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PHOSPHORUS AVAILIBILITY IN ANNUAL AND PERENNIAL
CROPPING SYSTEMS

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COURTNEY MALOOF GALLAHER

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PHOSPHORUS AVAILABILITY IN ANNUAL AND PERENNIAL CROPPING
SYSTEMS

By

Courtney Maloof Gallaher

A THESIS

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

MASTER OF SCIENCE

Department of Crop and Soil Sciences

2007

ABSTRACT

PHOSPHORUS AVAILABILITY IN ANNUAL AND PERENNIAL CROPPING
SYSTEMS

By
Courtney Maloof Gallaher

Understanding mechanisms that promote efficient nutrient cycling is key to
creating more sustainable agricultural landscapes. Many legumes have a unique ability to
mobilize soil P. Phosphorus cycling was quantified at the Long Term Ecological
Research (LTER) experiment at Kellogg Biological Station, in four systems initiated in
1989: 1) conventional corn-soybean-wheat, 2) organic corn-soybean-wheat with two
years of red clover cover crops, 3) continuous alfalfa, and 4) a successional system, taken
out of farmland in 1989. These treatments allow for comparisons between cropping
systems with annual and perennial legumes and different intensities of legumes. In 2006,
soybeans were planted as an assay of P bioavailability in the conventional, organic and
alfalfa systems. Soil samples from the four systems, from 1992, 2001 and 2006, were
analyzed for particulate organic matter phosphorus (POM-P), total organic P, total soil P,
and soil extractable P (Bray P) to examine changes in soil P pools over time.
Phosphorus bioavailability was greatest in the alfalfa system. Changes in soil pools
occurred over time, with more P being stored in the organic and plant available soil pools
in systems with a greater presence of legumes. Overall, these findings argue for an
integrated approach to phosphorus nutrient management in low-input or organic
agricultural systems, which utilize a variety of legumes to improve the bioavailability of

P and build up P in soil pools with rapid turnover, such as POM-P.

To Shaun, with gratitude for your never ending love support on this journey, and to my
parents for always encouraging me to follow my dreams.

iii

ACKNOWLEDGMENTS

I would like to thank my advisor, Sieglinde Snapp, for her time and patience. Her
guidance has taught me many of the skills necessary to complete a successful research
project. I would also like to thank her for her assistance in revising my thesis and helping
me to see it to completion. In addition, I wish to thank her for the many opportunities
she provided me in addition to my masters research that have helped to shape my future
academic career. I would also like to thank the members of my M.S. research
committee, Phil Robertson and John Biernbatun, for their expertise and feedback in
helping me to develop and complete my project. I greatly appreciate their guidance in
helping me to clarify my research objectives and results.

My sincere thanks to all the members of my lab, Kitty Oneil, Lowell Gentry,
Claire McSwiney, Brook Wilke, Tracy Beedy, Wezi Mhango, Marthe Diallo, Edgar Po,
Karen Cichy, Jennifer Hebert, Samantha Taffner, Carey Hammel, Rebecca Titus, and
Rich Price for their support. A special thanks to Kitty for her endless patience in helping
me with field and lab work, to Sam for her many hours of help in the laboratory, to
Jennifer for continuing to work on my samples despite an instrument that was difficult to
use, and to Lowell, Claire, Tracy and Wezi for being sounding boards for all my ideas.

I would like to thank all the KBS LTER staff members for their help in supporting
my project, in particular Drew Corbin for helping me to think through all aspects of my
project, and to Joe Simmons for his help with my field work and extensive knowledge of
the LTER field operations. I greatly appreciate the opportunity to work at such a well run

research station.

iv

Thank you to Marica Jn-Baptiste, Senthil Subramanian, Terry Schultz, and Steve
Vanocker for their thoughtful comments and dedication to helping me improve the
quality and clarity of my literature review.

Thank you to Rita House, Jodie Schonfelder, Darlene Johnson, and Sandie
Litchfield for their secretarial support and assistance in navigating my degree and
fellowship requirements. Thank you to Cal Bricker, who spent many hours repairing my
computer. Thank you to Jim Kells for his support as the Department Chair.

Thank you to Amy Wehrman, Sarah Halter, Audrey DeRose-Wilson, Courtney
Jones, Jeff Evans, Lauren Brown, Cass Hauserman, Dan Sklansky, Sandra Japuntich,
Steve Kennedy, Ruth Fisk, Tim Boring, Meleia Egger, Mariah Branche, Lucy Openja,
Dahab Hussein, and many others for the friendship and support they have given me over
the last three years. I am lucky to have such a wonderful and supportive group of friends.

Funding for this research was made possible by an NSF Graduate Research
Fellowship, and the NSF Long Term Ecological Research Program at Kellogg
Biological Research Station in Hickory Corners, Michigan.

Finally, my sincere thanks to my husband, Shaun Langley, my parents Cindy and
Dan, my sister Hayley, my dog Harold, and my other relatives for their love and support.
You have always believed in me, even when I didn’t believe in myself. Your love has
carried me through my master’s degree experience, and helped me to become who I am

today.

TABLE OF CONTENTS

 

 

 

 

LIST OF TABLES - - - VII
LIST OF FIGURES - - - - - - ...... VIII
INTRODUCTION -

CHAPTER 1. LITERATURE REVIEW - - - - - 4
THE PHOSPHORUS CYCLE ................................................................................................ 4
PLANT UPTAKE OF PHOSPHORUS ...................................................................................... 5
CROP ROTATIONS TO MANAGE SOIL PHOSPHORUS FERTILITY ........................................... 7
POSITIVE ROTATIONAL EFFECT OF LEGUMES ON P CYCLING ............................................. 8
PARTITIONING SOIL PHOSPHORUS POOLS ...................................................................... 10
EXTRACTABLE INORGANIC PHOSPHORUS ....................................................................... 1 1
TOTAL SOIL PHOSPHORUS .............................................................................................. 12
PARTICULATE ORGANIC MATTER ................................................................................... 13

CHAPTER 2. THE INFLUENCE OF LEGUMES ON THE BIOAVAILABILITY
OF PHOSPHORUS AND THE PARTITIONING OF SOIL PHOSPHORUS

 

POOLS - - - -- -- - - 16
INTRODUCTION .............................................................................................................. 16
METHODS ...................................................................................................................... 21

Site description and agronomy ................................................................................. 21
Soybean bioassay ...................................................................................................... 22
Soil Sampling ............................................................................................................ 25
Total and extractable inorganic soil phosphorus ..................................................... 25
Phosphorus budget .................................................................................................... 26
Particulate organic matter ........................................................................................ 26
RESULTS ........................................................................................................................ 28
Total and extractable inorganic soil phosphorus ..................................................... 28
Phosphorus Budget ................................................................................................... 30
Phosphorus Budget ................................................................................................... 33
Particulate organic matter ........................................................................................ 35
Soybean bioassay ...................................................................................................... 38
Soybean bioassay ...................................................................................................... 42
DISCUSSION ................................................................................................................... 46
Total and extractable inorganic soil P ..................................................................... 46
Phosphorus budget .................................................................................................... 4 7
Particulate organic matter ........................................................................................ 48
Soybean Bioassay ...................................................................................................... 50
CONCLUSION .................................................................................................................. 52
BIBLIOGRAPHY--." - -- _ - - - - -- 53

 

vi

LIST OF TABLES

TABLE PAGE
2.1 ANOVA for total and extractable inorganic soil P (0—20”) .................... 31
2.2 Total soil phosphorus (means +/- SEM) .......................................... 32
2.3 Extractable inorganic soil P (Bray P) (means +/- SEM) ........................ 32
2.4 Phosphorus budget for the conventional, organic and alfalfa

treatments of the LTER from 1997 to 2005 ...................................... 34
2.5 AN OVA results for particulate organic matter .................................. 39
2.6 POM as a percentage of soil, by weight (means +/— SEM) ..................... 40
2.7 Tissue P concentration of POM (means +/- SEM) .............................. 40
2.8 Total amount of P in the POM pool (means +/- SEM) ... ....................... 41
2.9 ANOVA for soybean tissue P concentrations ................................... 45
2.10 Soybean tissue P concentration (means +/- SEM) ............................... 45

vii

LIST OF FIGURES

FIGURE PAGE
2.1 Description of microplot areas within treatments of the LTER ............. 24
2.2 Extractable inorganic P in 1992, 2001 and 2006 ................................ 29
2.3 Extractable inorganic P in 2006 .................................................... 30
2.4 Particulate organic matter (as a percentage by weight of soil) ................ 36
2.5 Phosphorus concentration of particulate organic matter ........................ 37
2.6 Total P in the POM pool ........................................................... 38
2.7 Biomass of soybeans collected from microplot areas .......................... 43
2.8 Tissue P concentration of soybeans ............................................... 44

viii

Introduction

Phosphorus (P) is an essential nutrient for plants, needed for the production of
ATP, DNA, and RNA and other cell constituents. It plays important roles in nearly all
phases of the plant life cycle, including photosynthesis, flowering, seed production and
maturation, and root growth. Phosphorus deficiency can cause severe stunting and
significant yield losses (Haven et al., 1999). Despite being one of the most important
plant nutrients, P is also one of the least mobile in soil and therefore highly unavailable to
plants. Soluble forms of P are fixed by other soil minerals, making less than 0.01% of
total soil P available to plants (Brady and Weil, 2000). Thus an understanding of soil P
biogeochemistry is important for deciding how best to manage different agroecosystems.

Natural ecosystems have evolved to efficiently cycle soil nutrients, while modern
agricultural systems are less efficient and rely heavily on external inputs, including
inorganic fertilizers, to supplement the crop’s nutrient demands. Depending on the
agricultural system, farmers encounter a number of problems related to effective nutrient
management. In conventional agricultural systems, excessive use of inorganic fertilizers
often leads to non-point source pollution, which can have very detrimental impacts on
aquatic ecosystems, leading to eutrophication (Carpenter et al., 1998).

In other agroecosystems, farmers must manage crops in nutrient deficient soils or
with low nutrient inputs. Smallholder farmers in many areas of the developing world
struggle to meet the nutrient demands of their crops; they may not have access to
compost or manure, and supplemental inorganic fertilizers can be prohibitively
expensive. In the US, many organic farmers use manure to provide nitrogen and

phosphorus to their crops. However, organic farmers without access to manure would

also greatly benefit if their agroecosystems utilized crop rotations that better promote P
cycling. Identifying combinations of plants, such as different cereals and legumes that
can more efficiently cycle or increase the availability of soil nutrients, is extremely
relevant in this context.

Legumes have long been recognized for their role in the nitrogen cycle, but it is
also important to recognize that some legume species have unique mechanisms to access
sparingly soluble P pools (Ae et al., 1990). Previous greenhouse and field trials have
shown that legumes improve the bioavailability of phosphorus to subsequent crOps
(Cavigelli and Thien, 2003; El Dessougi et al., 2003; Kamh et al., 1999; Nuruzzaman et
al., 2005). Preliminary studies have also suggested that phosphorus mobilized by
legumes is moved from less available inorganic pools to more available soil organic
pools (Maroko et al., 1999). However, the impact of different species and management
practices of legumes on these soil pools is not well understood. Ultimately, improving
our understanding of how legumes help to mobilize soil phosphorus and store it in
different soil pools can help farmers design more efficient cropping systems.

A long-term agricultural experiment, located at Kellogg Biological Station (KBS),
has maintained different cropping systems with varying legume inputs since 1989. Four
of the treatments are based on corn-soybean-wheat rotations, with conventional, no-till,
low-input and organic management, the last of which includes red clover as a leguminous
winter cover crop. A continuous alfalfa treatment and a successional treatment, old
farmland that is returning to prairie, have also been established. Focusing on four of
these treatments (conventional, organic, continuous alfalfa, and successional) provides a

contrast between the number of years legumes are present in a crop rotation as well as the

duration of legumes present (annual vs. perennial). These four treatments provide the
context for answering questions about the impact of legumes on both availability of P to
subsequent crops, and about the extent of the movement of P between different soil pools.
In order to answer questions about the ability of legumes to enhance soil
phosphorus availability and cycling over time, we designed a study to look at 1) the
bioavailability of phosphorus in the conventional, organic and alfalfa treatments to a
subsequent soybean crop, and 2) to characterize different soil phosphorus pools from
three time points in the conventional, organic, alfalfa and successional treatments of the

LTER site.

Chapter 1. Literature Review

The Phosphorus Cycle

The global phosphorus cycle is the only major biogeochemical cycle in which the
atmosphere does not play a significant role. Phosphorus primarily occurs in nature as a
phosphate ion in rocks or ocean sediments. Chemical or biological weathering moves P
into the soil solution, and makes it available for plant uptake. Decomposing plant or
animal biomass returns P to the soil, increasing P in the organic pool (Fig. 1) (Newman,
1995)

In natural ecosystems, P is recycled efficiently between plants and soil, thus any P
losses are primarily a result of runoff or erosion. In contrast, in agricultural systems
additions and losses are directly impacted by different management strategies (Bundy and
Sturgul, 2001). Fertilizer and livestock manure are the main sources of P inputs into
agricultural systems, and P lost from these systems is primarily the result of runoff or
crop removal. In a study of Wisconsin cropland, total fertilizer P additions have declined
by 30% in recent decades, but crop removal of P has increased since 1970, largely
because of increased crOp yields, acreage and changes in crops grown, and was consistent
with a national trend (Bundy and Sturgul, 2001). Phosphorus loss from an agricultural
system can be minimized by practices such as decreasing fertilizer rates, point application
of P, incorporating crop residues or managing fields to reduce runoff.

Although farmers may apply substantial amounts of P fertilizer to a field, this P is
not readily available for plant uptake because soluble inorganic P is rapidly fixed by other

inorganic soil components. In acid soils, P complexes with aluminum (Al) and iron (Fe)

and in alkaline soils P is fixed by calcium (Ca) (Brady and Weil, 2000). In acid soils, it is
estimated that over 90% of P fertilizer added to a soil will be fixed by other soil minerals
(Bundy and Sturgul, 2001). Some legume species are able to access these soil P stores

and can improve overall P cycling in an agroecosystem.

Plant uptake of Phosphorus

Although the total amount of P in soil is large (300-5000 mg/kg), it is difficult
for plants to access since available P concentrations in soil solution tend to be extremely
low (Brady and Weil, 2000). Soil solution concentrations of inorganic P (Pi), the form of
P most readily available to plants, are commonly luM and rarely exceed IOuM (
(Bielesky and Ferguson, 1983). The two most common available forms of inorganic P
found in soil solution are HzPO4' and HPO42', with the H2PO4' most common at pH less
than 7, and HPO42' above pH 7 (Schachtrnan et al., 1998).

Mycorrhizal fungi aid in plant P uptake since P is relatively immobile in soil
and the mycorrhizal mycelium markedly extend the surface area of the root systems for
uptake. More than 90% of terrestrial plants form symbiotic relationships with
mycorrhizal fimgi (Bolan, 1991; Smith and Read, 1997). This symbiosis is based upon
the mutualistic exchange of plant carbon for soil mineral nutrients scavenged by these
fungal hyphae. Phosphorus uptake by plants infected by mycorrhizae can be 3 to 5 times
higher than in non-infected plants. (Smith and Read, 1997).

Although mycorrhizal associations remain an important factor in legume uptake

of soil P, many legumes have additional adaptations which allow them to chemically

modify the rhizosphere in order to access sparingly soluble sources of P. Legume roots
may increase P solubility of the dominant P fraction either through acidification (Ca-P) or
alkalinization (Al-P, Fe-P) of the rhizosphere via root exudates (Gahoonia and Nielsen,
1992; Riley and Barber, 1971). Under P deficient conditions, the non-mycorrhizal white
lupin, develops proteoid roots that excrete citrate in order to solubilize calcium-bound
soil phosphate (Dinkelaker et al., 1989; Gardner et al., 1983). Although pigeon pea
(Cajanus cajan L.) is mycorrhizal, it has been shown that additional soil P taken up by
this plant is due to its ability to utilize iron-bound phosphate (F e-P) by excreting piscidic
acid, which chelates Fe (Ae et al., 1990). Additional studies have shown that pigeon pea
is able to access aluminum bound phosphate (Al-P) using a similar mechanism.
Groundnut (Arachis hypogaea L.) has also been shown to access sparingly soluble soil
phosphorus pools by excreting organic acids, including malonic, oxalic and piscidic acid
(Otani et al., 1996). In P limiting conditions, alfalfa (Medicago sativa) acidifies the
rhizosphere by excreting malic, citric and succinic acids which solubilize iron-bound P
(Liption et al., 1987; Masaoka et al., 1993). Red clover (T rifolium pretense), a common
leguminous cover crop, has also been shown to excrete carboxylic acids which mobilize
sparingly soluble soil P by lignan exchange and occupation of P sorption sites (Gerke,
1995)

Some legumes are less efficient at mobilizing sparingly soluble soil P. For
example, soybean develops fibrous root systems which exude only trace levels of organic
ions in response to P limited conditions (Ohwaki and Hirata, 1992). However, these root
systems appear to be better adapted to acquire more readily available soil P than other

legumes (Braum and Helmke, 1995).

Crop rotations to manage soil phosphorus fertility

In view of the potential of different legumes to improve mobilization of phosphorus,
it is important to consider their overall role in maintaining soil fertility as part of a crop
rotation. Soil fertility is a fiindamental factor in determining the overall productivity of a
farming system. Conventional and organic farming systems approach the management of
soil fertility in different ways, with conventional agriculture using solutions based on
input substitution, such as inorganic fertilizer to improve soil fertility, and organic
systems using longer term, preventative solutions such as crop rotation (Stockdale et al.,
2001). Since one of the greatest challenges faced by organic farming systems without
access to manure is the management of nutrient supplies, within these systems, there is
generally an emphasis on alternative strategies to build soil fertility, such as through the
use of green manures and grain legumes (Watson et al., 2002). These legumes are
traditionally used to manage nitrogen fertility, but given the potential of legumes to
mobilize soil phosphorus, enhanced phosphorus fertility could be an additional benefit.
However, this potential benefit has not been much investigated.

The high crop yields achieved in most developed countries today are highly
dependent on phosphorus fertilizer, primarily from fertilizers derived from rock
phosphate (Newman, 1997). Organic systems that operate without the use of phosphate
inputs often run a P deficit, as calculated by system budgets (Berry et al., 2003), yet a few
authors have found that annual P deficits have caused no decline in the content of
extractable P over 10 to 20 years (Kafflca and Koepf, 1989; Oehl et al., 2002). Building

simple nutrient budgets based on crop rotations can be a useful tool to understand

potential long-term deficits and to develop long-term strategies for phosphorus nutrient
management. However, it is important to recognize potential problems with the over
simplification of nutrient budgets, which reflect assumptions about transfers from

available to unavailable pools that do not necessarily accurately predict crop available P.

Positive rotational effect of legumes on P cycling

The unique ability of some legume species to acquire sparingly soluble soil P has
the potential to improve the P use efficiency of crops grown subsequently in a rotation.
Decomposition of organic P in legume tissues provides a relatively labile form of P to the
next crop, thus supplementing the small soluble inorganic P pool with a larger pool of
mineralizable soil organic P in systems under organic or low fertilizer management
(Tiessen et al., 1994).

Crops grown following legumes are often higher yielding with more vigorous
grth because of these organic P pools. A greenhouse study by Cavigelli and Thien
(2003) found that sorghum yields were significantly higher following the incorporation of
legume residues in an experiment involving alfalfa, red clover, yellow sweet clover
(Melilotus oflicinalis), white lupin (Lupinus albus L.), winter pea (Pisum sativum L. ssp.
arvense) and hairy vetch (Vicia villosa), as well as winter wheat used as a control. Yield
increases were highest following alfalfa, which had the highest tissue P content prior to
incorporation, consistent with alfalfa having best facilitated movement of P into the soil
organic pool.

In another greenhouse study, the authors (Nuruzzaman et al., 2005) investigated

the effect that grain legumes have on P uptake of a subsequently grown wheat crop. In

this study, the legumes white lupine, field pea (Pisum sativum) and faba bean (Viciafaba)
all significantly improved the growth of a subsequent wheat crop as compared to the
growth of wheat after wheat. Tissue concentrations of P in wheat were 6%, 13% and
19% higher in wheat following field pea, white lupine and faba bean respectively. In
addition, carboxylic acids were present in significantly higher concentrations in the
rhizosphere soils of all the legume treatments. Because the system was not nitrogen
limited the authors concluded that the improved yields were likely due to mobilization of
sparingly soluble soil P.

Kamh et al. (1999) carried out a greenhouse study in P-limited soils from northern
Nigeria to examine the effect of nine legume species on a subsequent maize crop in an
effort to characterize legume P efficiency in comparison to maize. Legume species
studied included Mucuna pruriens (velvet bean), Lablab purpureus (hyacinth bean),
Glycine max (soybean), Phaseolus vulgaris (common bean), Cajanus cajan (pigeon pea),
Chamaecrista rotundifolia (round-leafed cassia), Clitoria ternatea (butterfly pea), and
Centresoma purbescens (centro). Of these species, velvet bean, common bean and
pigeon pea were the most efficient at providing soil P to a subsequent maize crop, and
were nearly as effective as the application of P fertilizer, at rates of 15 to 150 kg/ha P.
Because adequate mineral N was supplied to all the treatments, the authors also
concluded the improved maize yields following legumes were a result of mobilization of
sparingly soluble soil P.

In a subsequent greenhouse study, the same authors (Kamh et al., 2002) compared
the effect of the same legume species on maize growth using two P-limited, high P-fixing

soils from Nigeria and Guinea. Their results were similar to their previous study, but

further indicated that on high-P fixing soils grth of leguminous plants cannot alone
provide sufficient P nutrition to a subsequent maize crop. These results were confirmed
in a field trial with the same soils (Horst et al., 2001). However Horst el al. (2001)
suggested that growing leguminous crops can still contribute to the overall sustainability
of a system by reducing the need for external inputs and shifting P cycling towards more
plant available P pools.

Intercropping P—efficient legumes with cereal crops may allow for efficient
cycling of P and increased yields in a similar manner to legume rotations. El Dessougi et
a1. (2003) examined P cycling in a greenhouse study where maize was intercropped with
white lupine, sugar beet, oilseed rape or groundnut. Dry matter yield of maize from the
maize and groundnut intercrop was shown to be three times higher than maize grown
alone, where neither system received P fertilizer. In addition, soil fractionation indicated
that intercropped maize had higher available P fractions. Because all treatments had
sufficient available N, improved maize yields were most likely due to improved P

availability solubilized by groundnut.

Partitioning Soil Phosphorus Pools

Phosphorus is stored in three different soil pools designated primary inorganic,
secondary inorganic, and organic (Walker and Syers, 1976). These soil pools all release
biologically available P to soil solution but at very different rates, depending on soil
conditions such as soil texture, pH, moisture content, plant and microbial activities.

(Sanyl and De Datta, 1991). The primary inorganic pool includes primary soil minerals,

10

such as apatite, which is a source of soluble P early during ecosystem development.
Phosphorus released from the weathering of primary minerals is incorporated into plant
biomass and cycled back into organic P or secondary inorganic P pools. The secondary
inorganic P pool consists of secondary minerals that form as a result of chemical and
biological weathering. Over time, as primary minerals are depleted of P and most of the
P is found in organic and secondary inorganic P pools, the productivity of an ecosystem
is dependent on its ability to cycle P between organic matter and living organisms (Miller
et al., 2001). Soil P pools are difficult to quantify in a biologically relevant way, and thus
different laboratory methods, such as extractable inorganic phosphorus, particulate
organic matter phosphorus (POM-P), and total soil phosphorus, have been developed to

partition soil P pools and relate them to plant available P.

Extractable inorganic phosphorus

Three methods are commonly used today to measure extractable inorganic
phosphorus, which is a measure of plant available soil phosphorus. These methods are
dependent upon four basic reactions between P and the extracting solution: 1) dissolving
action of acids, 2)anion replacement to enhance desorption, 3) complexing of cations
binding P, and 4) hydrolysis of cations binding P (Elrashidi, 2005). In acidic soils, the
primary sources of P are Al- and Fe- phosphates, while calcium phosphates are the
dominant P mineral in alkaline and calcareous soils. Bray-1 and Melich 3 are most
appropriate for acidic to neutral soils while the Olsen method is an effective predictor of

plant available P in alkaline or calcareous soils.

11

The Bray-1 method, developed by Bray and Kurtz (1945), uses a combination of
HCl and NH4F as extractants to remove easily acid soluble forms of P, such as Al- and
Fe-phosphates. In 1953, A. Mehlich developed an extraction method (Melich-1) that
used a combination of HCl and H2804 acids to extract P from soils in the north-central
region of the US. (Mehlich, 1953). Due to the sulfate ions in solution, Melich-l.
dissolves Al- and Fe-phosphates as well as P adsorbed to colloidal surfaces in soils. In
the 1980’s, Mehlich further refined his initial methods to develop an extractant
appropriate for multi-elemental analysis (Mehlich, 1984). The Mehlich 3 extractant is a
combination of acids and salts, including dilute acetic acid (HOAc), nitric acid (HNO3),
ammonium fluoride (NH4F) and ammonium nitrate (NH4NO3) as well as the chelating
agent ethylenediaminetetraacetic acid (EDTA). Although versatile, the Mehlich soil tests
are only suitable for use in acid and neutral soils. In 1954, Olsen developed a soil P test
to extract inorganic P from calcareous, alkaline or neutral soils that uses a 0.5 M sodium
bicarbonate (N aHCO3) solution at a pH of 8.5. The sodium carbonate in solution causes
calcium carbonate to precipitate, thus increasing the dissolution of Ca-phosphates. In
addition, the sodium bicarbonate extractant also removes dissolved and adsorbed P from

calcium carbonate and Fe-oxide surfaces (Olsen et al., 1954).

Total soil phosphorus

Total soil phosphorus can be partitioned into either inorganic or organic forms.
There are several commonly accepted methods for determination of total soil P (P,),

including the sodium carbonate (N azCO3) fusion method and the perchloric acid (HClO4)

12

digestion method (Olsen and Sommers, 1982). The sodium carbonate fusion method is
most reliable, but it is very time consuming and requires the use of platinum crucibles
which are expensive and require special handling. The perchloric acid soil digestion is
more suitable for determination of phosphorus in large numbers of samples, although
samples must be digested under a perchloric acid fume hood (Olsen and Sommers, 1982).
A more recent study has suggested that determination of total soil P using the standard
Kjeldahl digestion, currently used for total soil N analysis, can also yield reliable results

(Taylor, 2000).

Particulate organic matter

Understanding soil organic matter (SOM) cycling is relevant to understanding
phosphorus cycling because productivity may be reduced in well fertilized soils where
there has been a loss of SOM (Aref and Wander, 1997; Johnston, 1991). In low-input or
organic farming systems, SOM is an essential source of important plant nutrients. Soil
organic matter levels can be improved using processes that increase carbon inputs and
reduce carbon losses, including a variety of management practices such as pasture (Sbih
et al., 2003), amending with compost (Stone et al., 2001), cover crops (Stevenson et al.,
1998), and no-till systems (Frey et al., 1999).

Analysis of SOM fractions has been undertaken as a means to detect changes in
processes of carbon assimilation in relationship to management practices. Historically,
SOM characterization focused on extraction methods that separated organic matter from

the soil mineral matrix, allowing for HPLC, GC, or elemental analyses (Wander, 2004).

13

These methods were judged based on their ability to isolate pure, reproducible and
homogenous components for analysis (Stevenson, 1994). More recent efforts at
characterizing SOM have focused on fractionating SOM into labile, slow and recalcitrant
pools, relating carbon retention to Motion. The labile, or active, organic matter pool
represents material recently incorporated into the soil and is thought to be of high nutrient
supply potential. Thus, this labile pool has the potential to greatly contribute to nutrient
cycling (Wander, 2004).

Particulate organic matter (POM) is commonly used to assess labile SOM pools.
POM has been shown to be responsive to changes in management practices, and some
research indicates it is better than other fractions at representing labile SOM, such as
carbon mineralization potential (Alvarez and Alvarez, 2000; Conteh et al., 1998;
Franzluebbers et al., 2000). POM can vary significantly with plant inputs, seasonality,
soil depth, and patterns of sampling so care must be taken to standardize sampling
regimes (Wander and Traina, 1996; Wander et al., 1998). Nutrient analysis of POM
provides an indicator of labile nutrient pools. Particulate organic matter phosphorus
(POM-P), a measure of total P in the organic matter collected, could be a useful an
indicator of labile soil phosphorus.

The two basic methods used to characterize soil POM separate organic matter
from the soil mineral matrix either by size or density. The POM or coarse fraction (CF)
typically refers to organic matter that is sand-sized or larger. Size-based methods
separate organic matter from soil using different sizes of sieves. These methods
generally focus on large residues, varying in size from 100-2000 pm, that can be clearly

identified as plant residues (Wander, 2004). Density separations separate sand from

14

organic matter by dispersing a soil sample in a liquid ranging in density from 1.6- 2.6
g/cm3. Lower densities tend to favor recovery of larger POM particles whereas use of
denser liquids results in the recovery of a wider range of particle sizes (Ladd and Amato,
1980). Liquids commonly used for density separations include sodium or potassium
iodide, sodium polytungstate (N aPT), and silica gels (Wander, 2004). Some methods of

POM measurement combine size and density separations.

15

Chapter 2. The influence of legumes on the bioavailability of phosphorus and the
partitioning of soil phosphorus pools

Introduction

Effective nutrient management presents a variety of challenges for farmers in
different agricultural systems. Conventional agricultural systems, which rely heavily on
inorganic fertilizers, have developed as the dominant agricultural system in the US. only
in the last one hundred years. Nutrient management in these systems is based on
inorganic fertilizers to supplement nutrient deficiencies, while organic or many
smallholder systems are managed through the use of long-term crop rotations (Stockdale
et al., 2001). Management of phosphorus presents unique challenges, because although
the total amount of P in soil is large (300-5000 mg/kg), it is difficult for plants to access
since available P concentrations in soil solution tend to be extremely low (Brady and
Weil, 2000). Farmers may apply substantial amounts of P fertilizer to a field, but this P is
not readily available for plant uptake because soluble inorganic P is rapidly fixed by other
inorganic soil components. Phosphorus complexes with aluminum (Al) and iron (Fe) in
acidic soils, and with calcium in alkaline soils (Brady and Weil, 2000). In acid soils, it is
estimated that over 90% of fertilizer added to a soil will be fixed by other soil minerals
(Bundy and Sturgul, 2001). Thus an understanding of soil P biogeochemistry is
important when deciding how best to manage different agroecosystems, especially in P
limited conditions.

Plant species mediation of nutrient cycling mechanisms requires in-depth study,

as a key regulator of nutrient cycling in intensively managed systems. Highly reactive

16

nitrogen and excess phosphorus application will continue to be relied upon to support
cropping system productivity as long as inefficient cycling remains-the norm. Natural
ecosystems have evolved to efficiently cycle soil nutrients, while modern agricultural
systems are less efficient and rely heavily on external inputs, including soluble inorganic
fertilizers, to supplement the crop’s nutrient demands. Some legume species are able to
access these soil P stores, although the impact on overall P cycling in an agroecosystem
has not been studied in detail. Identifying combinations of plants, such as different
cereals and legumes that can more efficiently cycle or increase the availability of soil
nutrients, may be extremely relevant in this context.

Mycorrhizal associations are one factor in legume uptake of soil P, but many
legumes have other adaptations which allow them to chemically modify the rhizosphere
in order to access sparingly soluble sources of P. Legume roots increase P solubility of
the dominant P fraction through mechanisms such as acidification (Ca-P) or
alkalinization (Al-P, Fe-P) of the rhizosphere via root exudates (Gahoonia and Nielsen,
1992; Riley and Barber, 1971). Genotype influences the mechanisms of plant-soil P
interaction. For example, under P deficient conditions, the non-mycorrhizal white lupine
develops proteoid roots that excrete citrate in order to solubilize calcium-bound soil
phosphate (Dinkelaker et al., 1989; Gardner et al., 1983). In P-limiting environments,
alfalfa (Medicago sativa) acidifies the rhizosphere by excreting malic, citric and succinic
acids that solubilize iron-bound P (Liption et al., 1987; Masaoka et al., 2004). Red
clover (T rifolium pretense), a common leguminous cover crop, has also been shown to
excrete carboxylic acids which mobilize sparingly soluble soil P by lignan exchange and

occupation of P sorption sites (Gerke, 1995). Other legumes appear to be less efficient at

17

mobilizing sparingly soluble soil P. For example, soybean develops fibrous root systems
which exude only trace levels of organic ions in response to P limited conditions (Ohwaki
and Hirata, 1992). However, these root systems appear to be better adapted to acquire
more readily available soil P than other legumes (Braum and Helmke, 1995). Species
effects on mechanisms of accessing sparingly soluble P have only begun to be elucidated.
The impact of different species and management practices of legumes on these
soil pools is currently not well understood. In numerous greenhouse and field trials,
legumes have been shown to improve the bioavailability of phosphorus to subsequent
crops (Cavigelli and Thien, 2003; El Dessougi et al., 2003; Kamh et al., 1999;
Nuruzzaman et al., 2005). In additional studies, phosphorus mobilized by legumes was
moved from less available inorganic pools to soil organic pools, or to readily available
plant available phosphorus pools (Maroko et al., 1999). For example, with a nutrient
budget conducted on farm in Malawi the potential of the legume pigeonpea residues to
improve P nutrition in intercropped corn was demonstrated (Snapp et al., 1998).
hnproving our understanding of how legumes help to mobilize soil phosphorus and store
it in different soil pools can help farmers design more efficient cropping systems.
Phosphorus in the soil is stored in three different pools, designated primary
inorganic, secondary inorganic, and organic (Walker and Syers, 1976). The rate at which
these pools release biologically available P to the soil solution varies depending on soil
conditions such as soil texture, pH, moisture content, plant and microbial activities.
(Sanyl and De Datta, 1991). Because soil P pools are difficult to quantify in a
biologically relevant way, different laboratory methods, such as extractable inorganic

phosphorus, particulate organic matter phosphorus (POM-P), and total soil phosphorus,

18

have been developed to partition soil P pools and characterize relationships to plant-
available P. Extractable inorganic P, a measure of plant-available P which describes
easily exchangeable iron and alumintun phosphates, is quantified in acidic to neutral soils
using the Bray-1 or Melich-3 extraction methods. Particulate organic matter (POM) is a
measure of soil organic matter that is commonly used to assess labile SOM pools, which
responds to changes in management practices more rapidly than other labile SOM
fractions (Alvarez and Alvarez, 2000; Conteh et al., 1998; Franzluebbers et al., 2000).
Nutrient analysis of POM provides an indicator of labile nutrient pools. Although few
studies have been published relating total phosphorus in POM to P nutrient cycling,
particulate organic matter phosphorus (POM-P) could be a useful indicator of labile soil
phosphorus (Salas et al., 2003).

In this study, we explored the ability of legumes to enhance soil P cycling and
availability over time, by examining how legumes in three long-term crop rotations
enhanced bioavailability of phosphorus to a subsequent crop. In addition we
characterized different soil phosphorus pools from three points in time within four
treatments of a long-term agricultural trial to look at the dynamics of soil phosphorus
over time.

In the Long Term Ecological Research (LTER) row-crop trial, located at
Michigan State University’s Kellogg Biological Station (KBS) in SW Michigan, USA,
different cropping systems have been maintained with varying legume inputs since 1989.
For this study, we collected soils from a conventionally managed corn-soybean-wheat
rotation, an organic corn-soybean-wheat rotation with two years of winter red clover as a

cover crop, a continuous alfalfa treatment, and a successional system (old farmland

l9

allowed to return to prairie). In addition, soybeans were planted as a bioassay into
microplots within the conventional, organic and alfalfa systems. These treatments were
chosen because they contain different numbers of annual and perennial legumes ranging
from few legumes (conventional) to intermediate legumes (organic) to continuous
legumes (perennial alfalfa). The overall objectives of this study were to I) examine the
bioavailability of phosphorus in the conventional, organic, and alfalfa treatments to a
subsequent soybean crop, and 2) to characterize movement of phosphorus between
different soil pools in conventional, organic, alfalfa and successional treatments of the

LTER.

20

Methods

Site description and agronomy

This study was conducted at the Long Term Ecological Research (LTER)
experiment at the WK. Kellogg Biological Station (KBS) in southwest Michigan, (42°
24' N, 85° 24' W, elevation 288 m). Soils at the LTER site developed on glacial outwash
deposited 12,000 years ago, and are primarily Kalamazoo (fine-loamy, mixed, mesic
Typic Hapludalfs) and Oshtemo (coarse-loamy, mixed, mesic Typic Hapludalfs) series
(Crum and Collins, 1996). Average annual rainfall at KBS is around 890 m y'1 with
about half falling as snow, and the mean annual temperature is 9.7 °C. (2006).

Established in 1989, the LTER is designed to test hypotheses using a randomized
complete block design testing treatments that vary in management intensity. Four
treatments were selected that ranged in legume intensity from annuals to perennials: 1) a
conventionally managed corn-soybean-wheat rotation that receives standard levels of
chemical inputs, 2) a certified organic corn-soybean-wheat rotation, with red clover cover
crop planted following wheat, 3) perennial alfalfa, and 4) an early successional treatment,
where old farmland has been allowed to return to prairie.

Treatments in the LTER are replicated six times with a plot size of 1 ha, but we
chose to use four of the replicates (reps l, 2, 4 and 5) based on the uniformity of soil

characteristics among the replicates (Kravchenko et al., 2006).

21

Soybean bioassay

Soybeans were used as an assay of P bioavailability following the conventional,
organic and alfalfa cropping systems, in expectation that a legume crop could access
sparingly soluble P pools enhanced through historical legume residue inputs. Soybeans
were planted June 19 and 20, 2006 into microplots (10 x 15 m) at the north end of each 1
ha replicate (reps 1, 2, 4 and 5) in the conventional, organic and alfalfa treatments. Each
microplot was divided into three sub-plots (see fig. 1) in order to test the effects of long
term cropping system history, short term recent legume effects and the addition of
inorganic P fertilizer. In subplot A soybeans were planted in both 2005 and 2006. In
subplots B and C, soybeans were planted in 2006 only, but either B or C was randomly
selected to receive 30 kg per hectare phosphorus applied as triple super phosphate (TSP)
fertilizer in June 2005. Alfalfa was killed using glyphosate and incorporated three weeks
prior to planting soybeans.

Precipitation in 2006 was 54.9 cm, distributed as follows over the months of the
growing season; 15.7 cm in May, 4.3 cm in June, 10.7 cm in July, 15.7 cm in Aug, and
8.9cm in September. Total rainfall over the 2006 growing season was within the range of
the 10 year average 50.3 +/- 13 cm (mean +/- std. dev.).

Soybean biomass was harvested September 8, 2006 from three 0.5 m row-lengths
in each sub-plot. Each row-length contained 5-8 plants, and row-lengths were
composited and dried as one sample. Soybean biomass was dried at 60°C for a minimum
of 72 hours, weighed, ground to pass through a 1mm sieve and thoroughly mixed, and
analyzed, using a cold digestion, for total tissue P content by A&L Great Lakes

Laboratories in Fort Wayne, Indiana. A small amount of sample (~0.2 grams) was

22

weighed into a digestion vessel, and the samples were digested in a microwave with
hydrogen peroxide and concentrated HCl. The digests were diluted to 20 mls (1:100

digestion) and then transferred to an ICP for mineral analysis.

23

Fimre 2.1 Description of microplot areas within treatments of the LTER

40m

 

ir

 

 

 

 

 

 

 

 

 

Microplot area within the conventional, organic, alfalfa and successional treatments of
the LTER. Soybeans were grown in subplot A in 2005 and 2006, and were grown in
subplots B and C in 2006 only. In addition, either B or C was randomly selected in
2005 to receive 30 kg/ha triple superphosphate fertilizer.

24

Soil Sampling

Composite soil samples were collected June 5 and 6, 2006 from the microplot areas at
depths of 0-20 cm and 20—50 cm. Samples were air dried and ground to pass through a
2mm sieve. Additional soil samples from 1992 and 2001 were obtained from the LTER
archives. The unground archived samples were collected at a depth of 0-20 cm. The
years 1992 and 2001 were chosen to represent the same phase of the rotation of the

rotation as the 2006 samples, all of which were sampled following corn.

Total and extractable inorganic soil phosphorus

Soil samples from 1992, 2001 and 2006 were measured for total P to allow investigation
of potential net losses between the treatments. Soils were air dried and ground to pass
through a 2mm sieve. Samples were analyzed for total soil phosphorus, using the
Kjeldahl total P method, by the Central Analytical Laboratory at Oregon State University
(Taylor, 2000).

Soil samples were also analyzed for extractable inorganic soil phosphorus as a
measure of plant available phosphorus. Soil samples were ground to pass through a 2mm
sieve and analyzed for Bray-P 1 by A&L Great Lakes laboratories in Fort Wayne, IN

(NCR-13 and North Dakota Agric. Exp. Stn. l, 1980).

25

Phosphorus budget

A phosphorus budget was assembled for the years 2000 to 2005 in the conventional,
organic, alfalfa and successional treatments of the LTER. Phosphorus inputs, in the form
of P fertilizer, were calculated based on fertilizer application records in the LTER field
log (2007). Exports from the system included phosphorus removed in harvested plant
biomass. Calculations for the P removed from crops in the conventional and organic
systems were based on grain yield data for the years 2000 to 2005, available on the LTER
data archives (LTER data catalog, 2007). The amount of phosphorus removed in alfalfa
cuttings was calculated based on biomass data available in the LTER archived data
catalogs. Phosphorus tissue and grain concentrations for alfalfa and soybean were based
on analysis of total tissue P in eight composite samples from the LTER in 2005 and 2006,
respectively. Grain phosphorus concentrations for corn and wheat were based on average

literature values (USDA, SR19).

Particulate organic matter

Particulate organic matter (POM) was analyzed from un-ground soil samples (0-
20 cm) fiom 1992, 2001 and 2006. POM was collected in two steps; first, sand and
organic matter were separated from the soil based on a size separation, and then a density

separation was used to isolate the particulate organic matter.

Size separation: Twenty five (25) grams of soil were shaken with 30 mL of 0.05 M NaCl

in 50 mL centrifuge tubes for 2 hours. Samples were passed through a 53pm sieve and

26

rinsed with distilled water until a clear solution was obtained. Materials retained on the
sieve were a mixture of sand and POM. This mixture was washed, using distilled water,

into a 500 mL glass canning jar and placed in an oven to dry overnight at 60°C.

De_nsitv separation:
Dried sand and POM_ mixtures were carefully scraped from the glass jars into 50-mL
conical centrifuge tubes containing 35 mL of sodium polytungstate of density 1.85
Mg/m3. Suspended material was mixed by slow reciprocal shaking by hand (10 strokes),
and the tube was placed in a vertical position and centrifuged at 2500 x g for 30 minutes.
POM, floating at the top of the tube, was carefully poured onto a 20 um mesh nylon filter
using a vacuum filtration system, and rinsed thoroughly with water to remove excess
sodium polytungstate. Materials retained on the 20 um filter were washed, using distilled
water, into a pre-weighed aluminum weighing pan and dried for 24h at 60°C. Once dry,
pans were weighed and then POM was ground by hand using a porcelain mortar and
pestle. Sodium polytungstate was recycled using the methods described by (Six et al.,
1999)

Ground POM was analyzed for total tissue P content by A&L Great Lakes
laboratory in Fort Wayne, IN using the method described above for soybean tissue

samples.

27

Results

Total and extractable inorganic soil phosphorus

Total soil phosphorus ranged from 210 to 564 ppm P, with an average value of
348 ppm across treatments. There were no significant differences in the total P content of
soil by cropping system (p-value = 0.684), nor did the application of P fertilizer in 2005
impact total soil P concentrations in 2006 (p-value = 0.989). In addition, the total amount
of P in soil stayed relatively stable over time (Tables 2.1 and 2.2).

Significant changes in extractable inorganic phosphorus (Bray-P) were seen over
time. The high levels of inorganic P measured in the 1992 soil samples reflect the fact
that the LTER experiment began on soils that had anthropogenically elevated levels of
soil P due to extensive application of P fertilizer when the farmland was in a com-
soybean rotation (Daroub et al., 2001). Extractable inorganic phosphorus changed
significantly over time in soils sampled at 0-20 cm (Figure 2.2), with decreases in plant
available phosphorus occurring in all treatments between 1992 and 2001 (p-value =
0.0309) (Tables 2.1 and 2.3). If the year 2006 is considered alone, significant differences
can be seen in the amount of plant available phosphorus between treatments, at depths of
0-20 cm and 20-500m (p-value = 0.006), with more plant available phosphorus in the
organic and successional treatments than in the conventional system (Figure 2.3). The
application of phosphorus fertilizer to subplots in 2005 had no significant impact on plant

available phosphorus in 2006.

28

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experiment at KBS, Hickory Comers, MI. The four systems include two management
systems (conventional and organic) for three year sequences of corn-soybean-wheat, a
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to prairie.

30

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Average annual P budgets were negative for each single rotation period in the
conventional, organic and alfalfa cropping systems, as well as over a nine year period
from 1997 to 2005. Over the nine year period, the net deficits were 12.0, 64.2, and 28.0
kg/ha P for the conventional, organic and alfalfa cropping systems, respectively (Table
2.4), which is a net negative per year of 1.3, 7.1, and 3.1 kg/ha P for the conventional,
organic, and alfalfa systems. This indicates that in all three systems, P removal in
harvested biomass exceeded the amount of P added in the form of inorganic fertilizer.
This net negative balance was not reflected in the measured extractable inorganic soil P,

or in the phosphorus uptake of crops from these systems.

33

 

 

 

 

 

 

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34

Particulate organic matter

The amount of particulate organic matter in soil, a measure of the active organic
matter pool, increased significantly over time in the organic, alfalfa and successional
systems of the LTER, but remained relatively constant in the conventional treatment.
Treatment differences in the percentage of POM in soil were noticeable as early as 1992,
just three years after the start of the LTER experiment and these trends continue through
2006 (Figure 2.4). In addition to long-term cropping system effects, recent crop history
also significantly influenced the amount of POM present in soil (p-value= 0.01).

Phosphorus concentrations of POM (POM-P) ranged from 0.039 to 0.088 percent,
with a mean concentration of 0.053 across all treatments. Treatment differences in the
phosphorus concentration of POM were significant (p-value = 0.0024) as were changes in
the P concentration of POM over time (p-value = 0.0036). The P concentration of POM
decreased in the conventional, alfalfa and successional treatments from 1992 to 2006,
while it remained fairly constant in the organic treatment (Fig. 2.5). The application of
fertilizer in 2005 did not significantly impact the phosphorus concentration of POM in
2006 (p-value = 0.1403). There was an unclear correlation between treatment and POM-
P concentration (Figure 2.6). Because there has been a significant increase in the
quantity of POM over time, more soil phosphorus is being stored in the POM pool for the

organic and alfalfa cropping systems.

35

Figure 2.4 Particulate organic matter (as a percentage by weight of soil)

'3 , _-
.2 11992
3 l-zoo1
0‘' l
0206
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O
n.

 

Conventional Organic Alfalfa Successional

Treatment

 

 

 

Figure 2.4 Averages plus standard errors of particulate organic matter, as a
percentage of soil by weight, in four cropping systems fiom 1992 to 2006.

36

Figure 2.5 Phosphorus concentration of Particulate Organic Matter

 

0.08

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POM Phosphorus (% P)

P
o
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Conventional Organic Alfalfa Successional

Treatment

 

 

Figure 2.5 Averages plus standard errors of phosphorus content as a
percentage of particulate organic matter (POM) in soil in four cropping
systems of the LTER from 1992 to 2006.

37

 

Figure 2.6 Total P in the POM pool

 

 

%

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Conventional Organic Alfalfa Successional
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Figure 2.6 Averages plus standard errors of the total about of P found in the
particulate organic matter (POM) pool in soils from the conventional, organic, alfalfa
and successional cropping systems at the LTER from 1992 to 2006.

38

 

 

 

 

 

 

 

 

 

 

 

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41

Soybean bioassay

Average biomass of soybeans harvested from microplots ranged from 4.0-4.5
Mg/ha, but was not statistically different by cropping system (p-value = 0.31) or due to
the application of phosphorus fertilizer (p-value= 0.3224) (Figure 2.7). The long term
cropping system history did, however, influence tissue phosphorus concentrations of
soybeans. Tissue phosphorus concentrations of the soybean pods ranged from 0.41 to
0.48 percent P, and the P concentration of the stems and leaves ranged from 0.12 to 0.2
percent P. This range is consistent with literature values (USDA, SR19). There were
significant differences in the tissue phosphorus concentrations of the soybeans between
the conventional, organic, and alfalfa treatments (p-value= <0.001), with the
concentration of P following alfalfa being the highest (see Figure 2.8).

We also examined the different soybean plant tissue types to look at allocation of
P within the plant. As expected, phosphorus concentrations of the soybean pods were
much higher than in the stems and leaves (p-value = <0.001), but the P concentrations of
both tissue types followed the same overall trend in terms of treatment differences.

Short term history did not influence soybean biomass or P tissue concentrations.
This is reflected in soybeans planted microplots where soybeans were grown for 2 years,

vs. 1 year following the general crop rotation of the LTER (p-value= 0.535) (Fig. 1).

42

Figure 2.7 Biomass of soybeans collected from microplot areas

 

Biomass (Mg/ha)

 

 

 

 

 

Time Type

 

stems

{la dén'éhuonéfi
l organic 1
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Figure 2.7 Averages plus standard errors of biomass of soybean pods and stems and
leaves collected from soybean microplots in three cropping systems of the LTER in

2006.

43

 

Figure 2.8 Tissue P concentration of soybeans

 

0.6

0.5

0.4

0.3

Tissue P cone. (“/o P)

0.2

0.1

Pods

Tissue Type

 

Stems and leaves

l
; I Organic

[ u Alfalfa

imaging;

 

Figure 2.8 Averages plus standard errors of tissue phosphorus concentrations of

soybeans grown as bioassays of phosphorus availability in conventional, organic, and

alfalfa cropping systems of the LTER in 2006.

44

 

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5
4

Discussion

Total and extractable inorganic soil P

Total soil phosphorus was measured to investigate differences among treatments
and over time, as this is important to understand when comparing movement of
phosphorus between soil pools. Unlike other studies of long-term cropping systems,
(Oehl et al., 2002; Richards, 1995) which showed declines in Pt over time, the total
amount of phosphorus remained relatively constant between all the treatments from 1992
to 2006 (Table 2.1), despite modest additions of phosphorus fertilizer in 2002 in the
conventional treatment, and 2003 and 2005 in the conventional and alfalfa treatments
(Fi g. 2.8). Short term management, growing soybean for one versus two years, or adding
P fertilizer in 2005, did not alter the total soil P in 2001 and 2006 (Table 2.1).

Extractable inorganic phosphorus (Bray P) declined in all treatments from the
1992 levels (Figure 2.2), which were anthropogenically high due to the large amounts of
fertilizer applied to farmland prior to the start of the LTER experiment in 1989. Over
time, some of this phosphorus appears to have moved into other soil pools, whereas no
detectable changes were found in the total amount of soil phosphorus from 2001 to 2006.
Differences in allocation of P to different soil pools were noted as early as 1995, in an
earlier study of this experiment, with a greater magnitude of soil P in the organic fraction
of the alfalfa and successional treatments than in the conventional and organic (Daroub et
aL,2001)

Extractable inorganic P from 2006 soil samples did not reflect the same pattern

observed with the bioavailability of phosphorus, as measured through the soybean

46

bioassay (Figures 2.3 and 2.8). Discrepancies between extractable Pi and measured
bioavailability have been found in other studies as well (Cavigelli and Thien, 2003;
Nuruzzaman et al., 2005). Bray P levels in 2006 were more available in the organic com
rotation than the alfalfa system, while based on the bioassay, soybean tissue P
concentrations were highest following the alfalfa system. This indicates the value of
multiple measures of plant available pools as well as soil extractions represent plant-
nutrient availability to variable degrees.

Phosphorus inputs to the conventional, organic and alfalfa treatments have varied
over the years, with only the conventional and alfalfa treatments receiving inorganic P
fertilizer. Despite this, extractable inorganic phosphorus was higher in the organic
treatment than in the conventional or alfalfa treatments (Figure 2.3). This is consistent
with the idea that long term management is an important determinant of nutrient
availability. In cases such as this site, with coarse soil and moderately high soil P, P

fertilizer inputs may have limited effects on P availability.

Phosphorus budget

Based on a nine year phosphorus budget for the LTER (Table 2.4), there was a net
deficit for phosphorus in the conventional, organic and alfalfa cropping systems. This
deficit was reflected in the extractable inorganic phosphorus (Bray P) tests, which
declined over time. Decisions about P fertilizer applications in the conventional and
alfalfa systems are based on P soil test levels, and as the soils started out at an

anthropogenically high level, fertilizer applications to these systems have been small,

47

resulting in a negative net balance. However, it is important to note that plant available
P was highest in the organic system in 2006, which showed the greatest net loss of P over
the nine year period. This illustrates some of the problems inherent in creating nutrient
budgets, which often do not accurately reflect P transfers into organic P or sparingly
soluble P pools. Watson et al. (2002) argue that studying trends in soil organic matter
and nutrient analyses, particularly in organic systems, is far more useful that using
chemical analyses that provide snapshots in time, because of the long-term approaches to
nutrient management in most organic systems. Results from our study demonstrate
changes in soil organic matter pools and Pi over time in the alfalfa and organic systems,

which are not adequately explained by a simple nutrient budget.

Particulate organic matter

In agreement with previous authors (Alvarez and Alvarez, 2000; Carter, 2000),
we found that particulate organic matter (POM) in our study was highly responsive to
changes in management practices. In 1992, just three years after the start of the LTER
experiment, the organic corn-soybean-wheat, alfalfa and successional treatments had
elevated quantities of POM, as a percentage of soil by weight as compared to the
conventional com-soybean—wheat rotation. This trend grew more pronounced over time
(Figure 2.4). In addition, POM samples collected from the subplots with either one or
two years of soybean grown in the microplot experiment in 2006 demonstrated that short
term management strategies impact POM. Quantities of POM differed significantly
between microplots with soybeans grown just one year following the main rotation vs.

those with two years of soybean (Table 2.5).

48

In the organic treatment, red clover is established as a winter cover crop in the
wheat, once in three years. The years chosen for analysis of POM followed the corn
phase of the rotation, and thus came a full year after red clover was incorporated. Any
residual red clover biomass was likely small in comparison to corn stalk biomass entering
the conventional and organic rotations. Smaller quantities of POM in the conventional
treatment may be due to increased mineralization with the addition of inorganic nitrogen
fertilizers. Differences between these two treatments also appear to illustrate that both
quantity and quality of the residue is important when considering ways to build soil
organic matter. Consistent with Drinkwater et al. (1998), leguminous cover crops appear
to be an important addition to a cropping system in terms of building soil organic matter.
The greater quantity of POM in the alfalfa and successional treatments is likely due to the
diversity and quality of residue inputs in these systems, as well as the presence of
continuous living plant cover over the winter compared to the conventional system (one
in three years of winter cover) and organic (two in three years of winter cover).

Particulate organic matter phosphorus (POM-P) was not a useful predictor of the
bioavailability of phosphorus, as there was little correlation with the soybean tissue P
concentrations (Figures 2.5 and 2.8). The phosphorus concentration of POM was much
higher in 1992 than in 2001 and 2006 for the conventional, alfalfa and successional
treatments. This correlates well with the elevated quantity of extractable inorganic
phosphorus in these years, which dropped significantly in subsequent years (Figure 2.2).
Most likely this is because POM is highly influenced by recent vegetation, and thus
plants which take up more available soil P will contribute to P-enriched POM. The

concentration of P in POM in the organic treatment remained relatively constant over

49

time, despite a significant decline in the amount of plant available phosphorus from 1992
to 2001. This is not readily explainable.

Although in general the concentration of P in POM declined over time, the total
amount of P stored in the POM pool increased over time, most notably in the organic and
alfalfa treatments, as a result of increases in the total quantity of POM in soil. Thus
more phosphorus is stored in an organic pool with rapid tin-hover, which should impact
plant availability of P. These results agree with the bioavailability results from the
soybean bioassay as well as the data on plant available phosphorus (Bray P) (Figure 2.6).
Thus it appears that it is possible to enrich the P content of POM with the application of P
fertilizer, but in terms of moving more soil P into the POM pool it is necessary to
consider longer-term management and rotation practices that enhance the quantity of

POM in soil.

Soybean Bioassay

The ability of legumes to enhance bioavailability of phosphorus to a subsequent
crop has been demonstrated in previous greenhouse studies (Cavigelli and Thien, 2003;
Kamh et al., 2002; Nuruzzaman et al., 2005), but little research has been performed
relating this back to field studies. In their paper, Cavigelli and Thein (2003) argue in
favor of long-term field studies that examine the effects of green manures on
bioavailability of phosphorus, since the capacity of native and agricultural systems to
supply P to plants from organic P mineralization develops over time and may not be

detected in shorter field studies.

50

In our experiment, P was more bioavailable to soybeans following perennial
alfalfa than following the conventional or organic com-soybean-wheat rotations. There
was no significant difference in the amount of soybean biomass collected from the
conventional, organic and alfalfa treatments, but the tissue concentrations of P were much
higher in soybeans from the alfalfa plot. This suggests that the presence of a perennial
legume, as well as long term management history positively impacted the availability of
phosphorus. Notably, the management and rotational history appears to have been more
important than application of phosphorus fertilizer in determining the availability of
phosphorus for plant uptake. Both conventional and alfalfa treatments had phosphorus
fertilizer applications in the last five years, but only in the alfalfa systems was P highly
available, despite consistent P removal in alfalfa hay biomass. There were no significant
differences in P concentration between soybean subplots that did or did not receive P
fertilizer. Short term management history (one vs. two years of soybeans) also did not
affect the bioavailability of P.

Our findings are consistent with previous research by Cavigelli and Thien (2003)
who demonstrated phosphorus was more bioavailable to sorghum following
incorporation of legume residues such as alfalfa and red clover. In their study, alfalfa had
the highest P concentration prior to incorporation, and yields of sorghum were highest
following alfalfa suggesting that it was best able to facilitate movement of P into the soil
organic pool. Previous research conducted in the alfalfa treatment of the LTER (Daroub
et al., 2001) documented an increase in the organic phosphorus fraction of the soil over
time, which they attributed to the turnover of the extensive alfalfa root system (Daroub et

al., 2001).

51

Conclusion

Two lines of evidence are consistent with a legume role in improving P
bioavailability to a subsequent crop in the alfalfa and organic cropping systems.
Phosphorus was most bioavailable to soybeans following alfalfa, and equally bioavailable
to soybeans following the conventional and organic rotations. Unlike the conventional
system, the organic rotation never received inorganic phosphorus inputs and thus it is
plausible that increased P availability was aided by the red clover cover crop in the
rotation. Extractable inorganic P (Bray P) was most related to long-term crop rotation,
while in contrast, the phosphorus concentration of POM seemed to be related to fertilizer
applications. However, the total amount of P located in the POM soil pool was closely
related to the long-term management of the cropping system, with increasing quantities
of P moving into the POM pool over time in the organic and alfalfa treatments. This is
due to the increase in the total quantity of POM in the organic and successional
treatments. Apparently, the quality not quantity of residue input was an important factor
in increasing the percentage of soil POM.

Overall, these findings argue for an integrated approach to phosphorus nutrient
management in low-input or organic agricultural systems, which uses a variety of
legumes to improve the bioavailability of P and build up P in soil pools with rapid
turnover, such as POM-P. Our results also demonstrate that organic systems which

integrate legumes can be sustainable over a long time period with regard to soil P.

52

BIBLIOGRAPHY

2006. LTER site description [Online] http://www.lter.kbs.msu.edu/siteDescription.htm
(verified 4-23-07).

2007. LTER Field Log [Online]
http://www.lter.kbs.msu.edu/Data/table.isp?Product=KBSOO4-001 (verified 4-8-
07).

 

NCR-13 and North Dakota Agric. Exp. Stn. l, 1980. Recommended chemical soil test
procedures for the North Central region, Vol. No. 221 (Rev.). North Central
Regional Publication.

Ae, N., J. Arihara, K. Okada, T. Yoshihara, and C. J ohansen. 1990. Phosphorus uptake by
pigeon pea and its role in cropping systems of the Indian subcontinent. Science
248:477-480.

Alvarez, R., and CR. Alvarez. 2000. Soil organic matter pools and their associations with
carbon mineralization kinetics. Soil Science Society of America Journal 64: 184-
189.

Aref, S., and M.M. Wander. 1997. Long-tenrr trends of corn yield and soil organic matter
in different crop sequences and soil fertility treatments, p. 153-197 Advances in
Agronomy, Vol. 62. Academic PRess, San Diego.

Berry, P.M., E.A. Stockdale, A. Sylvester-Bradley, L. Phillips, K.A. Smith, E.I. Lord,
C.A. Watson, and S. Fortune. 2003. N, P, and K budgets for crop rotations on
nine organic farms in the UK. Soil Use and Management 19:112-118.

Bielesky, R.L., and I. Ferguson. 1983. Phosphate pools, phosphate transport, and
phosphate availability. Annual Review of Plant Physiology 24:225-252.

Bolan, NS. 1991. A Critical-Review on the Role of Mycorrhizal Fungi in the Uptake of
Phosphorus by Plants. Plant and Soil 134:189-207.

Brady, NC, and RR. Wei]. 2000. Elements of the Nature and Property of Soils.
abridged 12th ed.

Braum, S.M., and RA. Helmke. 1995. White Lupin Utilizes Soil-Phosphorus That Is
Unavailable to Soybean. Plant and Soil 176195-100.

Bray, R.H., and LT. Kurtz. 1945. Determination of Total, Organic, and Available Forms
of Phosphorus in Soils. Soil Science 59:39-45.

Bundy, LG, and SJ. Sturgul. 2001. A phosphorus budget for Wisconsin cropland.
Journal of Soil and Water Conservation 56:243-249.

53

Carpenter, S.R., N.F. Caraco, D.L. Correll, R.W. Howarth, A.N. Sharpley, and V.H.
Smith. 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen.
Ecological Applications 8:559-568.

Carter. 2000.

Cavigelli, M.A., and SJ. Thien. 2003. Phosphorus bioavailability following incorporation
of green manure crops. Soil Science Society of America Journal 67:1186-1194.

Conteh, A., G.J. Blair, and U. Rochester. 1998. Soil organic carbon fractions in a
Vertisol under irrigated cotton production as affected by burning and
incorporating cotton stubble. Australian Journal of Soil Research 36:655-667.

Crum, J .R., and HP. Collins. 1996. KBS Soils [Online]
http://www.lterkbs.msu.edu/Soil/Qaracterization/ (verified 3-12-07).

Daroub, S., B.G. Ellis, and GP. Robertson. 2001. Effect of cropping and low-chemical
input systems on soil phosphorus fractions. Soil Science 166:281-291.

Dinkelaker, B., V. Romheld, and H. Marschner. 1989. Citric-Acid Excretion and
Precipitation of Calcium Citrate in the Rhizosphere of White Lupin (Lupinus-
Albus L). Plant Cell and Environment 12:285-292.

Drinkwater, LE, P. Wagoner, and M. Sarrantonio. 1998. Legume-based cropping
systemshave reduced carbon and nitrogen losses. Nature VOL 396

El Dessougi, H., A.Z. Dreele, and N. Claassen. 2003. Growth and phosphorus uptake of
maize cultivated alone, in mixed culture with other crops or after incorporation of
their residues. Journal of Plant Nutrition and Soil Science-Zeitschrift Fur
Pflanzenemahrung Und Bodenkunde 166:254-261.

Elrashidi, M.A. 2005. Testing methods for phosphorus and organic matter. NRCS
bulletin, Technical Reference. Soil Survey Laboratory, Lincoln, NE.

Franzluebbers, A.J., J .A. Stuedemann, H.H. Schomberg, and SR. Wilkinson. 2000. Soil
organic C and N pools under long-term pasture management in the Southern
Piedmont USA. Soil Biology & Biochemistry 32:469-478.

Frey, S.D., E.T. Elliott, and K. Paustian. 1999. Bacterial and fungal abundance and
biomass in conventional and no-tillage agroecosystems along two climatic
gradients. Soil Biology & Biochemistry 31 :573-585.

Gahoonia, TS, and NE. Nielsen. 1992. The Effects of Root-Induced Ph Changes on the
Depletion of Inorganic and Organic Phosphorus in the Rhizosphere. Plant and
Soil 143:185-191.

54

Gardner, W.K., D.A. Barber, and D.G. Parbery. 1983. The Acquisition of Phosphorus by
Lupinus-Albus L .3. the Probable Mechanism by Which Phosphorus Movement in
the Soil Root Interface Is Enhanced. Plant and Soil 70:107-124.

Haven, P.H., R.F. Evert, and SE. Eichhom. 1999. Biology of Plants. 6th ed. W.H.
Freeman and Company Worth Publishers.

Horst, W.J., M. Kamh, J .M. J ibrin, and V0. Chude. 2001. Agronomic measures for
increasing P availability to crops. Plant and Soil 237:211-223.

Johnston, A.E., (ed.) 1991. Fertility and soil organic matter, pp. 1-314. The Royal Society
of Chemistry, Wlksham, Wiltshire.

Kafflra, S., and H.H. Koepf. 1989. A case study in the nutrient regime in sustainable
farming. Biological Agriculture and Horticulture 6:89-106.

Kamh, M., W.J. Horst, F. Amer, H. Mostafa, and P. Maier. 1999. Mobilization of soil and
fertilizer phosphate by cover crops. Plant and Soil 211:19-27.

Kamh, M., M. Abdou, V. Chude, F. Wiesler, and W.J. Horst. 2002. Mobilization of
phosphorus contributes to positive rotational effects of leguminous cover crops on

maize grown on soils from northern Nigeria. Journal of Plant Nutrition and Soil
Science-Zeitschrift Fur Pflanzenemahrung Und Bodenkunde 165:566-572.

Kravchenko, A.N., G.P. Robertson, X. Hao, and D.G. Bullock. 2006. Management
Practice Effects on Surface Total Carbon: Differences in Spatial Variability
Patterns. Agron J 98:1559-1568.

Ladd, J .N., and M. Amato. 1980. Studies of Nitrogen Immobilization and Mineralization
in Calcareous Soils .4. Changes in the Organic Nitrogen of Light and Heavy
Subfractions of Silt-Size and Fine Clay-Size Particles During Nitrogen Turnover.
Soil Biology & Biochemistry 12:185-189.

Liption, D.S., R.W. Blanchar, and D.G. Blevins. 1987. Citrate, malate, and succinate
concentrations in exudates from P-sufficient and P-stressed Medicago sativa L.
seedlings. Plant Physiology 85.

Maroko, J .3., RI. Buresh, and RC. Smithson. 1999. Soil phosphorus fractions in
unfertilized fallow-maize systems on two tropical soils. Soil Science Society of
_ America Journal 63:320-326.

Masaoka, Y., M. Kojima, S. Sugihara, T. Yoshihara, M. Koshino, and A. Ichihara. 1993.
Dissolution of Ferric Phosphate by Alfalfa (Medicago-Sativa L) Root Exudates.
Plant and Soil 156:75-78.

Masaoka, Y., M. Kojima, S. Sugihara, T. Yoshihara, M. Koshino, and A. Ichihara. 2004.
Dissolution of Ferric Phosphate by Alfalfa (Medicago-Sativa L) Root Exudates.
Plant and Soil 156:75-78.

55

Mehlich, A. 1953. Determination of P, Ca, Mg, K, Na, and NH4. . North Carolina Soil
Test Division, North Carolina Department of Agriculture, Raleigh, NC.

Mehlich, A. 1984. Mehlich-3 Soil Test Extractant - a Modification of Mehlich-2
Extractant. Communications in Soil Science and Plant Analysis 15: 1409-1416.

Miller, A.J., E.A.G. Schuur, and O.A. Chadwick. 2001. Redox control of phosphorus
pools in Hawaiian montane forest soils. Geoderma 102:219—237.

Newman, El. 1995. Phosphorus Inputs to Terrestrial Ecosytems. J oumal of Ecology
83:713-726.

Newman, El. 1997. Phosphorus Balance of Contrasting F arming Systems, Past and
Present. Can Food Production be Sustainable? The Journal of Applied Ecology
34: 1334-1347.

Nuruzzaman, M., H. Lambers, M.D.A. Bolland, and E.J. Veneklaas. 2005. Phosphorus
uptake by grain legumes and subsequently grown wheat at different levels of
residual phosphorus fertiliser. Australian Journal of Agricultural Research
56:1041-1047.

Oehl, F ., A. Oberson, H.U. Tagrnann, J .M. Besson, D. Dubois, P. Mader, H.R. Roth, and
E. Frossard. 2002. Phosphorus budget and phosphorus availability in soils under
organic and conventional farming. Nutrient Cycling in Agroecosystems 62:25-35.

Ohwaki, Y., and H. Hirata. 1992. Differences in Carboxylic-Acid Exudation among P-
Starved Leguminous Crops in Relation to Carboxylic-Acid Contents in Plant-
Tissues and Phospholipid Level in Roots. Soil Science and Plant Nutrition
38:235-243.

Olsen, SR, and LE. Sommers. 1982. Phosphorus, p. 403-448, In A. L. Page, ed.
Methods of Soil Analysis Part II: Chemical and Microbiological Properties.
American Society of Agronomy, Madison, WI.

Olsen, S.R., L.E. Sommers, F.S. Wantabe, and LA. Dean. 1954. Estimation of available
phosphorus in soils by extraction with sodium bicarbonate, Circulation no. 939,
USDA, Washington, DC.

Otani, T., N. Ae, and H. Tanaka. 1996. Phosphorus (P) uptake mechanisms of crops
grown in soils with low P status .2. Significance of organic acids in root exudates
of pigeonpea. Soil Science and Plant Nutrition 42:553-560.

Richards. 1995.

Riley, D., and SA. Barber. 1971. Effect of Ammonium and Nitrate Fertilization on
Phosphorus Uptake as Related to Root-Induced Ph Changes at Root-Soil
Interface. Soil Science Society of America Proceedings 35:301-&.

56

Salas, A.M., E.T. Elliott, D.G. Westfall, C.V. Cole, and J. Six. 2003. The Role of
Particulate Organic Matter in Phosphorus Cycling. Soil Science Society of
America Journal 67: 1 81-189.

Sanyl, SK, and SK. De Datta. 1991. Biogeochemistry: An Analysis of Global Change
Academic Press, San Diego.

Sbih, M., A. Ndayegamie, and A. Karam. 2003. Evaluation of carbon and nitrogen
mineralization rates in meadow soils from dairy farms under transit to biological
cropping systems. Canadian Journal of Soil Science 83:25-33.

Schachtrnan, D.P., R.J. Reid, and SM. Ayling. 1998. Phosphorus uptake by plants: From
soil to cell. Plant Physiology 116:447-453.

Six, J ., P.A. Schultz, J .D. Jastrow, and R. Merckx. 1999. Recycling of sodium
polytungstate used in soil organic matter studies. Soil Biology & Biochemistry
31:1193-1196.

Smith, SE, and DJ. Read. 1997. Mycorrhizal Symbiosis Academic Press, San Diego,
Ca.

Snapp, 83., PL. Mafongoya, and S. Waddington. 1998. Organic matter technologies for
integrated nutrient management in smallholder cropping systems of southern
Africa. Agriculture, Ecosystems and the Environment 71 :185-200.

Stevenson, F.C., F.L. Walley, and C. van Kessel. 1998. Direct vs. indirect nitrogen-15
approaches to estimate nitrogen contributions from cr0p residues. Soil Science
Society of America Journal 62:1327-1334.

Stevenson, F.J. 1994. Humus chemistry, genesis, composition, and reactions. 2nd ed.
John Wiley & Sons, New York.

Stockdale, E.A., N.H. Lampkin, M. Hovi, R. Keatinge, E.K.M. Lennartsson, D.W.
Macdonald, S. Padel, F.H. Tattersall, M.S. Wolfe, and CA. Watson. 2001.
Agronomic and environmental implications of organic farming systems, p. 261-
327 Advances in Agronomy, Vol 70, Vol. 70.

Stone, A.G., S.J. Traina, and H.A.J. Hoitink. 2001. Particulate organic matter
composition and Pythium damping-off of cucumber. Soil Science Society of
America Journal 65:761-770.

Taylor, MD. 2000. Determination of total phosphorus in soil using simple Kjeldahl
digestion. Communications in Soil Science and Plant Analysis 31:2665-2670.

Tiessen, H., J .W.B. Stewart, and A. Oberson. 1994. Innovative soil phosphorus
availability indices: Assessing organic phosphorus, In J. L. Havlin and J. S.
Jacobson, eds. Soil testing: Prospects for improving nutrient recommendations.
SSSA Special Publication No. 40, SSSA and ASA, Madison, WI.

57

SR19. USDA Food Search for Windows, version 1.0, SR19.

Walker, T.W., and J .K. Syers. 1976. Fate of Phosphorus During Pedogenesis. Geoderma
1521-19.

Wander, M.M. 2004. Soil organic matter fractions and their relevance to soil function, p.
67-102, In F. Magdoff and R. R. Weil, eds. Advances in Agroecology. CRC.

Wander, M.M., and SJ. Traina. 1996. Organic matter fractions from organically and
conventionally managed soils .2. Characterization of composition. Soil Science
Society of America Journal 60: 1087-1094.

Wander, M.M., M.G. Bidart, and S. Aref. 1998. Tillage impacts on depth distribution of
total and particulate organic matter in three Illinois soils. Soil Science Society of
America Journal 62:1704-1711.

Watson, C.A., D. Atkinson, P. Gosling, L.R. Jackson, and F.W. Ryans. 2002. Managing
soil fertility in organic farming systems. Soil Use and Management 18:239-247.

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