991. In situ labeling of legume

residues with a foliar application of a 15N enriched urea solution. Commun. In Soil
Sci. Plant Anal. 22:437-447.

126

CHAPTER 4

COVER CROP ROOT AND SHOOT NITROGEN CONTRIBUTIONS TO
SUCCEEDING CORN CROP IN SITU

ABSTRACT

Winter rye (Secale cereale) cover cropping can reduce nitrogen leaching
from the soil profile and serve as a nitrogen source for a succeeding crop of corn
(Zea mays, L.). A field study was conducted on Kalamazoo loam soil (coarse-
loamy, mixed, mesic Typic Hapludalf) to quantify N absorption by a living and
resultant contributions of decomposing rye root and shoot residues through soil N
pools and into corn.

Two open-ended PVC cylinder lysimeters were inserted through the Ap
and into the center of the Btg horizons. Rye cover crops in these cylinders were
labeled in situ with 15N by foliar applications of solutions containing (‘5NH4)2SO4
(%99 atom % 15N). Corn was planted into the cylinders after herbicide
desiccation of rye.

The average two-year recovery of rye root-derived 15N by a successive
corn crop was 13%. Recovery of rye shoot-derived 15N by com for two years
averaged 8%. During the two-year study rye roots contributed nearly 12 kg N ha'1
to the succeeding corn crop and rye shoots contributed nearly 4 kg N ha'1 N to
succeeding corn crops. Rye roots plus shoots contributed slightly more than 15

kg N ha'1 during the two-year study. Rye root N contributions were three-fold

127

greater than shoot N, supporting the importance of considering root contributions
by a cover crop when developing N balance summaries for cover crop and

successive cash crop rotations.

INTRODUCTION

Conservation tillage systems that leave crop residues on the soil surface
are becoming very popular. The use of cover crop with a conservation tillage
system can increase ground residue cover, reduce soil erosion, increase water
infiltration, conserve soil water content and also serve as a sink source for plant
nutrients. Rye (Secale cereale, L.) is regarded as one of the best suited crops for
winter cover use because of its adaptability, tolerance to extreme cold and ease
of establishments. Rye produces large amounts of mulch for succeeding no till
com (Ditsch et al., 1993). A better understanding of the processes involved in
crop residue decomposition and N release in these systems is needed to develop
more efficient residue and fertility management practices. Crop residues
contribute significant amounts of N to the succeeding crop. Throughout the past
decade, a large number of researchers have focused on winter rye cover crop
uptake on residual soil N, release of N from cover crop residue (Ranells and
Wagger 1997, Wagger 1989a), and N uptake by a summer crop (Karlen and
Doran, 1991, Wagger 1989b). However, there is still lack of information on
quantities of N contributed from above and below ground rye cover crop residues

to succeeding corn plants. Plant roots control the concentrations and fluxes of

128

soil N by absorbing soil water and soluble N compounds (Frensch, 1996).
Released N in situ from decomposing plant roots and shoots contribute N to the
succeeding plant (Seiter and Honlvath, 1999). Amount of N deposited in the
rhizosphere of wheat accounted up to 20% of total plant N (Janzen, 1990 and
Janzen and Bruinsma, 1993). The amount of N deposited from pea residue was
48% of belowground N and from barley was 71% of total belowground N at
maturity (Jensen, 1996). Excretion of plant available N forms and mineralized
forms of N from rhizodeposits may be reabsorbed by current or succeeding
plants.

Most studies on 15N labeled residue-decomposition and associated
nitrogen transferred to the following crop have used dried leaves, stems and
sometimes roots. Although some researchers have determined N uptake from
root residues by succeeding plants, they first extracted roots from pots or
microplots and then incorporated them into the soil (Harris and Hesterrnan, 1990,
and Norman et al, 1990). Hubbard and Jordan (1996) reported 15N recovery of
corn from labeled soil plus a wheat root mix but they could not identify the direct
recovery from roots only. Stevenson et al., (1998) used an indirect approach for
estimating legume root-derived N uptake by succeeding plants. Few studies used
in situ labeling of plant materials such as foliar N fertilization in field studies
(Zebarth et al. 1991, Jordan et al., 1996) to determine plant uptake of N from
foliar N fertilization. Seiter and Horwath (1999) best approached the question of
root N contributions to successive plants by injecting 15N into trees of

agroforestry cultures and determined the corn uptake of root-derived N.

129

This research was conducted to specifically identify N contributions by
aboveground and belowground residues of rye cover crops to succeeding com
plants using 15N tracers under field conditions. The objective of this study was to

determine recovery of N from rye roots and shoots by the succeeding corn crop.

130

MATERIALS AND METHODS

A two-year, field experiment was conducted from 1997 to 1999 on 16
Microplots (6 by 10 m) established in August 1994 by Rasse (1997) on a
Kalamazoo loam soil (coarse-loamy, mixed, mesic Typic Hapludalf) at the
KBS/LTER site in southwestem Michigan. The same four treatment, consisting of
a control (C), rye roots only (RR), rye shoots only (RS) and rye roots and shoot
mulch (RRS) were distributed in a randomized block experiment as was
explained in previous Chapters. Some of the soil horizon properties and weather

data for the experiment site are presented in Tables 4.1 and 4.2.

Experiments with 15N

Two open-ended PVC cylinders, 30 cm in diameter and 60 cm in depth,
were pushed through the Ap horizon and into the center of the Btz horizon in
each plot by the help of front fork loader of a tractor as explained in Chapter 2
(Kavdir, 2000). In spring and fall of 1998, approximately 45 rye seeds were
planted into each cylinder of the rye treatment plots. The soil surface in each
cylinder was covered with plastic sealed around the walls and each row by
nontoxic clay sealant. Pine wood shavings were placed on the plastic to absorb
15N labeled spray mist materials preventing them to contact with soil surface.
Cylinders with no cover crops also covered plastic cover and pine shavings to

receive the same treatments.

131

Rye plants were labeled with 15N by means of foliar applications of
solutions containing 6.39 g (‘5NH4)2SO4 containing 99 atom% 15N dissolved in 9
L of distilled water in May. Three or four split applications of this solution were
applied to prevent run off or leaf damage by toxicity. Each time equal amounts of
15N solution (approximately 125 ml) were applied to each rye planted chambers
manually using graduated fine mist spray bottles. Plants within the PVC were
covered by clothes-baskets, with nested sides, and securely anchored to the soil
at night or before rain events to prevent foliar losses. These covers were
removed during sunny days.

Following a two week translocation period, the rye plants were spray-killed
with 2% Roundup (glyphosate) Ultra, without ammonium sulphate, in early May
of 1998 and 1999. Rye shoots were cut at the soil surface, wood shavings were
vacuumed and the clay sealants and plastic soil covers removed from each 15N
Iysimeter. Biomass samples of harvested rye shoots were weighed and
subsampled for analyses. Rye shoots were placed on the soil surfaces inside the
PVC cylinder lysimeters for all RS and RRS treatments.

Corn seeds (6-8) were hand planted into each PVC cylinder. Metal
screens with 1 cm openings were placed on top of the soil and secured with nails
to prevent residue loss by wind or animal consumption of com seeds or rye
shoots. Each chamber received 500 ml water from the soil surface. Chambers
were thinned to two plants 2 days after emergence. Thinned corn plants were left
on the soil surface of the chambers. No N fertilizers were applied to PVC

lysimeters.

132

In the first year of experiment two corn plants were grown in each 15N
Iysimeter. However, one of these plants was always smaller than the other one
and showed weak growth. Therefore, during the second year one corn plant was

grown in each 15N Iysimeter.

Soil sampling and analyses

Soil sub samples were taken before 15N labeling and at harvest to
determine initial and final 15N contents of the soil. At harvest, September 1998,
surfaces of the soil were divided into four equal parts: one part was taken for soil
analyses and another part was taken for root extractions, using a square shovel.
The remaining two parts were left in the chambers. During the second year, soil
core samples were taken from the chambers to depths of 150 cm, using a
hydraulic Gidding’s probe (Giddings Machines 00., Ft. Collins, CO) to measure
soil and com root N and 15N. Soil samples were split in half, vertically. One half
was air dried, ground and sieved through 53 um screen. Samples smaller than
53 um size were collected and analyzed for total N and C by means of the dry
combustion technique using a CHN analyzer (Carlo Erba, Italy) and %‘5N by
using Isotope Ratio Mass Spectrometer Model 2020 (Europa Scientific, Crewe,

UK).

133

Rye and corn root and shoot sampling and analyses

Rye root and shoot sub samples were taken before and after the 15N
labeling to determine initial and final 15N contents of plant shoots and roots. Soil
core samples were taken from the top 15 cm depth of soil by pressing PVC
tubing (117 cm3) into the soil to sample rye roots before and after 15N application.
This soil was mixed with distilled water and the slurry poured through a 53 um
screen, which retained the roots, which were washed with water. Fine white roots
and plant residues were picked from the sand, which remained on the screen,
using a tweezers. Fresh root biomass was recorded and calculated for each
chamber. Both roots and shoots were oven dried at 70°C for 24 h. Corn roots
were extracted from the soil matrix by sieving soil through a 1 mm screen
washed by distilled water before drying.

At harvest, corn plants were cut 1 cm above soil surface and separated
into ears, leaves and stems. Each plant part was weighed separately, dried and
reweighed before grinding to pass through a 0.5 mm sieve. Ground samples
were thoroughly mixed before 5-7 mg subsamples, weighed to 5 decimal places,
were transferred into small tin capsules and placed into the autosampler. Total C
and N were determined by the dry combustion method (Kirsten, 1983) using a
C/N/S analyzer, NA 1500 series 2 (Carlo Erba Stumentazione, Milano, Italy) and
%‘5N using Isotope Ratio Mass Spectrometer Model 2020 (Europa Scientific,

Crewe, UK).

134

Calculations

N derived from labeled residue (concentrations)

t «7 15N
(%Ndfr): am" 0 excess °°"‘"“““ x 100 [1]

atom % 15N excess label“, ”mo, 0,5,0,”

 

where:

Atom %‘5N excess of corn (ear, leaf, stem or root):
(atom 15N of corn grown in labeled soil) — (atom
unlabelled control)

‘5” of corn grown in

Atom %‘5N excess of rye root and/or shoot =
(atom 15N of labeled plants) - (atom ‘5" of plants before labeling)

Recovery of 15N from rye roots and/or shoots = [2]

% Ndfr / 100 X N content of succeeding corn
N return from rye shoots and/or roots

 

N dr, kg ha": % Ndfr x N accumulation in succeeding corn plant [3]

where:
The N accumulated (kg N ha") in ear, leave, stem and total com plant
from labeled rye shoots and/or roots are equal to % Ndfr times %N of corn times

the biomass (kg) of com per ha.

Calculations were modified from Stevenson et al. (1998).

135

Statistical analyses

Treatment effects on %Ndfr, %‘SN recovery and ngdfr ha'1 were
estimated by a PROC-GLM (ANOVA) procedure using Statistical Analysis
System (SAS Institute, 1999). Fisher's LSD test was used to separate means of
measurements. Correlation analyses were used to determine relationship

between plant and soil parameters. All significant tests were set at the 0.05 level.

136

RESULTS AND DISCUSSIONS

Corn N recovery from rye shoots

Recovery of rye shoot-derived 15N by above and belowground parts of the
corn plant was 8.6% (17.2% for 2 corn) in 1998 and 7.6% in 1999 (Figures 4.1
and 4.2). Partitioning of total recovered rye shoot derived N in various parts of
com showed that 24.3 to 48.1% was translocated to the ears (Figures 4.3 and
4.4). Com was more mature at harvest in1999 and resulted the greater recovery
of 15N by the corn ears than the other plant parts. Corn roots had the lowest 15N
recovery, 4 % and 2.6 % in 1998 and 1999, from rye shoot-derived 15N (Figures

4.1 and 4.2).

Corn N recovery from rye roots

Recovery of rye root-derived 15N by the entire com plant was 11.2% in
1998 and 14.7% in 1999 (Figures 4.1 and 4.2), which was higher than that
recovered from rye shoots. Recovery of rye root derived 15N by 2 com plants in
1998 was 22.4%. Corn ears recovered the greatest amount of 15N from rye roots,
53.4 % in 1998 and 51.4 % in 1999 of total recovered N by whole corn plant
(Figures 4.3 and 4.4). Stevenson et al.(1998), reported 2 to 4% recovered from
wheat residues and 3 to 5% of 15N were recovered from pea residues. They
suggested that crop roots were also very important N contributors to soil N pools

and should not be ignored when modeling soil N.

137

 

 

%

 

)
5
D
m
D
S
L

treatment

 

 

15
ntage of N recove

 

 

 

fmt
mamm
ELSR
/.
y
0

 

30v >668. 22

 

Figure 4.1. Perce

 

nd

OtS a

red from rye shoots, ro

ar, leaf, stem and root at harvest in 1998, n=4.

roots+shoots by com e

138

20

 

15-

 

 

 

15N recovery (%)
8

 

 

 

 

 

 

 

5 _
LSD (0.05)
treatment
0
Shoot Root Root+shoot
- Ear
- Leaf
Stem
[:1 Root

 

 

 

Figure 4.2. Percentage of “N recovered from rye shoots, roots and
roots+shoots by corn ear, leaf, stem and root at harvest in 1999, n=4.

139

 

 

 

 

 

...—lama m0
r mo .n
amSR Men.
W 3m
48

ms

 

 

 

 

15N recovery (%)

25—

 

 

 

Shoot Root Root+shoot

 

- Ear

- Leaf
Stem
:1 Root

 

 

 

Figure 4.4. Partitioning of total recovered N from rye shoots, roots and
roots+shoots by com ear, leaf, stem and root at harvest in 1999, n=4.

141

When they used indirect approach, 15N labeled fertilizer and non labeled
residue application, recovery of 15N by wheat from pea residues increased up to
11%. This increase was due to root derived nitrogen to the soil N pool. In our
study, using the direct approach (15N labeled residue) was used and gave us a
better tool to determine root N contributions to the successive com plants.
Utilization of N derived from rye roots by com was significantly greater than for
rye shoots (p<0.05). Similar results were found for N derived from shoots and
roots of wheat by Thomsen et al. (1996). They reported that decomposition and
15N mineralization of wheat roots were faster and the uptake of wheat root
derived 15N was greater than 15N from wheat shoots. Similarly in our research,
earlier mineralization of 15N resulted in more 15N recovery by com plant from rye
roots. In a previous Chapter 3, it was reported that N derived from rye roots was
greater than N derived from rye shoots in the exterior layer in June 1998 (Kavdir,
2000). It was also reported that there was a direct relationship between changes
in the ratio of total N of external layers and internal regions of soil aggregates
and com biomass from the beginning to the end of com growing season. Rye
root N contents in PVC 15N lysimeters had an average value of 57.5 kg N ha'1
while shoot N content in PVC chambers had 58.7 kg N ha'1 in 1999. Therefore, it
can be concluded that rye roots conserve as much N as rye shoots. However,
the contributions of rye root-derived N to the succeeding corn was six times

greater in 1998 and twice as much as from rye shoots in 1999 (Table 4.3).

142

Table 4.1. Thickness, pH and bulk density of Ap, 8t, and Bt2 horizons at LTER
Microplots at Kellogg Biological Station, Ml.

 

 

Horizon Depth th Bulk Density
cm 9 cm'3
Ap 0-31 5.5 1 .5
Btt 31 -49 5.7 1 .7
Btg 49-65 5.3 1 .8

 

tData were taken from KBS-LTER web site, www.kbs.msu.edu

Table 4.2. Weather data for 1998 and 1999 of LTER Microplots at Kellogg
Biological Station, MI.

 

 

Year tPrecipitation *Yearly Mean *Yearly Max *Minimum Air
(mm) Air Temp (C) Air Temp (C) Temp

1998 812.3 10.7 34.2 -16.5

1999 624.2 11.1 35.5 -24.3

 

tData were taken from KBS-LTER web site, www.kbs.msu.edu

143

Table 4.3. Contributions of rye shoot, root and root+shoot to com N contents in
1998 and 1999 at LTER Microplots of a Kalamazoo loam, n=4.

 

 

 

 

1998
Treatment
Ear Leaf Stem Total
kg N ha'1 kg N ha‘1 kg N ha'1 kg N 'ha'1
Shoot 0.58bt 0.36b 1 .47b 2.41 b
Root 6.79a 4.33a 3.89a 15.01 a
Root+shoot 6.50a 4.85a 4.1 53 1 5.50a
1999
Treatment Ear Leaf Stem Total
kg N ha'1 kg N ha'1 kg N ha“1 kg N ha'1
Shoot 2.42b 1.51 a 0.91 a 4.84a
Root 5.78a 1 .76a 0.78a 8.32a
Root+shoot 6.78a 5.90a 2.09a 14.76a

 

tValues followed by the same letter within same column (in each year) and
between treatments are not significantly different at p>0.05 according to Fisher's
LSD.

144

Residue and soil contact are important considerations for the best
decomposition of plant material as well as the quality of plant material.
Schomberg et al.(1994) reported that decomposition rate of incorporated
residues into the soil was faster when they were placed on the soil surface. Thus
continuous contact of root residues with soil particles in contrast to surface
placed rye shoot residues caused much faster decomposition of dead rye roots
despite of greater C:N ratios of root residues (Kavdir, 2000, Chapter 3). Rye root
and shoot N values in 15N lysimeters were approximately 2 times greater than
those for the outside of the chambers due to greater population of rye in
chambers. We can conclude that com utilized rye root-derived N more than rye
shoot derived N due to early mineralization of roots. The retention of rye derived
N was retained primarily on exterior layers of soil aggregates causing rye N to
become much more available for growing com roots (Kavdir, 2000, Chapter 3)

Root plugging of some of the macropores also reduced leaching of
mineralized N and more 15N was retained on the surfaces of aggregates
(Chapters 2 and 3). In Chapter 3, (Kavdir, 2000) it was reported that contributions
of rye root-derived N were greater than that of rye shoot-derived N to the soil
aggregates. Both concentration and gradient of recently derived rye N increased
with increasing aggregate sizes. In both years recovery from the plots where rye
roots were present was greater than the only shoot applied plots.

There was a direct linear correlation (r2=0.68) observed between 15N ratio

of exterior layers to interior regions of soil aggregates and 15N uptake by com

145

plant (Kavdir, 2000 ,Chapter 3). Recovery rates of 15N from rye roots plus shoots
were the highest in both years (Figures 4.1 and 4.2). Com above and
belowground parts recovered 15.8% and 17.0% of rye root plus shoot derived
15N in 1998 and 1999 respectively.

Harris and Hesterrnan (1990) reported com recovery of N from alfalfa was
29% for root-derived N and 21% for shoot-derived N. Compared to rye,
recoveries of N from alfalfa shoots and roots by com were greater. However,
contributions of rye roots to the retention of N and timely release of this N to

succeeding corn crop can not be overlooked.

Retention and loss of rye root and shoot derived N from soil

Soil retained 40, 62 and 61% of rye shoot, root and root plus shoot-
derived N in 1998. Recovery of N from shoots, roots, and root plus shoots of rye
by soil were 21, 60 and 70 % respectively in 1999 (Figures 4.5 and 4.6).
Unaccounted portions of 15N in treatments with labeled shoots and roots were
assumed to be lost from the soil by leaching. Plots with rye shoots applications to
bare soil lost 42% of the rye shoot-derived 15N in 1998 and 70% of the rye shoot-
derived 15N in 1999 from the soil profiles (Figures 4.5 and 4.6).

Hubbard and Jordan (1996) observed greater 15N losses from labeled
wheat roots and soil when wheat shoot residues were placed on the soil surface.
Leaching and some denitrification of N could be the reason for greater N loss
from rye shoot mulching of surface soils. Application of surface placed plant

mulches decreases raindrop effects, reduces evaporation and increases

146

saturated hydraulic conductivity. Saturated hydraulic conductivities of soils
increased four-fold when applications of rice straw mulches to the soil surface
were increased from 0 to 12 Mg ha'1 (Lal et al., 1980).

Rasse (1997) reported that denitrification rates were higher when alfalfa
shoots were applied to bare soil than when alfalfa was planted in Microplots at
KBS. Alfalfa shoot applications to bare soils increased denitrification losses by
nearly 250 g N (ha d)". Denitrification losses were not measured in this study.

Nitrogen isotope quantities remaining in the rye shoot residues, at the end
of the corn harvest, were very low (Figures 4.5 and 4.6). One of the reasons
behind the low 15N in the remaining residue may be due to uncontrolled
conditions in the field experiments such as soil disturbance by animals and
weather conditions. Once corn crop reached to certain growth size, metal nets on
the soil surfaces were removed to obtain better corn growth. Thus the residue
biomass covered at the end of the experiment may have been smaller than the
actual amounts added.

Total N (kg N ha") recovered from rye residues were calculated by using
the equation [3]. Recovery of N was 2.41, 15.01 and 15.50 kg N ha'1 in 1998 and
4.84, 8.32 and 14.76 kg N ha'1 in 1999 from shoots, roots and root plus shoots of
rye (Table 4.3). These similar quantities of 15 kg N ha‘1 for both years confirms
that substantial contributions of N are absorbed by a winter rye cover crop and
passed along to successive corn production.

We can conclude that rye cover crop shoots and roots supplied 15.50 kg

N ha‘1 to the succeeding corn plants under limited N conditions of a Kalamazoo

147

loam soil. Contributions of rye root-derived N was greater than that of rye shoot-
derived N to the succeeding corn plant. Most of the rye-derived N, absorbed by
com, was distributed primarily to the ears with descending quantities deposited in
the leaves, stems and roots. Rye root contributed to N to the succeeding com
plant by reducing N leaching from the soil profile and releasing N via root
exudates and mineralized N from dead roots. Therefore active plant roots should
be maintained between two successive cash cropping to maximize soil N

availability and reducing N leaching.

148

15N recovery (%)

 

 

 

LSD (0.05)
treatment

 

 

Shoot Root Root+shoot

 

 

- Soil
{:23 Lost
- Residue

 

 

Figure 4.5. Percentage of 15N from rye shoots, roots and
roots+shoots retained by soil, 15N remained in residue and
lost from Iysimeter soil profile of a Kalamazoo loam soil at
harvest in 1998, n=4.

149

15N recovery (%)

 

100
95 ~
90 n

80—
75-
70—
65-

45-
4o-
35-

25-
20)
15-

 

LSD (0.05)
treatment

   
 
 

 

 

Shoot Root Root+shoot

 

- Soil
Lost
- Residue

 

 

 

Figure 4.6. Percentage of 15N from rye shoots, roots and roots+shoot:
retained by soil, 15N remained in residue and lost from Iysimeter soil
profile of a Kalamazoo loam soil at harvest in 1999, n=4.

150

REFERENCES

Ditsch, 0.0., M. A. Alley, K.R. Kelley and Y1. Lei.1993. Effectiveness of winter
rye for accumulating residual fertilizer N following corn. Journal of Soil and Water
Conservation. 48:125-132.

Frensch, J., T.C. Hsiao and E. Steudle. 1996. Water and solute transport along
developing maize roots. Planta. 198:348-355.

Harris, G.H., and 0.8. Hesterrnan. 1990. Quantifying the nitrogen contribution
from alfalfa to soil and two succeeding crops using nitrogen-15. Agron. J. 82:129-
134.

Hubbard, V. C. and D. Jordan. 1996. Nitrogen recovery by com from 15N labeled
wheat residues and intact roots and soil. Soil Sci. Soc. Am. J. 60:1405-1410.

Janzen H. H. 1990. Deposition of nitrogen into the rhizosphere by wheat roots.
Soil Biol. Biochem., 22:1 155-1 160.

Janzen H. H. and Y. Bruinsma. 1993. Rhizosphere N deposition by wheat under
varied water stress. Soil Biol. Biochem. 25:631-632.

Jensen, E. S. 1996. Rhizodeposition of N by pea and barley and its effect on soil
N dynamics. Soil Biol. Biochem. 28:65-71

Jordan, D., R.R. Bruce and DC. Coleman. 1996. Nitrogen availability to grain
sorghum from organic and inorganic sources on sandy and clayey soil surfaces
in a greenhouse pot study. Biol. Fertil. Soils. 21: 271 -276.

Karlen, D.L. and J.W. Doran. 1991. Cover crop management effects on soybean
and com growth and nitrogen dynamics in an on-farm study. Am. J. of Alter. Ag.
6: 71 -82.

Kavdir, Y. 2000. Distribution of cover crop N retained by soil aggregates in rye-
com agroecosystem. Thesis (PhD). Michigan State University. Dept. of Crop and
Soil Sciences. 157 p.

Lal, R., D. de Vleeschauwer and R. M. Nganje. 1980. Changes in properties of a
newly cleared tropical Alfisol as affected by mulching. Soil Sci. Soc. Am. J.
44:827-833.

Norman, R.J., J.T. Gilmour and ER. Wells. 1990. Mineralization of nitrogen from

nitrogen 15 labeled crop residues and utilization by rice. Soil Sci. Soc. Am. J.
54:1351-1356.

151

Ranells, N.N. and M. G. Wagger. 1997. Nitrogen-15 recovery and release by rye
and crimson clover cover crops. Soil Sci. Soc. Am J. 61:943-948.

Rasse. D. P. 1997. Alfalfa and com root modifications of soil nitrogen flux and
retention. Thesis (Ph.D.), Michigan State University. Dept. of Crop and Soil
Sciences. 149 p.

Schomberg, H. H., P. B. Ford and W. L Hargrove. 1994. Influence of crop
residues on nutrient cycling and soil chemical properties. In: Managing
agricultural residues. Unger P.W. Ed., Lewis Publishers. p:99-121.

Seiter, S. and W. R. Horwath. 1999. The fate of tree root and pruning nitrogen in
a temperate climate alley cropping system determined by tree injected 15N. Biol
Fertil Soils. 30:61-68.

Stevenson F.C., F. L. Walley and C. van Kessel. 1998. Direct vs. indirect
nitrogen-15 approaches to estimate nitrogen contributions from crop residues.
Soil Sci. Soc. Am. J. 62:1327-1334.

Thomsen, l.K., J.M. Oades and M. Amato. 1996. Turnover of 15N in undisturbed
root systems and plant materials added to three soils. Soil Biol. Biochem.
28: 1 333-1 339.

Wagger, M.G.1989a. Time of desiccation effects on plant composition and
subsequent nitrogen release from several winter annual cover crops. Agron. J.
81:236-241.

Wagger, M.G. 1989b. Cover crop management and nitrogen rate in relation to
growth and yield of no-till com.Agron. J.81:533-538.

Zebarth, B.J., V. Alder, and R.W. Sheard. 1991. In situ labeling of legume

residues with a foliar application of a 15N enriched urea solution. Commun. In Soil
Sci. Plant Anal. 22:437-447.

152

SUMMARY AND CONCLUSIONS

Rye cover crop reduced inorganic N leaching from com-based
agroecosystems, during a two-year field study. Rye roots recovered and retained
greater quantities of N than rye shoots reduced N leaching from the soil profile.
Soil tillage also influenced the amount of inorganic N lost below the rooting zone
of a Kalamazoo loam soil.

Two-stage applications of Roundup herbicides to Roundup ready com
grown in NT systems successfully reduced N leaching during a wet year. After
band spraying of rye, living rye roots between rows reduced soil nitrate leaching
by plugging some of the soil macropores and absorbing soil nitrates in both N
fertilized and non-fertilized plots. Strategic band placement of herbicide provided
a slow-release, during plant decomposition, of a rye-based starter N while N
absorption by the standing rye cover crop continued across more than 65% of
the soil surface between the young corn rows.

Negative correlations were observed between inorganic N contained in
soils and associated root lengths, volumes and surface areas in Ap horizons of
all treatments. These negative correlations appeared to result from the greater
root populations and moreefficient N uptake by rye roots. Nearly 90 and 40 kg
ha‘1 in CT-F and NT-F were retained in soil profiles due primarily to the plugging
of soil pores by roots. Therefore, using two-stage herbicide applications in NT

management systems, kept as many active rye roots as possible in the soil and

153

reduced N leaching while retaining more N in the soil profile for the succeeding
corn crop.

Incorporation of rye into soil by tillage resulted in possible breakdown of
soil aggregates. Nitrogen in the centers of large soil aggregates became more
available for N mineralization and to the succeeding com plants. Regardless of
greater N leaching from CT treatments, greater corn yields were observed in CT
than NT treatments. Presence of rye did not significantly increase or decrease
corn grain yields. A direct linear correlation (P<0.05) between rye biomass N
content and grain yield r2=0.75 in 1998 and r2=0.75 in 1999 was observed for all
non-fertilized and rye cover planted plots. Under non-N fertilized conditions,
additions of N via rye cover crop roots and shoots mineralization increased corn
grain yields.

Rye cover crop root contributions to the reduction of N leaching has been
underestimated by most of the researchers. Results from these studies indicated
that surface area, volume and length of rye root were much more important
factors than rye root biomass to uptake N from the soil profile. These greater
morphological root parameters are indicators of greater root plugging and
absorption than is root biomass. Rye roots also contribute to the stabilities of
external concentric layers of soil aggregates ranging in size from 2.0-9.5 mm
across.

Separating individual soil aggregates into three different layers by SAE
chambers greatly increased the sensitivity of identifying 15N dynamics within soil

aggregates and associated measurements of short term contributions of cover

154

crop N and C in soil aggregates and their concomitant effects on plant N
nutrition. Soil aggregates from rye cover crop treatments were much more
resistant to erosion forces to the external and transitional concentric layers than
soil aggregates from no rye cover crop treatments. Soil eroded from external and
transitional layers of soil aggregates layers was reduced from 2-10 fold by rye
cover crop. Smaller soil (2.0-4.0 mm) aggregates were much more resistant to
erosion than larger (6.3-9.5) mm across aggregates. Transitional layers of soil
aggregates were more resistant to erosion than exterior layers of soil aggregates.

Rye root and shoot derived nitrogen had accumulated on the exterior
layers of soil aggregates 4.0 to 6.3 mm and 6.3 to 9.5 mm across, 17 days after
rye shoots were application to the soil surface. Greater root-derived N
accumulated on the exterior layers of soil aggregates. Rye roots contributed
more N to the soil aggregate surface layers than did rye shoots. Gradients of
recently derived rye N increased with aggregate sizes. These results supported
that roots grow preferentially around the surfaces of soil aggregates, through
associated macropores, rather than through the internal regions of soil
aggregates. Organic materials derived from rye roots and shoots were
homogeneously distributed across soil aggregates 2.0-4.0 mm across resulting in
minimum 15N gradients within smaller aggregates.

High correlations were discovered between changes in the ratios of total N
contents in external layers and internal regions of soil aggregates and the
production of corn biomass from the beginning to the end of corn growing

season. More positive ratios produced greater corn biomass. Thus, it is clear that

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uptake of N is more efficient from the surfaces of the soil aggregates larger than
4 mm.

Concentric gradients of total N were found for soil aggregates larger than
4 mm. These gradients appeared to develop following the formation of soil
aggregates. Concentric gradients of rye root and shoot derived N increased with
increasing aggregate size and changed with time. The location of N in a soil
aggregate was important for corn plant utilization. There seemed to be a constant
rate of migration or flux of 15N ions, mostly organic in nature, originating from rye
roots and shoots into soil aggregates. Early in the season, more 15N migrated to
the interior regions of the smallest aggregates, 2-4 mm across, but was limited to
only surface and transitional layers of the larger aggregates, 6.3-9.3 mm across.
At harvest, more of the 15N located within interior regions of the smallest sized
aggregates had been withdrawn by com root activity while more 15N remained
within the interior regions of the medium to larger sized soil aggregates 4—6.3
mm across. These 15N gradients suggested that the preforrnation and of soil
macroaggregates and functional fluxes of N are very dynamic processes utilizing
many biogeochemical reactions. Therefore, the soil aggregate hierarchy model,
proposed by Oades and Waters (1991) cannot be the only model for the
formation of soil structure within a Kalamazoo loam soil. If this were the only
model of aggregate formation then macroaggregates and microaggregates would
exhibit similar N gradients as the smaller aggregates would obtain the uniform N
concentrations before aggregating into larger hierarchies of larger aggregates.

These larger aggregates should contain similar concentrations of N throughout.

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In contrast, we identified concentration gradients of plant-derived N across soil
aggregates greater than 4 mm. The location of the N, especially on soil
aggregate surface layers, highly contributed to N uptake by the contemporary
corn crop. Rye root and shoot derived N, located within exterior layers of larger
soil aggregates decrease with time with corn root uptake of N. Average recovery
of rye root 15N by above and belowground parts of the com plant was 13 %.
Recovery of rye shoot derived 15N by above and belowground parts of com was
8 % (average of two year). Therefore these studies clearly demonstrate that
active reduce N leaching from the soil and the strategic management of rye cover
crops deposit highly available N to soil aggregate surfaces which is preferentially

absorbed by roots of a successive corn crop.

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