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MANAGING CROPPING SYSTEMS TO ENHANCE THE ACTIVE SOIL
NITROGEN POOL AND CONTROL ITS MINERALIZATION

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Jose E. Sanchez

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MANAGING CROPPING SYSTEMS TO ENHANCE THE ACTIVE SOIL
NITROGEN POOL AND CONTROL ITS MINERALIZATION

By

Jose E. Sanchez

A DISSERTATION

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Crop and Soil Sciences

2000

ABSTRACT

MANAGING CROPPING SYSTEMS TO ENHANCE THE ACTIVE SOIL
NITROGEN POOL AND CONTROL ITS MINERALIZATION
By

Jose E. Sanchez

Sustainable nitrogen (N) management requires practices that enhance soil fertility and
minimize N loss. This study explores the effects of substrate diversity and living roots on
N mineralization, and the inﬂuence of management on the seasonal pattern of soil nitrate
(N03) levels. In part 1 a “diverse system”, consisting of a corn (Zea mays L.)—corn-
soybean (Glycine max L.)-wheat (T riticum aestivum L.) rotation with cover crops and
fertilized with composted dairy manure was compared to a corn monoculture with
conventional fertilizers. The speciﬁc objectives were to determine if a diverse system
would induce higher mineralization rates than would a continuous corn monoculture, and
to determine whether these two systems differ in their ability to mineralize added
substrate. Net mineralized N in the diverse system was 90 and 40% higher than that of
the monoculture at 70 and 150 days of laboratory incubations respectively. A
comparable response was found in situ where a 70% higher net mineralization was
observed at 70 days. The active N pool increased over time, but the ability of soil
organisms to break down additional substrate did not change as a result of diversity. In
part 2, soil from the diverse system was used to determine if living corn roots inﬂuence N
mineralization. Corn roots increased the mineral-N supplying capacity of soil from the

diverse system by more than 50%. This increase appears to be caused by a stimulation of

N mineralization, but a previous increase in the active N pool seems to be necessary
before signiﬁcant stimulation can be realized. In part 3, soil N03 levels were measured
throughout six years from April through December in the previously described rotation
and continuous corn both receiving compost or fertilizer. The objectives were to
determine if N03 ﬂuctuation patterns could be used as a suitable indicator of soil fertility
changes within or between seasons, and to determine the leaching potential resulting
from the management strategies. Seasonal soil N03 levels were sensitive to management.
Soil N03 levels were generally higher in plots receiving fertilizer than those receiving
compost. The exceptions were the cover cropped 1St year corn and all soybean plots.
Soil NO; levels responded not only to fertilizer applications but also to an increased
active N pool. The leaching potential in fertilized corn was always signiﬁcantly higher
than in corn receiving compost. In soybean and wheat, no difference between fertility
sources was observed. A comparison between leaching potential and N03 levels in the
following spring revealed that fertilized corn may be susceptible to higher leaching
losses. The seasonal N03 patterns were sensitive indicators of fertility status. Fertility
can be increased by application of conventional fertilizer and also by soil conditioning
practices that increases the mineralizable N pool size. Farming practices enhancing
substrate diversity can dramatically increase the mineralizable N pool size. The use of
rotations, cover crops, and organic amendments from animal sources enhanced the
mineralizable N pool. This in combination with a crop specially selected for its ability to
stimulate N mineralization has the potential to increase yields, decrease fertilizer

requirements, and thus have a beneﬁcial impact on soil and water quality.

ACKNOWLEDGEMENTS

I want to express my sincere gratitude to Dr. Richard Harwood for his guidance and
permanent support during these past three wonderful years as a doctoral student.
Appreciation is also extended to Drs. Richard Leep, Darryl Warncke, and Kenneth
VerBurg for their encouragement and for serving on the guidance committee. This study
was integrated with the study of several other people, so debts of gratitude are owed to
many. I thank Dr. Eldor Paul for his research input, and Drs Tom Willson, John Durling,
Marcus Jones, Manuel Colunga, Peter J eranyama, and Yasemin Kavdir for their
assistance throughout this study. I especially thank Jeff Smeenk, Elaine Parker, and
Kadir Kizilkaya for their technical support and friendship. I greatly appreciate the
logistical assistance provided by Greg Parker, Jim Bronson, Brian Graff, Garrett
Ponciroli, and the staffs of the MSU Soil Testing Lab and the Department of Crop and
Soil Sciences. I also would like to thank my fellow graduate students Ann-Marie
Fortuna, Da Ouyang, and Jane Boles. Thanks to Anne Conwell for her encouragement
and always being there to help me. I appreciate the support provided by Dr. James Jay. I
am thankful to the CS. Mott Foundation and the taxpayers of the great State of Michigan
for the ﬁnancial support that made this study possible. Finally Iwant to thank my family,

present and future, for their incomparable support and understanding.

iv

PREFACE
Chapter 1 in this dissertation was written in the style required for publication in
the Soil Science Society of America Journal. Chapter 2 was written in the style required
for publication in the Agronomy Journal. Chapter 3 was written in the style required by

the Journal of Environmental Quality.

TABLE OF CONTENT

LIST OF TABLES ......................... ' .................................................................................. viii

LIST OF FIGURES ........................................................................................................... ix
CHAPTER 1: SUBSTRATE DIVERSITY AFFECTING N MINERALIZATION

IN LONG-TERM CROPPING SYSTEMS .................................................................... 1

ABSTRACT ............................................................................................................... 1

INTRODUCTION ..................................................................................................... 2

MATERIALS AND METHODS: .............................................................................. 4

Site Description ........................................................................................... 4

Laboratory Incubation ................................................................................. 5

In Situ Incubation ........................................................................................ 6

Mineralized N Calculations ........................................................................ 7

Data Analysis .............................................................................................. 8

RESULTS ................................................................................................................ 10

Laboratory Incubation ............................................................................... 10

In Situ Incubation ...................................................................................... 11

Relating Laboratory and In Situ Incubations ............................................ 11

DISCUSSION: ......................................................................................................... 12

CONCLUSIONS ...................................................................................................... 1 5

REFERENCES ........................................................................................................ 16

CHAPTER 2: CROPPING EFFECT ON NITROGEN MINERALIZATION ........ 25

ABSTRACT ............................................................................................................. 25

INTRODUCTION ................................................................................................... 26

vi

MATERIALS AND METHODS ............................................................................. 28

Site Description ......................................................................................... 28

Field Measurements .................................................................................. 29
Laboratory Measurements ........................................................................ 31
Nitrogen Balance Calculation ................................................................... 32
RESULTS ................................................................................................................ 33
Nitrogen Mineralization ............................................................................ 33

Carbon Mineralization .............................................................................. 34
DISCUSSION .......................................................................................................... 35
CONCLUSIONS ...................................................................................................... 38
REFERENCES ........................................................................................................ 39

CHAPTER 3: THE IMPACT OF MANAGEMENT ON RESIDUAL NITRATE

FLUCTUATIONS IN LONG-TERM CROPPING SYSTEMS ................................. 49
ABSTRACT ............................................................................................................. 49
INTRODUCTION ................................................................................................... 50
MATERIALS AND METHODS: ............................................................................ 52
Site Description ......................................................................................... 52
Sampling and Laboratory Procedures ....................................................... 53

Data Analysis ............................................................................................ 53
RESULTS AND DISCUSSION .............................................................................. 56
Nitrate Levels Inﬂuenced by Management ............................................... 56

Nitrate Leaching Potential ........................................................................ 58
REFERENCES ........................................................................................................ 6O

vii

LIST OF TABLES

CHAPTER 1: SUBSTRATE DIVERSITY AFFECTING N MINERALIZATION IN
LONG-TERM CROPPING SYSTEMS

Table 1.

Table 2.

Table 3.

Selected chemical properties of soil from the “diverse” and monoculture
cropping systems, and the organic materials added in the laboratory and
in situ incubations. .................................................................................... 19

Net mineralized N ﬁ'om added materials for two cropping systems at two
incubation dates, expressed as percentage of added N. ............................ 20

Selected climatic data and amount of water added during the time period
when in situ incubations were performed. ................................................ 21

CHAPTER 2: CROPPING EFFECT ON NITROGEN MINERALIZATION

Table 1.

Table 2.

Table 3.

N content of soils and plants immediately before and 70 days after
planting, and calculation of N supplied by each soil. ............................... 43

Mineral-N concentration in the soil proﬁle at the end of the experiment
(70 days aﬁer planting). ............................................................................ 44

Distribution of root biomass dry-weight and root N in the soil proﬁle
based on recovered roots at the end of the experiment (70 days after
planting). ................................................................................................... 45

CHAPTER 3: THE IMPACT OF MANAGEMENT ON RESIDUAL NITRATE
FLUCTUATIONS IN LONG-TERM CROPPING SYSTEMS

Table 1.

Table 2.

Analysis of variance for soil N03 levels as affected by year (1994-1999),
fertility source, crop, cover crop, and month of the year. ......................... 64

Soil N03 levels as affected by crop, fertility source, cover crop, and time
of the year. ............................................................................................... 65

viii

LIST OF FIGURES

CHAPTER 1: SUBSTRATE DIVERSITY AFFECTING N MINERALIZATION IN
LONG-TERM CROPPING SYSTEMS

Figure 1.

Figure 2.

Figure 3.

Net mineralized N after 70 and 150 days of laboratory incubations for soil
sampled from for the “diverse” and monoculture cropping systems.

Lower and upper case letters indicate signiﬁcant differences in mineral-N
at day 70 and 150 respectively (P < 0.05, Tukey-Kramer test). ............... 22

Net mineralized N after 70 days of in situ incubations for the “diverse”
and monoculture cropping systems. Bars followed by the same letter are
not signiﬁcantly different (P < 0.05 , Tukey-Kramer test). ....................... 23

Level of agreement between laboratory and in situ incubation across
cropping systems and addition of organic materials. Individual data points
are the LS-means of 32 observations. ...................................................... 24

CHAPTER 2: CROPPING EFFECT ON NITROGEN MINERALIZATION

Figure 1.

Figure 2.

Figure 3.

Cumulative mineralized N using laboratory incubations in soils sampled
before planting (May 1999) from a 4-year rotation and continuous corn at
the LFL. Units are based on a 10 cm sample depth. Bars with the same
letter are not signiﬁcantly different in mineral-N at day 70. The After-
wheat soil was deﬁned as “conditioned” soil in this study. ...................... 46

Cumulative mineralized N during laboratory incubation for soil sampled at
planting and 70 days after planting. Field plots used in this study were
from the “conditioned” soil (Aﬁer-wheat) at the LFL. Units are based on
a 10 cm sample depth. Letters indicate signiﬁcant differences in
mineralized N after 70 days of incubation. ............................................. 47

COz-C released during laboratory incubations for pre-planting and 70 days
aﬁer planting (70 DAP) soils. Individual data points are the least squares
means of 4 replications. ............................................................................ 48

CHAPTER 3: THE IMPACT OF MANAGEMENT ON RESIDUAL NITRATE
FLUCTUATIONS IN LON G-TERM CROPPING SYSTEMS

Figure 1.

Soil NO3-N levels in the 1St year corn plots of the rotation using data from
1994 to 1999. Monthly values have been adjusted to the middle of the
month. Error bars represent that portion of the standard errors above the
mean for each month. ................................................................................ 66

ix

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Soil NO3-N levels in the 21nd year corn plots of the rotation using data from
1994 to 1999. Monthly values have been adjusted to the middle of the
month. Error bars represent that portion of the standard errors above the
mean for each month ................................................................................. 67

Soil NO3-N levels in the soybean plots of the rotation using data from
1994 to 1999. Monthly values have been adjusted to the middle of the
month. Error bars represent that portion of the standard errors above the
mean for each month. ................................................................................ 68

Soil NO3-N levels in the wheat plots of the rotation using data from 1994
to 1999. Monthly values have been adjusted to the middle of the month.
Error bars represent that portion of the standard errors above the mean for
each month. ............................................................................................... 69

Soil NO3-N levels in the continuous corn plots using data from 1994 to
1999. Monthly values have been adjusted to the middle of the month.
Error bars represent that portion of the standard errors above the mean for
each month. ............................................................................................... 7O

CHAPTER 1

Substrate Diversity Affecting N Mineralization

in Long-Term Cropping Systems

ABSTRACT

This study explores the effect of substrate diversity on nitrogen (N)
mineralization. A “diverse system”, consisting of a corn (Zea mays L.)-com-soybean
(Glycine max L.)-wheat (T riticum aestivum L.) rotation with cover crops and fertilized
with composted manure was compared to a corn monoculture with conventional
fertilizers. N mineralization was measured in situ and in laboratory incubations as was
the ability of each soil to mineralize added compost and red clover (T rifolium pratense)
residue in the 6th and 7th years of rotation. Net mineralized N in the “diverse system” was
90 and 40% higher than that in the monoculture at 70 and 150 days of laboratory
incubations respectively. Comparable response was found in situ where a 70% higher net
mineralization was observed at 70 days. The 70- and 150- day mineralizable N pools
increased over time, but the ability of soil organisms to break down additional substrate
did not change as result of diversity. The high level of agreement between the laboratory
and in situ mineralization data is also discussed. A more diverse cropping system may
increase the soil’s capacity to supply N to a growing crop while maintaining desirable
levels of soil organic matter. This is essential for the overall long-tenn productivity and

sustainability of agricultural systems.

INTRODUCTION

Nitrogen mineralization is an important indicator of soil quality (Duxbury and
Nkambule, 1994; Seybold et al., 1998), and its management could represent an excellent
tool in achieving a sustainable N supply. Managing N mineralization efﬁciently is likely
to result in a more synchronized N release, and have the potential to reduce N fertilizer
dependence while promoting N recycling within agroecosystem boundaries.

N mineralization is regulated by abiotic factors such as soil moisture, temperature,
and texture; and by the supply of above- and below-ground substrates (Jenny, 1980). In a
practical sense, managing N mineralization implies managing organic inputs. The way in
which organic materials inﬂuence mineralization is closely related to their quality (Swift
et al., 1979). The quality of a particular material is deﬁned by its chemical composition,
including C/N ratio, lignin, and polyphenol contents (Tian et al., 1997). Substrates with
low N and high concentration of lignin and polyphenols decompose and release N slowly
(Comforth and Davis, 1968). In contrast, those rich in N with low lignin and polyphenol
concentrations decompose rapidly (Handayanto et al., 1997). Therefore, a well-balanced
diversity of materials entering the soil is expected to favor N availability for a growing
crop while maintaining desirable levels of soil organic matter.

It is accepted from an ecological point of view that enhanced species richness is
beneﬁcial for ecosystem performance (Kareiva, 1996, Tilman et al., 1996), but the
question remains whether that is the case for below-ground subsystems (W ardle et al.,
1996). Diversity can be primarily achieved with the use of crop rotations, cover crops,
and application of organic amendments (Gliessman, 1998). In this work we investigated

how substrate diversity inﬂuences N mineralization. Two cropping systems were

selected according to their level of diversity. The “diverse system” received residues of
corn, soybean, wheat, red clover, crimson clover (T nfolium incarnatum), and annual
ryegrass (Lolium multiﬂorum), in combination with composted manure during each
rotation cycle. This system was compared to a continuous corn monoculture where
commercial fertilizer was the fertility source. Speciﬁc objectives of this study were to
detennine if the “diverse system” would induce higher mineralization rates than the
continuous corn monoculture, and to determine whether these two systems differ in their
ability to mineralize added substrate. The ﬁrst objective deals with the potential of the
cropping system to supply N to a growing crop. The second asks whether there have
been important changes in microbial performance relating to the decomposing ability of
each microbial community.

In 1998 and 1999, laboratory and in situ incubation experiments were initiated to
determine N mineralization potentials. While in situ incubations were expected to
generate data that more closely reﬂect actual ﬁeld conditions, laboratory incubations
require less effort and cost. A regression analysis of N mineralization over time was used
to compare them. Learning more about the relationship between the two methods has the
potential to facilitate the calibration of predictive models based on commonly used

laboratory incubations (Smith et al., 1977; Bhogal et al., 1999).

MATERIALS AND METHODS
Site Description
This study was conducted in the Living Field Laboratory (LF L), a long-term
experiment established in 1993 at the W. K. Kellogg Biological Station located in
Southwest Michigan. The site experiences cold moist winters and warm humid summers.
Average precipitation has been 860 m y'1 (1988-1999), and potential evapotranspiration
typically exceeds precipitation from May through September. The soil is a mixture of
Kalamazoo and Oshtemo sandy loams (Typic Hapludalfs). The depth of the Ap horizon
is 20 to 25 cm, and pH ranges from 6.3-6.8. The LFL was designed to test various
combinations of rotation and cover crops under several agronomic management regimes
(Jones, 1996). The main goal is to test alternative strategies for achieving nutrient
cycling efﬁciency and reducing chemical input requirements. The experimental design is
a four-replication split-split-plot, randomized complete block, with main plots for each
management type. The plots used for this study were in the Integrated Compost and
Integrated Fertilizer management types. Both management types include minimal
application of pesticides, and banded herbicide plus cultivation for weed control. The
main difference is in their fertility source: Integrated Fertilizer uses commercial fertilizer
and Integrated Compost uses composted dairy manure. Subplots represent each crop in a
com-com-soybean-wheat rotation, plus continuous com. All crops except soybean are
grown with and without a cover crop. Cover crops include red clover ﬁost-seeded into
wheat, crimson clover interseeded into 1St year and continuous corn, and annual ryegrass

interseeded into 2“d year corn. Before planting, approximately 4 Mg ha“1 of composted

manure is added annually to all crops except soybean. Reduced tillage (chisel plow) is
used throughout the LF L.

This study used 1st year corn plots with cover crops of the Integrated Compost,
and the continuous corn plots without cover crop of the Integrated Fertilizer management.
The crop rotation in the Integrated Compost was selected as the “diverse system” because
residues of six plant species in combination with composted manure are incorporated into
the soil throughout the rotation cycle. These substrates have a wide range of C/N ratio.
The lowest C/N ratio provided by the red clover cover crop (14:1) and the highest by
wheat stubble (80:1). We used the 1‘“t year corn (immediately following wheat + clover)
plots because, historically, its grain yields has been comparable to those when fertilizer
was used (Jones, 1996). In contrast, the continuous corn plots in the Integrated Fertilizer
management only receives corn residues (C/N ratio of 60: 1) as substrate. For clarity we
will use the terms “diverse system” to describe the 1St year corn, and “monoculture” when
referring to continuous corn. The C, N, and C/N ratios of soil from these cropping
systems are described in Table 1.

In early May 1998 and 1999, soil samples were taken from the diverse and
monoculture cropping systems following tillage and immediately before planting and
used in long-term laboratory incubations. Immediately after planting, microplots were

established in the plots of the two cropping systems and were used for in situ incubations.

Laboratory Incubation

Soil samples were taken at 0-10 cm depth using a 1.9 cm diameter soil probe.
Thirty cores from each plot were composited, placed in plastic bag, and stored at
approximately 4 °C until processed. Soils were sieved through a 6 mm screen and sub-
sampled for moisture determination. Sixty-three 20 g dry weight equivalent aliquots of
each sample were weighed into 100 ml plastic specimen cups. Three rates of red clover
were added: 0 Mg ha’1 (Control), 2.5 Mg ha'1 (C10), and 5 Mg ha'1 (2Clo). The rates of
compost were 0 Mg ha‘1 (Control), 5 Mg ha'1 (Com), and 10 Mg ha'1 (2Com). The
combinations Com + Clo, Com + 2Clo, 2Com + C10, and 2Com + 2Clo were also
included to the experiment. Both compost and clover were dried, ﬁnely ground, and sub-
sampled for chemical analysis using the acid and neutral ﬁber detergent method (Goering
and Van Soest, 1970). Selected chemical properties of added clover and compost are
presented in Table 1. Calculations for the actual amount of material added to cups were
based on a soil bulk density of 1.3 and depth of 10 cm. The quantity of compost was
based on dried weight of non-sand material. Following the additions, each cup was
manually agitated to uniformly mix the soil and substrates. The soils were brought to 50
°/o of water holding capacity. The specimen cups were stored in plastic storage containers
that had a thin layer of water on the bottom to maintain humidity. These containers were
then placed in a controlled temperature room at 25 °C for 10, 20, 30, 50, 70, 100, and 150
days. At the end of each incubation interval, the corresponding samples were removed
and frozen temporarily to stop microbial activity. N-NO3' and N-NHI concentrations
were determined using the extraction technique described by Kenney and Nelson (1982)

and a Lachat automated colorimetric analyzer (Lachat Instruments Inc. Milwaukee, WI).

In Situ Incubation

Four microplots of 2 m2 were established in each plot and used for in situ
incubation experiments (Raison et al., 1987). Two levels of red clover: 0 Mg ha’1
(Control) and 5 Mg ha'1 (2Clo), two rates of compost: 0 Mg ha’1 (Control) and 10 Mg ha“1
(2Com), and the combination 2Com + 2Clo were used in this experiment. The clover
was dried, and applied unground. The treatments were randomly assigned to microplots,
the materials uniformly added to the soil surface, and incorporated using a long-tined
hand cultivator (Ben Meadows Co. Atlanta, GA). Fifteen PVC tubes (30-cm long and 5-
cm i.d.) were inserted 25 cm in the corn row of each microplot to prevent root in-growth.
After tube insertion, soil samples were taken at 0-10 cm from each microplot to measure
initial mineral N. Soil moisture was controlled in the cylinder, using temporary rain
shelters and periodic addition of water, to prevent extreme wetting-drying events, and to
minimize nitrate leaching and denitriﬁcation. Soil moisture was gravimetrically
determined every two weeks. The amount of water to be added was calculated by
subtracting the actual soil moisture from the estimated water holding capacity for the O-
10 cm depth. Half of the calculated amount was added at each of two different days
during the two-week period.

On days 14, 28, 42, 56, and 70 after insertion, three cylinders from each microplot
were randomly selected and removed. Samples were taken from 0-10 cm depth within
each cylinder using a 1.9-cm dia soil probe. The three cores were composited, placed in
plastic bag, and stored at approximately 4 °C until processed. Mineral N was determined

according to the method mentioned in the laboratory incubation section.

Mineralized N Calculations

Net N mineralization using laboratory and in situ incubations was calculated
using the difference between inorganic N content at the end of the incubation period and
at day 0. For the in situ incubation we assumed that deep N leaching was prevented and
gaseous N loss minimized. Linn and Doran (1984) reported that in a well-drained soil the
relative amount of anaerobic denitriﬁcation is negligible. Also, a recent long-term study
in an adjacent ﬁeld indicated that N loss due to denitriﬁcation ranged from 1.3 to 0.4 kg
ha'1 per year (Robertson et al., 2000). Our agronomic treatments were nearly identical to
that study.

The amount of net N mineralized from added substrate was calculated by
subtracting the mineralized N of the control from the treatments with added substrate.
The calculated amount is expressed as a percentage of the initial amounts of N added as
clover, and/or compost.

Data Analysis

The same statistical model was applied to the laboratory and in situ data sets.
Factors used in the model were: year, replication, cropping system, treatment, and
incubation time. We performed analysis of variance for each data set, using the SAS
Mixed procedure (SAS Institute, 1999) in which the data from each incubation interval
was treated as a repeated measurement of the corresponding experimental unit. The
optimal covariance structure was determined using Schwarz’s Bayesian Criterion (Littell
et at., 1997). The laboratory incubations data set was appropriately explained by a
compound symmetry (CS) covariance structure, while its heterogeneous version (CSH)

better corresponded to the in situ data set. CS assumes constant variance and covariance,

and CSH uses a different variance parameter for each diagonal element, and assigns the
square roots of these parameters in the off-diagonal entries (SAS Institute, 1999). The
appropriateness of CSH for in situ data set may be related to the inherent variability
commonly found in the ﬁeld.

The presented results focus on the 70- and 150-day N pools, obtained from
mineralization at those days. These incubation periods were selected due to their
agronomic importance. The 70-day N pool was expected to closely represent the portion
of organic-N available for mineralization during a growing season. The ISO-day pool
contains that of the 70-day plus a more resistant fraction and is likely to be available for
mineralization in a more intermediate term.

In situ incubations were expected to generate data that more closely reﬂect actual
ﬁeld conditions, but laboratory incubations are more widely used. Regressions for the
laboratory and in situ incubations were constructed using the least square means (LS-

means) of common treatments at incubations periods less than 70 days.

RESULTS
Laboratory Incubation

Results of laboratory incubation are shown in Figure 1. Net mineralized N in the
diverse system was 90 and 40% higher than the monoculture at 70 and 150 days of
incubation respectively. When substrates were added, the diverse system still
mineralized more N than the monoculture when comparing the same treatment and
incubation time. The net N mineralization was highest when 5 Mg ha'1 of clover was
applied to this cropping system. The lowest net mineralization was observed in the
monoculture without clover. The monoculture was able to produce as much mineral-N as
the diverse system when it received an extra 2.5 Mg ha'1 of clover. In general, the
addition of compost did not signiﬁcantly alter net mineralization.

Table 1 provides some important chemical properties of the soils and added
clover and composted manure. They are critical elements in explaining the observed
mineralization rates for the two materials. The laboratory information in Table 2 shows
the percentage of N (from added substrates) mineralized at 70 and 150 days of
incubation. There were no signiﬁcant differences between cropping systems in N
mineralization from added clover and/or compost. Clover additions released
considerable N during the ﬁrst 70 days, and modest amounts from 70-150 days of
incubation. The proportion of N mineralized from each substrate remained constant as
the rate of added substrate was doubled. The proportion of each substrate N mineralized
increased, as calculated from weighted average calculation, when the substrate were

combined (calculation not shown).

10

In Situ Incubation

Mineralized N for the two cropping systems at 70 days of in situ incubation is
shown in Figure 2. The net N mineralization in the diverse system was 70% higher than
that of the monoculture, agreeing with the pattern observed in the laboratory. When
substrates were applied, the diverse system also produced more N than the monoculture.
The highest N mineralizations were obtained when 5 Mg ha'1 of clover was added to the
diverse system. The monoculture without clover produced the lowest net mineralized N.
In the absence of clover, the monoculture required considerably more (compared to the
2.5 Mg ha'1 in the laboratory) clover to release comparable amount of N than the diverse
system. The addition of compost did not signiﬁcantly increase mineralized N in any
treatment.

In situ results of mineralized N from added substrates are included in Table 2.
Mineralized N values from the additions were consistent with those measured in the
laboratory. Thus, no signiﬁcant differences between the two cropping systems in
mineralization of added materials were observed. The low mineralization rates of
compost caused a dilution effect on the percentage of mineralized N from the combined
materials.

Relating Laboratory and In Situ Incubations

Figure 3 indicates the level of agreement between laboratory and in situ

incubation. Regardless of cropping history and addition of materials, net mineralized N

at 70 days in situ was 90% of that in the laboratory.

11

DISCUSSION

The striking differences in net mineralization between the two cropping systems
demonstrate the linkage between substrate diversity and the size of the active N pool.
Swift (1979) reported that quantitative variation in mineralized N occurs as a result of the
variety of organic inputs. Net mineralized N in the diverse system was undoubtedly
enhanced by the incorporation of residues from the three legumes, three grasses, and
composted manure. Individual contribution varies according to quantity and quality of
the substrate. For instance, legumes with narrow C/N ratios and abundant soluble
compounds are more likely to mineralize at higher rates than residues from grasses with
high C/N ratios. It is reasonable to expect that the legumes are primarily responsible for
the increased active N pool. Our data agree with those of Stanford and Smith (1972),
Chang and Juma (1996), and Franco-Vizcaino (1997), showing that crop diversity
including annual legumes increases the mineralizable N compared to monoculture. The
higher mineralization rates in the diverse system with added clover and/or compost were
inﬂuenced by the initial N pool size of this soil. Despite this accumulative effect across
treatments, the corn monoculture showed that the presence of clover can dramatically
increase its mineralization potential. This indicates that increasing diversity by
incorporating a high quality substrate may cause a considerable improvement in soil
fertility.

Net mineralized N from the additions was expressed as percentage of added N to
facilitate the comparison between both cropping systems. The similar ability of the corn
monoculture and the diverse system to mineralize additional organic N may be an

indicator of rapid changes in microbial biomass. N mineralization is a microbiologically

12

\f/

mediated process (Paul and Vorony, 1984), and the microbial biomass is largely
controlled by organic C content and by recent substrate additions (Paul and Clark, 1989).
Its appears that regardless of their initial status, the microbial biomass positively reacts to
the abundance of easily-decomposable compounds (e.g., from clover) by increasing its
size and activity (immobilization and mineralization). This response was reported in a
previous study performed at the same experimental site (Cavigelli, 1998). The sizeable
clover-N mineralized during the ﬁrst 70 days and the small or undetected amount
mineralized from 70-150 days suggest that N release may coincide with crop growth.
Results of mineralized clover-N are comparable to those reported by Ladd et a1. (1983),
Bremer and van Kessel (1992), and Biederbeck et a1. (1996). It is possible that the
addition of compost did not stimulate the microbial biomass as may happen with clover.
In a previous study, Willson et a1. (2000) found that microbial biomass did not increase
immediately following compost additions. Even if the ON ratio of clover and compost
were somehow similar, decomposition rates were different because different C
compounds exhibit different decomposition rates (Somda et al., 1991). We suggest that
the low, sometimes-negligible, release of compost-N was related to the presence of a
large quantity of more stable compounds. During the composting process manure-N is
stabilized through microbial assimilation and humiﬁcation and thus the end product
(compost) mineralizes N at a considerably slower rate (Castellanos and Pratt, 1981).
Under laboratory conditions, the monoculture required about 2.5 Mg ha'1 of
clover to release as much N as the diverse system (in absence of clover). But in situ,
much more clover was apparently needed. Under ﬁeld conditions, N mineralization for

the diverse system may have been enhanced by meso-, macro-, and mega-fauna activity.

13

These components of the soil biota are responsible for increased fragmentation of organic
materials. Fragmentation increases substrate surface area for further microbial
colonization, thereby speeding decomposition and mineralization (Linden et al., 1994).

F aunal biomass appears to be more extensive in diverse soils and is related to the amount
and quality of organic materials in the soil (Andren et al., 1990).

The combination of substrates always mineralized more N than have been
anticipated from the “weighted” means of their individual mineralization. This was
especially striking when 5 Mg ha'1 of compost was combined with 5 Mg ha'1 of clover,
where the percentage mineralized was nearly double that anticipated from their individual
amounts. We suggested the possibility of a synergistic relationship but this effect was
not seen in the more limited in situ experiment.

The results of the laboratory incubations were generally comparable to those of
the in situ experiment. This can be explained by the similarity of their environmental
conditions in situ. Selected climatic data obtained from the nearby Long-Term
Ecological Research weather station is shown in Table 3. Moisture was controlled in
both experiments and the average soil temperatures in the ﬁeld and in the laboratory were
also coincidently alike. The greater differences in N mineralizations were observed early
in the incubation period. Early season lower average temperatures probably inﬂuenced
the in situ N mineralization to be lower compared to those from the laboratory where
temperature was kept constant. The similarity of results generated by these two methods
agrees with those reported by Hadas et al. (1989).

These laboratory and in situ incubations are not intended to represent actual ﬁeld

mineralizations, but to provide evidence of how this process may be affected by substrate

14

diversity, since in situ mineralizations were done with plant roots being excluded.
Further studies are strongly recommended to quantify N release in presence of living
roots. Growing roots not only affect N mineralization by exposing organic materials to
decomposition and uptake of N, but rhizodeposit production represents a signiﬁcant
energy input to the soil microbial community (Bakken, 1990; Texier and Billes, 1990).
Sanchez et a1. (2000) in a previous study found that the presence of corn roots can
dramatically increase N mineralization in soils with a large active N pool.
CONCLUSIONS

Farming practices promoting diversity have the potential to increase dramatically
the mineralizable N pool and decrease fertilizer use. Among such practices, we suggest
the use of rotations, cover crops, and organic amendments from animal sources.
The ability of soil organisms to break down additional substrate did not change as a result
of diversity. This is essential for improving levels of soil organic matter because organic
materials entering a “diverse soil” will not decompose at higher rates. Maintaining
desirable levels of soil organic matter is closely related to the overall long-term
productivity and sustainability. The question of whether there is an additional gain in N
mineralization by adding clover and compost combined was not deﬁnitely answered by
this research. A possible synergistic effect was shown in the laboratory incubations but
was not found in situ. Laboratory incubation is a reliable tool to estimate N
mineralization in the ﬁeld as long as plant roots are excluded. Further studies are needed

to determine how mineralization of added substrates are affected by the presence of roots.

15

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Castellanos, J. Z. and P. F. Pratt. 1981. Mineralization of manure nitrogen-correlation
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Duxbury, J .M., and S.V. Nkambule. 1994. Assessment and signiﬁcance of biologically
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Franco-Vizcaino, E. 1997. Comparative soil quality in maize rotations with high or low
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Hadas, A., S. Poingenbaum, A. Feigin, and R. Portnoy. 1989. Nitrogen mineralization
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Ladd, J .N. M. Amato, R.B. Jackson, and J .H.A. Butler. 1983. Utilization by wheat crops
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Linden, D.R., P.F. Hendrix, D.C. Coleman, and P.C.J. van Vliet. 1994. Fauna]
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Robertson, G.P., E.A. Paul, and RR. Harwood. 2000 [in review]. Greenhouse gases in
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Seybold, C.A., M.J. Mausbach, D.L. Karlen. H.H. Rogers. 1998. Quantiﬁcation of soil
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Smith, S.J., Young LB, and GE. Miller. 1977. Evaluation of soil nitrogen
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41:74-76.

Somda, Z.C., P.B. Ford, and W.L. Hargrove. 1991. Decomposition and nitrogen
recycling of cover crops and crop residues. p. 103-105. In W.L. Hargrove (ed.)
Cover crops for clean water. Proc. Int. Conf., Jackson, TN. 9-11 April. Soil and
Water Conserv. Soc., Ankeny, IA.

Stanford, G., and SJ. Smith. 1972. Nitrogen mineralization potentials of soils. Soil Sci.
Soc. Am. Proc. 36:465-471.

Swift, M.J., O.W. Heal, and J.M. Anderson. 1979. Decomposition in terrestrial
ecosystems. Studies in ecology 5. Blackwell, Oxford.

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

Tian, G., L. Brussaard, B.T. Kang, and M.J. Swift. 1997. Soil fauna-mediated
decomposition of plant residues under constrained environmental and residue
quality conditions. p. 125-134. In G. Cadisch and KB. Giller (ed.) Driven by
nature: Plant litter decomposition. CAB International, Wallingford, UK.

Tilrnan, D., D. Wedin, and J. Knops. 1996. Productivity and sustainability inﬂuenced by
biodiversity in grassland ecosystems. Nature. 379:718-720.

Wardle, D.A., and KB. Giller. 1996. The quest for a contemporary ecological
dimension to soil biology. Soil Biol. Biochem. 28:1549-1554.

Willson, T.C., E.A Paul, and RR. Harwood. 2000 [in review]. Managing biologically
active organic matter fractions in sustainable crop production. Applied Soil
Ecology.

18

Table 1. Selected chemical properties of soil from the “diverse” and monoculture
cropping systems, and the organic materials added in the laboratory and in situ

 

 

 

 

incubations.
Material C/N Total C Total N Lignin Cellulose Hemi-
cellulose
g kg"
Diverse soil 11.5 15.0 1.3 - - -
Monoculture soil 11.0 11.0 1.0 - - -

Young red clover 10.6 433.0 41.0 106.0 100.0 94.0
Composted manurer 13.1 340.0 26.0 90.0 692.0 53.0

 

1' Chemical properties based on weight of non-sand materials. Sand had been added as
bedding in the dairy, and was present in the compost at approximately 50% as determined
by water column separation.

19

Table 2. Net mineralized N from added substrates for two cropping systems at two
incubation dates, expressed as percentage of added N.

 

Diverse system Monoculture
%

 

 

 

Laboratory incubation

Treatments 70 days 150 days 70 days 150 days
Com 0 0 2 2
C10 30 30 31 31
2Com 0 6 3 4
2Clo 30 34 32 32
Com + Clo ll 15 13 16
Com + 2Clo 21 30 19 29
2Com + C10 9 12 11 12
2Com + 2Clo 14 18 14 17
In situ incubation

2Com 6 - 1 -
2Clo 29 - 24 -
2Com + 2Clo 13 - 12 -

 

20

Table 3.

Selected climatic data and amount of water added during the time period when

in situ incubations were performed.

 

 

 

 

Average of soil temperature Accumulated Water added
at three depths (cm) precipitation during incubation
Year 1 4 11
°C cm
1998 25.1 27.7 24.0 19.2 13.3
1999 25.6 26.8 23.5 21.1 13.0
Average 25.4 27.3 23.8 20.2 13.2

 

21

 

250

200-

150 ‘

Net mineralized N (mg kg-l)
§

 

Figure 1.

 

DIVERSE SYSTEM - 0-70 days MONOCULTURE
I: 70-150 days

—' A

BC

 

 

BC
BC
CD
D D I

 

 

ya cafe“ 0° 0° 0° 0° ”0° 0° ester (90 0° 0° 0° 0° 0° 0°
xx 000 0"“

MATERIAL ADDED
Net mineralized N after 70 and 150 days of laboratory incubations for soil
sampled from for the “diverse” and monoculture cropping systems. Lower

and upper case letters indicate Signiﬁcant differences in mineral-N at day
70 and 150 respectively (P < 0.05, Tukey-Kramer test).

22

 

180
DIVERSE SYSTEM MONOCULTURE
I60 -
14o -

120q

100‘

Net mineralized N (mg kg -1)
8

 

     

60w
40-
20- l
0..
\
05$? .939 9'00 no?
.999

MATERIAL ADDED

Figure 2. Net mineralized N after 70 days of in situ incubations for the “diverse” and
monoculture cropping systems. Bars followed by the same letter are not
signiﬁcantly different (P < 0.05, Tukey-Kramer test).

23

 

 

100

0 Lab incubation (y = 24.1 + 0.92x )
O In situ incubation (y = 16.7 + 0.89x)

Net mineralized N (mg kg")

 

 

 

I I l

0 20 40 60 80

 

Incubation time

Figure 3. Level of agreement between laboratory and in situ incubation across cropping
systems and addition of organic materials. Individual data points are the LS-

means of 32 observations.

24

CHAPTER 2
Cropping Effects on Nitrogen Mineralization

ABSTRACT

The main objective of this study was to determine the effect of corn (Zea mays L.)
roots on nitrogen (N) mineralization. Rotations of com-com-soybean (Glycine max L.)-
wheat (Triticum aestivum L.) in combination with cover crops, and the application of
composted manure were used as “conditioners” to increase the active N-pool. Polyvinyl
chloride (PVC) cylinders were placed in the 1st year corn plots before planting to conﬁne
or exclude roots of corn and wheat. Soil moisture was controlled to prevent extreme
drying-wetting periods and to minimize N leaching and denitriﬁcation. The moisture
control regime also promoted shallower root growth, primarily in the Ap horizon. In situ
N balance was calculated, and changes in N- and carbon (C)-pool Sizes were determined
by laboratory incubations. Living corn roots increased the mineral-N supplying capacity
of conditioned soil by more than 50%. We suggest that this increase is caused by a
stimulation of N mineralization but that soil conditioning is necessary before signiﬁcant
stimulation can be realized. No Si gniﬁcant increase in available N was observed when
wheat was planted, indicating the possibility that this effect may vary dramatically among
plant Species. The possible effect of speciﬁc plant species in maintaining soil organic
carbon levels is discussed. This information is vital for the management of N in

sustainable cropping systems.

25

INTRODUCTION

Nitrogen mineralization is a microbiologically mediated process consisting of the
transformation of organic N to ammonium, and is a major contributor to the amount of
soil N available for plant uptake. Corn may derive up to 70% of its required N from the
mineralization of soil N (Parkin et al., 1998). The presence of plant roots has signiﬁcant
effects on the soil microbial population since conditions for microbial grth are
especially favorable in the rhizosphere (Barber and Lynch, 1977; Newman, 1985; Foster,
1988; Bazin et al., 1990). The soil microﬂora is heavily inﬂuenced by carbon sources
derived from rhizodeposition (Curl and Harper, 1990). These easily-decomposable
substrates are translocated from the above-ground parts of the plant to the roots and
subsequently into the surrounding soil as root respiration and root-derived material in the
form of root exudates, mucilage, and sloughed cells and tissues (Qian et al., 1997). The
role of rhizodeposits in the turnover of soil organic matter (SOM) has been controversial
for many years. A number of studies reported an increase in the decomposition rate
(Billes and Bottner, 1981; Fisher and Gosz, 1986b; Cheng and Coleman, 1990). Others
reported no change (Cuenca et al., 1983; Harmer and Alexander, 1985), or even a
reduction by the presence of plant roots (F uehr and Sauerbeck, 1968; Gadgil and Gadgil,
1975; Jenkinson, 1977, Sparling et al., 1982; Fisher and Gosz, 1986a; Staaf, 1988; Faber
and Verhoef, 1991). .

Contradictory effects have also been reported for N mineralization. A stimulatory
effect of plant roots has been observed in some studies (Bartholomew and Clark, 1950;
Molina and Rovira, 1964; Clarholrn, 1985a; Berg and Rosswall, 1987; Bottner et al.,

1991; Haider et al., 1993). Others have found that the net effect of plant roots is to

26

increase N immobilization (Huntjens, 1971; Texier and Billes, 1990; Qian et al., 1997).
The discrepancy of these results reﬂects the diversity of soils, plant Species, experimental
methods, and the complexity of the plant-soil interaction on C and N dynamics. Previous
workers did not attempt to identify ﬁeld conditions that enhance the stimulatory effect of
plant roots on N mineralization. However, the addition of N-rich plant residues
(Bhattacharyya et al., 1986; Wang and Bakken, 1989) or N fertilizer (Liljcroth et al.,
1990; Swinnen et al., 1995) may increase N mineralization in the presence of plant roots.
In this work we propose a strategy where previous soil conditioning could be used
to enhance root stimulation of N mineralization. Soil conditioning, as we deﬁne it, is the
process of improving the quantity and quality of the active organic N-pool. We use
practices such as long-term crop rotation, cover crops, and the addition of composted
manure to build up the active pool of organic N. The main objective of this study was to
determine the possible effect of corn roots in stimulating N mineralization. In situ N
balances were calculated for the bare soil, corn, and wheat treatments at 70 days after
planting. Mineralization potentials determined by laboratory incubations were used to
quantify changes in the organic-N and -C pools after cropping. According to the results
of our study, corn roots growing in a conditioned soil increase soil mineral-N supply by
more than 50%. Strategies aimed at developing sustainable cropping systems and N

management should consider this possible rhizosphere effect on SOM turnover.

27

MATERIALS AND METHODS
Site Description
This research is part of the Living Field Laboratory (LFL), a long-terrn
experiment established in 1993 at the Kellogg Biological Station in Hickory Corners,
Michigan. The soil is a mixture of Kalamazoo and Oshtemo sandy loam (Typic
Hapludalfs). The depth of the Ap horizon is 20 to 25 cm, and pH ranges ﬁ'om 6.3-6.8.
Average precipitation has been 860 m y'1 (1988-1999), and potential evapotranspiration
typically exceeds precipitation from May through September. The LFL is designed to
test various combinations of rotation and cover crops under several agronomic
management regimes (Jones, 1996). The main goal is to test alternative strategies for
achieving nutrient cycling efﬁciency and reducing chemical input requirements. The
experimental design is a four-replication split-split-plot, randomized complete block,
with main plots for each management type. The plots used for this study were in the
Integrated Compost management type. This type includes minimal application of
pesticides, banded herbicide plus cultivation for weed control, and composted dairy
manure as the nutrient source. Subplots represent each year in a com-com-soybean-
wheat rotation, plus continuous corn. All crops except soybean are grown with and
without a cover crop. Cover crops include red clover (Trifolium pratense) frost-seeded
into wheat, crimson clover (T rifolium incarnatum) interseeded into 1St year and
continuous corn, and annual ryegrass (Lolium multiﬂorum) interseeded into 2"d year corn.
Before planting, approximately 4 Mg ha'1 of composted manure is added annually to all

crops except soybean. Reduced tillage (chisel plow) is used throughout the LFL.

28

This study includes a ﬁeld experiment using root conﬁning or excluding cylinders
in the 1St year corn plots with cover crops. The C content of this soil is 13 g kg’1 with a
UN ratio of 10. Plots in 1St year corn (immediately following wheat + clover) were
selected because their soil has a larger Short-term (based on 70 days incubation) organic-
N pool compared to other crops in the rotation or to continuous corn (Figure 1). The 70-
day N-pool closely represents the portion of organic-N available for mineralization
during a growing season. In early May 1998, six PVC cylinders (70-cm long and 30-cm
i.d.) were pressed with minimal soil disturbance 60 cm into the soil immediately after
corn (cv. Pioneer 3751) planting. Two corn plants were allowed to grow in three of the
cylinders of each plot and the corn seeds were removed from the other three rings (Bare
treatment). Using new lSt year corn plots, the experiment was repeated in 1999 with the
addition of three cylinders planted with spring wheat (cv. Russ). Four grams (approx.
100 seeds) of wheat were planted in a radial distribution from the center of the rings.

Field Measurements

Immediately after cylinder insertion, soil samples were taken at 0-10, 10-25, 25-
50, and 50-60 cm depths from each plot. Six extra cores were taken adjacent to each
cylinder at 0-10 cm depth and combined for additional laboratory testing. Soil moisture
was controlled in the cylinder, using temporary rain Shelters and periodic addition of
water, to prevent extreme wetting-drying events, and to minimize nitrate leaching and
denitriﬁcation. We intended that controlling the soil moisture would also concentrate
root grth in the Ap horizon. Soil moisture was determined weekly using time domain
reﬂectometry (Tektronix 1502B) measurements. The amount of water to be added was

calculated by subtracting the actual soil moisture from the estimated water holding

29

capacity for the 0-10 cm depth. The calculated amount was divided in half and each part
was added at two different days of the week. The process was repeated every week.
Removable plastic Shelters were used as needed to exclude rain from entering the
cylinders.

The soil surface of each cylinder was mapped into 6 sections in a pie shaped
conﬁguration for sampling. Every 14 days, 3 soil cores (1.9 cm dia.) were taken at 0-10
cm depth from one section of each cylinder. Sections were randomly selected and
samples were taken along an imaginary radius. After each sampling, the open holes were
plugged using small pieces of %-inch PVC pipes (15-cm long). The objective of this
sampling was to monitor changes in N throughout the experimental period and to conﬁrm
soil moisture gravimetrically.

After 70 days, each cylinder and its intact soil core was removed using a front-end
loader. The PVC pipe was separated ﬁom the soil core by sawing longitudinally and
splitting the pipe, exposing the intact core. Soil samples at 0-10, 10-25, 25-50, 50-60 cm
depth were taken from the cores, and deep samples were taken from beneath the tubes
(60-70 cm depth) to determine N movement throughout the proﬁle. Bulk densities were
determined for the 0-10, 10-25 and 25-60 cm depths for nutrient concentration
corrections. Corn and wheat biomass were separated from the soil cores and analyzed for
total C and N using a Carlo Erba N A 1500 Series 2 N/C/S analyzer (CE Instruments

Milan, Italy).

30

Laboratory Measurements

Pro-planting and ﬁnal N-N03' and N-NHX concentrations were determined using
the extraction technique described by Kenney and Nelson (1982) and a Lachat automated
colorimetric analyzer (Lachat Instruments Inc. Milwaukee, WI). For clarity we will use
the terms “pre-planting” to describe the soil at day 0, and “ﬁnal” to describe the soil at
termination 70 days after planting, including the bare, corn, and wheat treatments.
During the preparation of ﬁnal soil, most remaining roots were removed using a 6 mm
sieve. Wheat roots were difﬁcult to separate due to their small size and senescence at the
time of sampling. The pre-planting and ﬁnal soil samples from the 0-10 cm depth of
each plot were composited by treatments and incubated to determine cumulative
mineralizable N and C in the laboratory. Three (20 g dry weight equivalent) aliquots of
each sample were weighed into 100 ml plastic Specimen cups and brought to 50 % of
water holding capacity. The specimen cups were stored in plastic storage containers that
had a thin layer of water on the bottom to maintain humidity. These containers were then
placed in a controlled temperature room at 25 °C for 30, and 70 days. At the end of each
incubation date, the corresponding samples were removed and frozen temporarily to stop
microbial activity. Inorganic N was determined using the previously described method.
Net N mineralization potential was calculated as the difference between inorganic N
content at the end of the incubation period and at day 0. Separate laboratory incubations
were used to determine cumulative C mineralization at 20, 30, 50, 70, 99, and 148 days

of incubation (Paul et al., 2000).

31

Nitrogen Balance Calculation
The mineral-N supplying capacity of soil was calculated using the equation:
NSC=NHarvested + A Nlnorganic (soil)

where NSC is the cumulative mineral-N supplied by soil during the 70 days in the ﬁeld,
Names,“ is the harvested plant biomass N, A Nlnorganic is the N result of subtracting initial
from ﬁnal soil N (N -NO3' + N-NI-LC).

This simpliﬁed equation assumes that deep N leaching was prevented and gaseous
N loss minimized. One can argue against our assumption concerning the lack of gaseous
N loss since no measurement was attempted. Accurate data for denitriﬁcation and
volatilization are rarely available for upland farming systems (Robertson, 1997). But in a
different location within Southwest Michigan, a short-term ﬁeld experiment using 15N on
a poorly drained soil Showed that denitriﬁcation amounted to only 1-2% of the
immobilization plus nitriﬁcation rates (Christensen et al., 1990). Linn and Doran (1984)
reported that in a well-drained soil the relative activity of anaerobic denitriﬁcation is
negligible. Also, a recent study in an adjacent ﬁeld indicated that N loss due to
denitriﬁcation ranged from 1.3 to 0.4 kg ha'1 per year (Robertson et al., 2000). Our
agronomic treatments were nearly identical to that study.

All data were analyzed using the Proc Mixed of SAS (SAS Institute, 1999).
Analyses of variance and the Tukey-Kramer test at the 95% conﬁdence level were

performed.

32

RESULTS
Nitrogen Mineralization

Data of in situ N balance is shown in Table 1. Mineral-N supplying capacity in
soil planted with corn was signiﬁcantly greater than bare (unplanted) soil for both years.
In 1999, N supplying capacity of soil with corn was Signiﬁcantly greater than soil with
wheat. No difference was found between bare and wheat soils. Table 2 Shows the
distribution of mineral-N throughout the proﬁle. The high concentration in the Ap and
the small amount in the C-horizon indicate that deep N leaching was insigniﬁcant. A
more detailed sampling in 1999 Shows that the concentration of N in the 60-70 cm depth
was less than 3 mg kg'1 in all treatments. As expected, more than 90% of the total corn
and wheat roots were collected from the 0—25 cm depth (Table 3). The information
provided in Tables 2 and 3 supports the effectiveness of the experimental method in
preventing leaching and concentrating root grth in the Ap horizon.

Mineralized N for the pre-planting soil and ﬁnal soil of each treatment at 30, and
70 day of laboratory incubation is Shown in Figure 2. The 70-day N-pool size for the soil
planted with corn was 70% smaller than that of the bare soil. This provides additional
evidence suggesting that the increased N supply in the presence of corn roots was
obtained primarily from the potentially mineralizable organic N pool. There was also a

signiﬁcant reduction in the N-pool size with wheat.

33

Carbon Mineralization

Cumulative COz-C released from pre-planting soil and the three treatments at 20,
30, 50, 70, 99, and 148 days of laboratory incubation is shown in Figure 3. There is no
Signiﬁcant difference in C02 evolution between soil planted with corn and the pre-
planting soil at any point in the incubation. This implies that the 148-day C-pool was
unchanged by the presence of corn roots. In contrast, the bare soil treatment produced
Signiﬁcantly less C02 than all other treatments at all dates. The reduction of the short-
term carbon pool in bare soil compared to the pre-planting soil was over 20%. After 30
days, COz-C evolved from wheat soil was signiﬁcantly lower than that of corn and pre-
planting soils. Thus, wheat roots also reduced the 148-day C-pool size, but this reduction

was smaller than that of bare soil.

34

DISCUSSION

The mineral-N supplying capacity of soil in the ﬁeld was increased over 50% in
the presence of corn roots compared to bare soil. We suggest that this increase is caused
by a stimulation of N mineralization. Although the strategy (soil conditioning prior to
crop stimulation) used in this study has rarely been describe in previous literature,
previous works provide a great deal of understanding of the processes governing N
mineralization. The effects of roots on soil include uptake of nutrients and water,
movement of soil caused by root penetration, and release of rhizodeposits (Priha et al.,
1999). Growing roots expose new organic material to decomposition and plant uptake
creates a nutrient gradient from the bulk soil toward the roots. A more direct root control
of N-mineralization is related to root rhizodeposit production (Clarholrn, 1985b). The
plant’s release of these organic materials represents a signiﬁcant energy input to the soil
microbial community (Bakken, 1990; Texier and Billes, 1990). Whipps (1985) reported
that in young corn, 47-69% of the total C transferred to the roots could be lost in the form
of rhizodeposition. Helal and Sauerbeck (1987) estimated that the amount of C released
by com roots from 7-30 days after planting was equivalent to more than 1000 kg ha'1 of
organic matter. This root-derived C acts as a “fuel” for a sequence of events that would
not occur in the absence of roots (Clarholrn, 1985b). As a result, microbial biomass will
increase, since soil microorganisms are typically carbon-limited (Paul and Clark, 1996).
Under these circumstances two major events will occur. First, N-mineralization is
stimulated because of the abundance of low C/N ratio substrate provided by the soil
“conditioning”. Secondly, N-immobilization is enhanced due mainly to the increase in

bacterial biomass, but it is counter-balanced by much greater mineralization. The

35

increased bacterial biomass is subj ect to predation by nematodes and protozoa, causing
an increase in microbial turnover (Elliott et al., 1979; Clarholm, 1985 a, b; Robinson et
al., 1989; Kuikman et al., 1990). During these simultaneous processes, corn roots
continue releasing organic compounds and removing any surplus of NO3' and NH],
continuously affecting N dynamics.

Martin and Puckridge (1982) estimated that wheat roots release about 1000 kg ha”1
of C during the growing season. While corn and wheat plants release considerable
amounts of C into the rhizosphere, no signiﬁcant increase in the available N for wheat was
observed. This difference may be related to the amount and quality of rhizodeposits
released into the soil. Merckx et a1. (1987) reported that materials released from corn
roots were incorporated in the microbial biomass to a higher degree than materials from
wheat roots. In addition, corn roots may possess a more vigorous growth habits than
wheat, allowing them to expose more organic material to decomposition and compete
more efﬁciently with microbes. Wedin and Tilman (1990) found dramatic divergences of
net N—mineralization rates for ﬁve different grass species growing in initially identical
soils, conﬁrming the potential for strong interactions between plant species and N cycling.

The 70-day N-pool size for the corn treatment was a third of the Size that of the
bare soil treatment. This implies that the additional mineral-N, supplied by the soil in
presence of corn roots, was obtained primarily from that particular pool. The Signiﬁcant
reduction in the Size of the 70-day pool for the soil planted with wheat was probably
caused by greater N mineralization during root growth. It is possible that the additional
N was not available for wheat plants due to microbial competition and lesser bacterial

predation by protozoa and nematodes compared to corn. The increase of the 70-day N-

36

pool for the bare soil when compared to the pro-planting soil may be due to the
transformation of a portion of the less active N-pool into more readily-available forms
during the May-June period with optimal soil moisture and temperature.

It is important to recognize the role of soil conditioning as prerequisite to enhance
the stimulatory effect of living roots on N mineralization. Soil conditioning practices
such as rotations, cover crops, addition of organic amendments, and conservation tillage,
not only increase the active N-pool Size but provide a beneﬁcial environment for plant
development and microbial activity (Willson et al., 2000).

The greater mineralization of soil N in the presence of growing corn is
undoubtedly linked to an increase in the mineralization of soil C. However, the
laboratory incubation data indicate that there was no decrease in mineralizable C after
corn growth, whereas mineralizable C decreased in wheat and bare soil treatments.
Rhizodeposition was sufﬁcient to compensate for the C mineralization in the soil planted
with corn, but not in the soil planted with wheat. It is likely that root-derived C is a
relatively more important energy source for microbial activity under corn than under
wheat. Our data agree with those of Shields and Paul (1973); Reid and G088 (1982, and
1983); and Haider et al. (1989, 1991, and 1993) showing that growing plants tend to

maintain soil C levels whereas bare soil tend to decrease it.

37

CONCLUSIONS

We conclude that com roots stimulate N-mineralization in the presence of a large
active organic N-pool, and this effect can vary dramatically among plant Species. This
could represent a very important step in designing sustainable cropping systems. The
combination of soil conditioning practices including rotations, cover crops, organic
amendments, and conservation tillage with a cash crop specially designed to stimulate N
mineralization have the potential to increase yields and decrease fertilizer requirements.
Further studies are needed to classify plant Species and cultivars according to their level

of stimulation in presence of varying N-pool sizes

38

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42

Table 1. N content of soils and plants immediately before and 70 days after planting,
and calculation of N supplied by each soil.

 

 

 

 

 

Treatments pre-planting Shoot N root N ﬁnal mineral N supplied by
mineral soil N (A) (B) (C) soil N (D) soil (E)
kg ha"1
1998
Bare 35.71L - - 148.0 112.3b
Corn 35.7 159.0 30.3 18.7 172.3a
1999
Bare 32.9 T - - 138.4 105.5c
Corn 32.9 149.1 26.9 21.0 164.1a
Wheat 32.9 82.4 10.2 56.7 116.4bc
E=B+C+D-A

Same letter within each year are not signiﬁcantly different in mineral-N at day 70.
1' A single bulk sample was used for all treatments.

43

Table 2. Mineral-N concentration in the soil proﬁle at the end of the experiment (70

 

 

 

 

days after planting).
Treatments
Soil depth Bare Corn Wheat

(cm) mg 1‘84

1998
0-25 35.6 3.5 -
25-60 5.9 1.3 -

1999
0-10 60.6 8.0 ’ 19.3
10-25 19.2 3.2 9.1
25-50 7.1 2.0 4.8
50-60 3.7 2.1 3.2
60-70 2.6 1.7 2.1

 

 

 

 

 

 

Table 3. Distribution of root biomass dry-weight and root N in the soil proﬁle based on
recovered roots at the end of the experiment (70 days after planting).

 

 

 

 

Soil depth Corn Wheat
(cm) kg ha"
Biomass N Biomass N
1998
0-60 3236 30.3 - -
1999
0-25 3333 25.2 625 10.0
25—50 205 1.4 13 0.2
50-60 41 0.3 2 0.1
0-60 3579 26.9 640 10.3

 

45

 

80

- 0-30 days
1:] 30-70 days

 

O\
O
1

 

 

 

 

 

Mineralized N (mg kg‘ 1)
b
o

20-

 

 

 

 

Figure 1. Cumulative mineralized N using laboratory incubations in soils sampled
before planting (May 1999) from a 4-year rotation and continuous corn at the
LFL. Units are based on a 10 cm sample depth. Bars with the same letter are
not Signiﬁcantly different in mineral-N at day 70. The After-wheat soil was
deﬁned as “conditioned” soil in this study.

46

 

80-

O\
O
1

Mineralized N (mg kg'l)
A
O

N
O
I

 

Figure 2.

- 0-30 days
1:1 30-70 days

 

 

 

 

 

 

 

 

 

 

     

 

pre-planting bare corn wheat

SOIL

Cumulative mineralized N during laboratory incubation for soil sampled at
planting and 70 days after planting. Field plots used in this study were from
the “conditioned” soil (After-wheat) at the LF L. Units are based on a 10 cm
sample depth. Letters indicate signiﬁcant differences in mineralized N after
70 days of incubation.

47

 

 

 

 

 

 

 

 

 

1200 -
1000 .
a
Tag 800 "
x
DD
5
Q 600 i
'N
O
o
400 ‘ -O- pro-planting
, ~o- bare (70 DAP)
.-0 -v— com (70 DAP)
200 - o' -v- wheat (70 DAP)
20 4o 60 80 100 120 140

Incubation days

Figure 3. COz-C released during laboratory incubations for pre-planting and 70 days
after planting (70 DAP) soils. Individual data points are the least squares

means of 4 replications.

48

CHAPTER 3

The Impact of Management on Patterns of Soil Nitrate Fluctuation

in Long-Term Cropping Systems

ABSTRACT

This study was done to assess how management practices inﬂuence the seasonal
nitrate (N 03) ﬂuctuation patterns. The speciﬁc objectives were 1) to determine if
ﬂuctuation patterns could be used as a suitable indicator of soil fertility changes within or
between seasons, and 2) to determine the leaching potential of several management
strategies. Composted dairy manure or fertilizer was applied to a rotation of corn (Zea
mays L.)-corn-soybean (Glycine max L.)-wheat (T riticum aestivum L.), and to continuous
corn, both with and without cover crops. For six years soil N03 levels were measured
from April through December. Nitrate levels and ﬂuctuation patterns were very sensitive
to management. In corn and wheat receiving fertilizer, N03 levels were generally higher
than those receiving compost. The only exception was the cover cropped 1St year corn.
Nitrate levels responded not only to fertilizer applications but also to an increased
mineralizable N pool. The leaching potential in fertilized corn was always signiﬁcantly
higher than in corn receiving compost. In soybean and wheat, no difference between
fertility sources was observed. A comparison between leaching potential and NO; levels
in the following spring revealed that fertilized corn may be susceptible to higher leaching
losses. No signiﬁcant differences were observed in the fertilized wheat and in all crops
receiving compost. Seasonal N03 ﬂuctuation provides useful information of soil N
fertility status and has the potential to be used in testing management practices that

enhance biological fertility and reduce leaching losses.

49

Efﬁcient N management implies an optimum crop uptake for maximum yields
while minimizing N losses. This is essential for enhancing the economics of crop
production and decreasing the risk of environmental degradation. Conventional cropping
systems that use high rates of N fertilizer have achieved high yields but have also
affected water quality (Keeney, 1989; HaSSink et al., 1991). Increased N losses to the
groundwater may have originated from excessive application of fertilizer and were
exacerbated by a reduction in the soil’s ability to hold water and residual N (Addiscott et
al., 1991). This decreased ability is related to the declining levels of soil organic matter
(McKenney et al., 1993). Thus, to reduce N03 leaching and contamination of
groundwater, it is necessary to incorporate farming practices that enhance soil organic
matter while reducing the fall N03 levels (Kuo and Sainju, 1998). These practices may
include the use of biological resources such as crop residues, Nz-ﬁxing plants, cover
crops, organic fertilizers, (Willson, 1998) and a ﬁnely tuned fertilizer management
program (Keeney, 1983). A variety of agronomic practices can be successfully integrated
to achieve desirable environmental beneﬁts (Russelle and Hargrove, 1989).

In this study, composted dairy manure or fertilizer was applied to a rotation of
com-com-soybean-wheat, and to continuous corn both with and without cover crops.
Our main purpose was to obtain a better understanding of how agricultural practices
inﬂuence the seasonal ﬂuctuation pattern of soil N03 levels. The ﬁrst speciﬁc objective
was to determine if a ﬂuctuation pattern could be used as a suitable indicator of changes
in soil N fertility within or between seasons. When fertilizer was used, N03 ﬂuctuation
was expected to closely follow fertilizer applications. Under compost with cover crops,

this ﬂuctuation was likely to be related to changes in N pool Sizes. The second objective

50

was to use fall NO; levels in assessing the nitrogen leaching potential. Management
practices that reduce N03 leaching potential can play an important role in minimizing
groundwater contamination (Khakural and Robert, 1993). The estimated leaching
potential was then compared to N03 levels of the following spring. Any Signiﬁcant
reduction in N03 levels was expected to explain a possible leaching loss. To accomplish
the described objectives, measurements of soil N03 were obtained monthly from 1994 to
1999 and used to generate curves describing the ﬂuctuation patterns. Analysis of

variance and covariance, and test for multiple comparisons were performed.

51

MATERIALS AND METHODS
Site Description

This ﬁeld study was conducted from 1994 to 1999 as part of the Living Field
Laboratory (LF L). The LFL is a long-term experiment established in 1993 at the W. K.
Kellogg Biological Station located in Southwest Michigan. Climate is described as cold
moist winters and warm humid summers. Average precipitation has been 860 m y'1
(1988-1999). Evapotranspiration potential typically exceeds precipitation from May
through September. The soil is a mixture of Kalamazoo and Oshtemo sandy loams
(Typic Hapludalfs). The depth of the Ap horizon is 20 to 25 cm, and pH ranges from 6.3-
6.8. The LFL was designed to test combinations of rotation and cover crops under
several agronomic management regimes (Jones, 1996). The main goal is to test
alternative strategies for achieving nutrient cycling efﬁciency and reducing chemical
input requirements. The experimental design is a four-replication Split-split-plot,
randomized complete block, with main plots for each management type. Integrated
Compost and Integrated Fertilizer management type plots were used for this study. Both
management types include minimal application of pesticides, and banded herbicide plus
cultivation for weed control. The main difference is the nitrogen source: Integrated
Compost uses composted dairy manure and Integrated Fertilizer uses commercial
fertilizer. For clarity we will use the terms “Compost” and “Fertilizer” to identify these
two types throughout the paper. Subplots represent each year in a corn-com-soybean—
wheat rotation, plus continuous com. All crops except soybean are grown with and
without a cover crop. Cover crops include red clover (T rzfolium pratense) frost-seeded

into wheat, crimson clover (T rifolium incarnatum) interseeded into 1St year and

52

continuous corn, and annual ryegrass (Lolium multiflorum) interseeded into 2nd year corn.
Corn plots treated with fertilizer received 10 kg ha'1 of N at planting. In late June,
sidedress N rates were determined according to the pre-sidedress nitrate test (PSNT)
(Magdoff et al., 1984). Fifty kg ha'1 of urea-N was applied in mid March to the fertilized
wheat. Soybeans received no fertilizer. In the compost plots, approximately 4 Mg ha'1 of
composted dairy manure was added annually before planting to all crOps except soybean.
Reduced tillage (chisel plow) was used throughout the experiment.
Sampling and Laboratory Procedures

During the six years, soils were sampled once in April, twice in May and June,
and once monthly from July to December. Sampling was generally performed at the
middle of each sampled month, except for May and June when they were carried out
early and late in the month. Occasionally, dry soil conditions did not allow sampling.
Five soil cores were taken at 0-25 cm depth using a 1.9 cm dia. probe, composited, and
dried at 35 0C for 72-96 h. Soils were then ground and the N-NO3' concentrations were
determined using the extraction technique described by Kenney and Nelson (1982) using
a Lachat automated colorimetric analyzer (Lachat Instruments Inc. Milwaukee, WI).

Data Analysis

The factors used in the statistical model were: year, replication, nitrogen source,
crop, cover crop, and month of the year. Both data points of May and June respectively
were averaged. We performed analysis of variance (AN OVA) and covariance, using the
SAS Mixed procedure (SAS Institute, 1999) in which the data from each month of the
year was treated as a repeated measurement of the corresponding experimental unit. The

optimal covariance structure was determined using Schwarz’s Bayesian Criterion (Littell

53

\I,

et at., 1997). Compound-symmetry (CS) was the covariance structure that best explained
our data set. CS assumes a constant variance and covariance (SAS Institute, 1999). After
using ANOVA to identify effects that produce Signiﬁcantly different N03 levels, the
Tukey-Kramer test was used to compare overall LS-means. In addition, curves were
constructed for each crop using monthly means and their corresponding standard errors.
The interpretation of results focused on the analysis of combined years Since our purpose
was to identify overall trends and/or patterns.

Nitrate leaching potential, as we deﬁne it for our sandy loam soil, is the fall (early
or late) N03 concentration that may be susceptible to leaching loss. The estimation of
leaching potentials required some additional adjustment of values. “Early fall” values
were generated by combining the N03 levels from September and October. Meanwhile
for the “late fall”, the November and December data were used. September-October will
be referred to as “early fall”, and “late fall” will represent November-December. The
leaching potential was assigned based upon the presence or absence of cover crops. In
cover cropped soils, the late fall N03 level was assumed as the leaching potential. It was
expected that the presence of plant cover would partially uptake water and residual N03
during most part of the fall, therefore reducing the N03 loading and downward movement
of water. Some authors have reported that leaching loss is closely related to soil N03
levels of late in the fall (Brown et al., 1993; Solberg, 1995). In the absence of cover
crops, the early fall N03 was assumed as the leaching potential. Water and N03 was
expected to gradually leave the Ap horizon from early fall when the soil begins to
recharge. Previous works agrees that N03 leaching from the root zone is likely to start in

that period and continue until early spring (Steenvoorden, 1989). This period in

54

Michigan is characterized by precipitation exceeding evapotranspiration (Cavigelli et al.,
1998). Using early fall values as leaching potential in uncovered soils therefore appears
to be more appropriate especially when using Shallow soil sampling. The estimated
leaching potentials were contrasted to those values of the following spring (April). It was
assumed that any reduction in N03 levels observed in early spring was primarily due to

loss by leaching.

55

RESULTS AND DISCUSSION

Nitrate Levels Inﬂuenced by Management

An analysis of variance (Table 1) indicated that time and the farming practices
evaluated in this study inﬂuence soil N03 levels. Timing, explained by the effect of year
and month, is inﬂuenced by the temporal variation of temperature and precipitation
predominant in Southwest Michigan. These climatic factors play a fundamental role on
microbial mediated N transformation processes (Swift et al., 1979; Tian et al., 1993) and
possible leaching losses (Paul and Clark, 1996). Timing may also be inﬂuenced by the
time of fertilizer application. As expected, two- and three-order interactions were
frequently statistically signiﬁcant, indicating that factors were not independent of one
another (e. g. sidedress of N is a not a random occurrence). The April-December N03
levels (Table 2) for crops with compost were Signiﬁcantly lower than those with
fertilizer. The only exceptions were soybean and the cover cropped lSt year corn. Cover
crops substantially enhanced N03 levels in lSt year corn regardless of the nitrogen source.
This increase was related to the N contribution by legume cover crops. Other researchers
have reported the beneﬁcial effect of legume cover crops in providing N to main crops
(Hesterman et al., 1986; Power, 1987; Oberle and Keeney, 1990; Rasiah, 1999). In
unfertilized soils, the N contribution from mineralization of soil organic materials is
essential because it is the primary source of mineral-N to non-legumes crops. In the 1St
year corn compost plots, the interacting action of cover crops with varied residues from
rotation and the compost itself may also be responsible for this increase. Sanchez et al.
(2000b) reported that the use of rotation, cover crop, and addition of composted manure

can dramatically increase the mineralizable N pool size. Rotation appears to play an

56

important role in soil fertility by maintaining or enhancing soil organic matter levels
(Campbell et al., 1992; Mitchell et al., 1991; Friedel et al., 1996; Franco-Vizcaino, 1997).
N contributions of composted manure may not be signiﬁcant in the Short-term since most
of the N exists in stable organic forms (Paul and Beaucharnp, 1993), but the long-term
effects of annual additions are likely to improve soil fertility. The presence of corn roots
may be another possible interacting element in enhancing N mineralization. Sanchez et
al. (2000a) found that the presence of corn roots can dramatically increase N
mineralization in soils with a large active N pool.

The monthly ﬂuctuations (Fig. 1 to 5) revealed striking differences between N03
responses to nitrogen sources. Soil N03 levels in fertilized corn (Fig. 1, 2, and 5)
dramatically increased by late June as a result of sidedress fertilizer application. The
cover cropped lSt year corn (Fig. 1) showed a lower N03 level than corn without cover.
This was expected since the cover cropped lSt year corn received considerably less
fertilizer based on PSNT recommendations. Soil N03 levels remained high until mid
August and then plummeted by mid September. This decrease coincides with the period
of maximum N uptake needed for grain ﬁlling. The N03 ﬂuctuation in the cover cropped
lSt year corn under compost appears to follow the course of N mineralization. Silgram
and Shepherd (1999) indicated that denitriﬁcation and volatilization are negligible, so
that the N released by mineralization will be reﬂected in the N03 levels. The
mineralizable N pool size is highest in spring and is smallest near physiological maturity
(EL-Haris et al., 1983; Willson, 1998). Other authors found that N mineralization can be
considerable after harvest (Bonde and Rosswall, 1987; Franzluebbers et al., 1994). This

may explain why most soils experience an additional increase in N03 level after crop

57

harvest. In corn using compost, this late increase was not pronounced and probably can
be attributed to a drastic reduction in the N pool size during corn growth. In soybean
(Figure 3), no differentiating patterns were observed between fertility sources. Even
though neither compost nor fertilizer and cover crops were applied during the soybean
growing season, we expected higher N03 levels in those plots historically receiving
compost and cover crops. The contribution of compost and legume cover crops in
providing N to succeeding crops has been well documented throughout the literature but
it appears that soybean does not stimulate N mineralization as does com. This may have
an enormous Signiﬁcance in providing evidence that an appropriate selection of crop
rotation in combination with cover crops and addition of organic amendments may lead
to a build up of the mineralizable N pool. The application of fertilizer in March
dramatically inﬂuenced the April N03 levels in fertilized wheat (Fig. 4) following
soybeans.
Nitrate Leaching Potential

Signiﬁcant differences in N03 levels during the growing season nearly always
lead to signiﬁcance in early and late fall leaching potentials. This strongly suggests that
the potential for N03 leaching loss increases with certain agricultural practices such as
fertilizer applications and continuous corn monoculture. Soil N03 levels for early fall,
late fall, and the following spring, are Shown in Table 2. The leaching potential in
fertilized corn was always signiﬁcantly higher than corn using compost. In soybeans and
wheat, no difference between fertility sources was seen. The same management practices
applied to all soybean plots may explain the similarity of leaching potentials. This may

indicate that leaching potential was not inﬂuenced by the previous cropping history. The

58

wheat’s ability to remove much of the soil N03 during its growing season may be
responsible for the lowest leaching potential of the rotation. Microbial immobilization
may also inﬂuence those low levels because of decomposition of the high C/N ratio
residue after wheat harvest.

Comparing the estimated nitrate leaching potential with those values obtained in
the following spring was expected to closely explain any possible leaching loss. A
signiﬁcant reduction in N03 levels was observed in all fertilized corn plots. These
suspected losses strongly agree with the leaching values reported by Smeenk et al. (2000)
measured from the same plots. These values for fertilized corn are within the range of
leaching loss reported by Paul and Clark (1996). No signiﬁcant differences between
leaching potential and spring N03 levels were observed for fertilized wheat and all crops
using compost. In cover cropped 1St year corn, minimal loss may have occurred despite
enhanced Spring N03 levels indicating that N use may be efﬁcient. Furthermore, Jones
(1996) reported that the use of rotation, cover crops, and composted manure can supply
adequate N to a growing 1‘” year corn crop. This may suggest that using management
practices that enhance the mineralizable N pool Size have the potential to increase yields
and reduce fertilizer requirements while reducing the risk for groundwater pollution. The
lowest leaching potential in wheat may indicate an additional environmental beneﬁt of
this small grain. Smeenk et a1. (2000) found that wheat leached the lowest amount of
nitrate. The comparison was not feasible for the soybean plots in the fertilizer
management since those plots received late winter (during wheat) application of fertilizer,

and that explain the high N03 values in spring.

59

We conclude that seasonal N03 ﬂuctuation patterns could be used as sensitive
indicators of N fertility status. Nitrogen fertility can be increased by application of
conventional fertilizer and also by soil conditioning practices that increases the
mineralizable N pool size. The potential for N03 leaching loss increases with certain
agricultural practices such as fertilizer applications and continuous corn monoculture.
Monitoring N03 levels throughout the season has the potential to be used to test

management practices that enhance biological fertility and reduce leaching losses.

60

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nitrogen in four cropping systems. Soil Sci. Soc. Am. J. 51:1508-1514.

Brown, R.E., G.E. Varvel, and CA. Shapiro. 1993. Residual effects of interseeded hairy
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Campbell, C.A., R.P. Zentner, H.H. Janzen, and KB. Bowren. 1992. Effect of crop
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Cavigelli, M.A., S.R. Deming, L.K. Probyn, and RR. Harwood (ed.). 1998. Michigan
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EL-Haris, M.K. V.L. Cochran, L.F. Elliot, and DP. Bezdicek. 1983. Effect of tillage,
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F ranco-Vizcaino, E. 1997. Comparative soil quality in maize rotations with high or low
residue diversity. Biol. Fertil. Soils. 24:32-38.

Franzluebbers, A.J., F.M. Hons, and DA. Zubuerer. 1994. Seasonal changes in soil
microbial biomass and mineralizable C and N in wheat management systems.
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Friedel, J .K., J .C. Munch, and W.R. Fischer. 1996. Soil microbial properties and the
assessment of available soil organic matter in a Haplic Luvisol after several years
of different cultivation and crop rotation. Soil Biol. Biochem. 28:479-488.

Hassink, J., J .H. Oude Voshaar, E.H. Nijhuis, and J .A. van Veen. 1991. Dynamics of the
microbial populations of a reclaimed-polder soil under a conventional and
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Hesterrnan, O.B., C.C. Sheaffer, D.K. Barnes, W.E. LueSchen, and J .H. Ford. 1986.
Alfalfa dry matter and nitrogen production, and nitrogen response in legume-com

rotations. Agron. J. 78:19-23.

Jones, M. 1996. Enhancing yield, proﬁtability and nitrogen mineralization in com-based
integrated cropping systems. Ph.D. diss. Michigan State Univ. East Lansing.

Khakural, BR, and PC. Robert. 1993. Soil nitrate leaching potential indices: Using a
simulation model as a screening system. J. Environ. Qua]. 22:839-845.

Keeney, DR. 1983. Transformations and transport of nitrogen. pp. 48-64. In F.W.
Schaller and G.W. Bailey (ed.) Agricultural management and water quality. Iowa
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Keeney, DR, and D.W. Nelson. 1982. Nitrogen: Inorganic forms. p. 643-698. In AL.
Page et al. (ed) Methods of soil analysis. Part 2. 2"d ed. Agrom. Monogr. 9.
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61

Keeney, DR. 1989. Sources of nitrate to groundwater. pp 23-34. In R.F. F ollet (ed.)
Nitrogen management and groundwater protection. Elsevier Press, New York.

Kuo, S., and U.M. Sainju. 1998. Nitrogen mineralization and availability of mixed
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McKenney, D.J., S.W. Wang, C.F. Drury, and W.I. Findlay. 1993. Denitriﬁcation and
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Mitchell, C.C., R.L. Westerrnan, J .R. Brown, T.R. Peck. 199]. Overview of long-term
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Oberle, S.L., and DR. Keeney. 1990. Factors inﬂuencing corn fertilizer N requirements
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Paul, J.W., and EC. Beauchamp. 1993. Nitrogen availability for corn in soils amended
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Smeenk, J., R.R. Harwood, A. Fortuna, J .E. Sanchez, T.C. Willson, M.E. Jones, and B.
Parker. 2000. Crop, cover crop, and management inﬂuences on seasonal nitrate

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Lansing.

63

Table 1. Analysis of variance for soil N03 levels as affected by year (1994-1999),
nitrogen source, crop, cover crop, and month of the year.

 

 

 

 

 

Source of variation 1994 1995 1996 1997 1998 1999 All years
Pr > F

Replication NS NS NS NS NS NS NS
Year NAT NA NA NA NA NA ***
Month *t* *t* *** tilt *iﬂt *** 1|"ka
Nitrogen source (N) turns and: tau tan mu tan aura
Crop (C) an: aunt um tau and: taint: III!”
Cover crop (CC) ** * M M NS an” an

Interaction effects

N x C all an: *1“: an talus anus anal:
N x CC NS * * NS NS at :1:

N x CC NS can an: M: aunt: aunt nus
N X C X CC NS * 1"" NS It“ NS an:

 

*, **, *** Signiﬁcant at the 0.05, 0.01, and 0.001 probability levels, respectively.
‘1 NA, not applicable.

64

Table 2. Soil N03 levels as affected by crop, nitrogen source, cover crop, and time of

 

 

the year.
Crop April-December Early fall Late fall Following spring
mg kg'1
1‘” year corn
Fertilizer + cover 11.8a‘I‘ 13.3AaI 9.0Ba 4.3Ca
Fertilizer only 9.8b 9.8Ab 7.2Aab 4.5Ba
Compost + cover 8.6b 5.5Ac 5.3Abc 3.4Aa
Compost only 5.8c 3.5Ac 3.3Ac 3.6Aa
2"d year corn
Fertilizer + cover 9.9a 13.8Aa 7.4Ba 3.7Ca
Fertilizer only 10.6a 14.3Aa 8.3Ba 4.9Ca
Compost + cover 5.0b 3.7Ab 3.7Ab 3.3Aa
Compost only 4.9b 3.0Ab 3.4Ab 3.4Aa
Soybean
Fertilizer + cover 5.6a 5.3Ba 5.2Ba 8.1Aa
Fertilizer only 5.9a 5.8Ba 4.83a 8.6Aa
Compost + cover 4.8a 5.8Ba 4.8Ba 3.7Bb
Compost only 4.6a 5.0Ba 5.SBa 3.8Bb
Wheat
Fertilizer + cover 5.0a 3.0Aa 2.9Aa 3.1Aa
Fertilizer only 5.9a 3.4Aa 2.9Aa 3.5Aa
Compost + cover 2% 3.7Aa 2.8Aa 3.3Aa
Compost only 2% 2.9Aa 2.5Aa 3.0Aa
Continuous corn
Fertilizer + cover 11.1a 1 1.8Aa 11.4Aa 4.1Ba
Fertilizer only 8.83 9.4Aa 7.6Ab 4.8Ba
Compost + cover 4.8b 3.7Ab 3.8Ac 3.4Aa
Compost only 4.6b 3.8Ab 3.6Ac 3.6Aa

 

 

 

T LS-means within a crop column (fertility source x cover crop) followed by a different
lowercase letter differ at the 0.05 probability level.

I LS-means within a row (time of the year) followed by a different uppercase letter differ
at the 0.05 probability level.

65

NO3-N (mg kg“)

40

 

35—

30-

25-

20-

 

+ compost + cover
—0— compost only
+ fertilizer + cover
—v— fertilizer only

 

 

 

Figure 1. Soil NO3-N concentrations in the 1” year corn plots of the rotation using data

from 1994 to 1999. Monthly values have been adjusted to the middle of the
month. Error bars represent that portion of the standard errors above the mean
for each month.

66

35

 

 

+ compost + cover
—0— compost only
+ fertilizer + cover
-V- fertilizer only

 

 

 

30 ~
25 -
_/"‘\
'2."
on 20 a
5
z.” 15 .
0
Z
10 -
5 _,
0
Figure 2.

Soil NO3-N concentrations in the 2“d year corn plots of the rotation using data
from 1994 to 1999. Monthly values have been adjusted to the middle of the
month. Error bars represent that portion of the standard errors above the mean
for each month.

67

10

 

 

+ compost + cover
-O— compost only
+ fertilizer + cover
-V— fertilizer only

\v
3&4? ‘F'x

' v

. \,

V

  

 

8 .1
Ten
34
no
E 6 n
25
on
Z
4 ..
2
Figure 3.

Soil NO3-N concentrations in the soybean plots of the rotation using data from
1994 to 1999. Monthly values have been adjusted to the middle of the month.

Error bars represent that portion of the standard errors above the mean for
each month.

68

 

 

 

 

 

 

14 - + compost + cover
—0— compost only
+ fertilizer + cover
‘2 ‘ -v— fentilizer only
_/I\ 10 "
'60
.34
cc
8 _
E
Z.
o” 6 i
Z
4 -
2 ..
0 I I I I I I I I I

Figure 4. Soil NO3-N concentrations in the wheat plots of the rotation using data from
1994 to 1999. Monthly values have been adjusted to the middle of the month.
Error bars represent that portion of the standard errors above the mean for
each month.

69

4o

 

 

+ compost + cover
—0— compost only
+ fertilizer + cover
—V'— fertilizer only

 

 

30 e
700
34
Of)
5 20 -
Z.
OM
Z
10 - "
. . .4.
O
O I I I I I I I I I
Apr. May Jun Jul. Aug. Sep. Oct. Nov. Dec.
Figure 5. Soil NO3-N concentrations in the continuous corn plots using data from 1994

to 1999. Monthly values have been adjusted to the middle of the month.
Error bars represent that portion of the standard errors above the mean for
each month.

70

 

 

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