Effects of the Quality of Organic Soil Amendments on the Soil Community and on Nitrogen Mineralization in an Agroecosystem in the Georgia Piedmont

Project Overview

Project Type: Graduate Student
Funds awarded in 2005: $8,576.00
Projected End Date: 12/31/2007
Grant Recipient: University of Georgia
Region: Southern
State: Georgia
Graduate Student:
Major Professor:
Carl Jordan
University of Georgia

Annual Reports


  • Agronomic: corn
  • Vegetables: greens (leafy)


  • Crop Production: nutrient cycling, organic fertilizers
  • Natural Resources/Environment: biodiversity
  • Soil Management: green manures, nutrient mineralization


    We studied how the chemical quality of surface-applied organic amendments and the soil foodweb interact to determine nitrogen mineralization. Quality had substantial effects on the microbial communities in soil. By affecting microbial populations quality also influenced the consumer populations. Modeling suggested that: communities generated by different quality amendments have differential abilities for mineralization; communities may be better at mineralizing substrates similar to those that generated them; community structure is less important in determining mineralization for intermediate quality substrates. In soils whose faunal community has been impoverished mineralization is less responsive to quality than in soils with a more complex community.


    The majority of agricultural soils in the Southern Piedmont region of the U.S. are degraded (Lal and Livari, 2004). Intense weathering combined with historic and contemporary agriculture further depleted soils that were naturally low in soil organic matter and bases. Farmers in the Southern Piedmont operate under these conditions, which inherently demand high fertilizer inputs. This demand is customarily met with the addition of synthetic fertilizers and, less commonly, the use of organic amendments imported from beyond the farm. Attaining more ecologically and economically sustainable food production systems demands strategies to build soil organic matter and nutrient availability for plants. Regular application of organic amendments to soil constitutes a central component of such strategies. Soil organic amendments such as green manures and crop residues can have an important role not only in building soil organic matter and in conservation of soil and water, but also in supplying nutrients—nitrogen in particular—to subsequent crops in rotations and to simultaneous crops (Gliessman, 1998).

    Nitrogen can be the most limiting factor in low-input and organic systems (Clark et al., 1999). Whenever the use of inorganic nitrogen is replaced with organic materials, the availability of N in soil depends largely on the biological processes of nutrient release, which makes understanding these processes increasingly important. Two important factors governing the rate of nutrient release are: (1) the biochemical quality of organic matter inputs, and (2) the functioning of the community of soil organisms, including microorganisms, protozoa, nematodes and arthropods. Furthermore, organic inputs quality and soil communities can interact in soil to determine the rates of release of nitrogen into the soil. The details of this interaction are for the most part ignored. The purpose of this project was to combine experimental and modeling approaches to study how these two factors interact to determine N release into the soil and to inform practical recommendations for enhancing nitrogen availability in soil.

    The role of organic inputs quality on nitrogen mineralization:
    Release of nitrogen occurs as a sub-process of the decomposition of organic materials. Biochemical quality determines the susceptibility of a substrate to attack by decomposers. Indices of litter quality such as initial N concentration, C:N ratio, Lignin:N and (Lignin + polyphenol ):N ratios are good predictors of nutrient release rates under varied environmental circumstances. In agroecosystems, the selection of organic amendments and the timing of applications determine the quality and release pattern of the decomposition substrate.

    The role of the soil community:
    In agricultural systems, soil biota play important roles in regulating organic matter decomposition and nutrient release (Andren et al., 1990). Hendrix et al. (1990) stressed the concept of soil biota as fundamental components of sustainable agroecosystems, particularly as regulators of nutrient release. The decomposition of organic matter is a biological process in which microorganisms (bacteria and fungi) play the most direct role. However, the complete soil community (microbes and fauna) and the direct and indirect interactions between them are involved in the process (Coleman et al., 1994). Members of the soil fauna include protozoa, nematodes, micro and macroarthropods. Interactions among soil community members regulate the availability of the nutrient supply that allows for aboveground growth (Wardle, 2002). Indeed, direct trophic interactions are responsible for a great fraction of nutrient release. For example, de Ruiter et al. (1994) estimated that protozoa feeding on bacteria were responsible for up to 95% of the total N released in two arable farming systems.

    Effect of quality on soil communities:
    Agricultural practices such as application of organic amendments can affect population size and dynamics of organisms in soil communities (e.g. Bulluck III, 2002, van Vliet et al., 2000). Furthermore, the quality of added organic amendments can differentially affect soil populations structure (Robinson et al., 1994; Yang et al. 2003) and function (Bending et al., 2002). Most studies looking at the effect of the quality of organic amendments on communities have focused on the communities inhabiting the layer of added organic material (e.g. Bjornlund and Christensen, 2005; Wardle et al., 2006) or on soil communities after substrate incorporation (e.g. Bending et al., 2002; Salamanca et al., 2006) but few have looked at the effects of unincorporated substrates. However, in natural or managed systems in which the amendment does not get incorporated into the mineral soil (e.g. no-till systems), it is crucial to distinguish between the communities living in the amendment layer and the mineral soil communities as (a) they utilize different organic matter pools as energy and nutrient resources and thus can play different roles in driving processes and (b) the populations that compose them can establish interactions that may influence carbon and nutrient cycling and long term organic matter accumulation (Fontaine et al., 2004). How the mineral soil communities respond to the chemical quality of organic inputs can have implications on the dynamics of nutrients and indigenous soil organic matter (Waldrop and Firestone, 2004). Understanding the short term effects of amendment quality on soil detrital communities is of special importance in integrated agricultural systems where organic soil amendments such as green manure and crop residues are seasonally applied as a strategy to enhance soil fertility. The first objective of this study was to investigate the short term effect that the chemical composition of one-time surface-applied amendments has on the soil microbial and micro and mesofauna community in the mineral soil. We used five different substrates and a mixture representing a gradient of different quality parameters and attempted to determine (a) the effect of substrate type on soil communities and (b) what biochemical parameters most influence the soil micro food web groups’ abundance during the first six months of decomposition.

    Interactive effects of soil faunal community and substrate quality on nitrogen mineralization:
    Given the soil community’s role in nutrient cycling, modifications in its structure can potentially affect nutrient release processes and therefore nitrogen availability (de Ruiter et al., 1994). Agricultural management practices such as tillage or pest management can affect the composition of the soil community. Among the members of the soil community, the soil fauna is particularly vulnerable extinction or homogenization due to management. It is well known that the composition of the soil fauna can influence the decomposition and mineralization of plant litter and soil organic matter (Bradford et al., 2002; Brussaard, 1998; Coleman et al., 2004; Edwards, 2000; Lavelle et al., 1993) and thus it would be expected that changes in the faunal community imply changes in the dynamics of decomposition and nutrient mineralization. In general, nutrient mineralization and decomposition increase with increasing body size of the soil animals and with increasing complexity of the faunal community. However, interactive effects of the soil fauna community structure and the quality of available substrates on mineralization and decomposition have been documented. Fauna can increase decomposition and mineralization rates of low quality litter (Couteaux et al., 1991; Tian et al., 1992), but the opposite has been observed as well (Schadler and Brandl, 2005). Gonzalez and Seastedt (2001) on the other hand found no fauna-litter interactions. Some studies have shown that the composition of the faunal community can affect the level of control of organic matter quality on soil processes. In particular, a more complex fauna seems to enhance the degree of control of substrate quality on decomposition (Schadler and Brandl, 2005; Smith and Bradford, 2003). The second objective of this study was to investigate the effect of the structure of the faunal community on nitrogen mineralization and in particular to explore one potential way in which fauna could affect nitrogen mineralization: by mediating the control that the quality of surface added substrates exerts on the structure of the micro-food web (the microbial groups and their direct predators). In a factorial arrangement, we exposed soil to surface-applied plant materials of contrasting chemical compositions and restricted the access of size-classes of fauna to mineral soil. After six months of decomposition, we assessed the effects of substrate type on the structure of microbial community in the mineral soil when fauna had been excluded and when it was present, and measured nitrogen mineralization and decomposition under both scenarios after 21, 91 and 165 days. We also estimated the abundances of microarthropods and nematode trophic groups in soil. We asked (a) does soil fauna mediate the effect of substrate type and quality on nitrogen mineralization, (b) does soil fauna mediate the effect of substrate type on the structure of the microbial community and the micro foodweb in the mineral soil, and (c) is the effect of fauna on mineralization associated with its effect on the microbial community and their predators?

    Effect of quality on nitrogen mineralization due to its effect on trophic interactions:
    It has been suggested that the effect of substrate quality on soil processes is driven by its effects on the soil organisms responsible for the processes (Wardle, 2002). By affecting the soil community structure, substrate quality can affect the trophic interactions occurring in the organic layer and mineral soil habitat. Trophic interactions in the soil food web have major effects on carbon and nutrient mineralization. Carbon mineralization can increase as a result of higher turnover rate activity and respiration of consumed populations due to grazing (Bardgett et al., 1993) while nitrogen mineralization occurs mainly due to excretion of excess nitrogen by the consumer (Woods et al., 1982). Furthermore, it has been suggested that the result of trophic transfers between the members of the soil food web can depend on the quality of resources (Bardgett, 2005; Herlitzius, 1983; Wardle, 2002). Thus, the quality of organic amendments has the potential to influence the dynamics of nutrients and carbon due to its inherent chemical degradability but also due to its effects on the trophic interactions among soil populations. These two factors may in turn interact to affect mineralization. The inherently complex nature of these interactions makes them difficult to assess via experimental means as this would require isolating the effect of the soil populations and that of substrate quality.

    The complexity of the decomposition process and biological interactions in soil makes modeling approaches not just desirable but necessary. Organism-oriented models, which explicitly incorporate soil organisms and their interactions with the biophysical environment, have a high explanatory value and permit the evaluation of the effects of intervention and management (Paustian, 1994; Smith et al., 1998). The organism-oriented modeling approach initiated by Hunt et al. (1987) has been applied to several natural and agricultural systems (Berg et al., 2001; de Ruiter et al., 1994a; Hassink et al., 1994; Schroter et al., 2003). In this approach food webs are constructed by aggregating species into functional groups and the structure and functioning of the food webs are analyzed in relation to nutrient cycling. Soil food web models have proven useful in predicting C and N mineralization rates (de Ruiter et al., 1994b), in explaining rates in terms of the relative contribution of groups of organisms and particular trophic interactions (Berg et al., 2001).

    For the third objective we use the soil food web model scheme of Hunt et al. (1987) to simulate carbon and nitrogen mineralization from surface applied substrates of differing chemical qualities and from mineral soil based on observed population sizes and the trophic interactions among the members of the soil food web. This model describes mineralization as being regulated not only by trophic interactions or chemical quality limitations, but by the interaction of both. We add simple features to describe differential chemical composition of the substrates and differential abilities of fungi and bacteria to degrade organic matter fractions. This approach allows us to isolate the effects of the changes in soil food web structure prompted by the added substrate from the effects of the quality of substrate on trophic transfers.

    We calibrated the model using the measured soil populations and nitrogen mineralization after the application of one substrate and then assessed its performance with the other plant materials. We then used the model to investigate (a) the importance of the soil community changes brought about by the quality of substrate on carbon mineralization and nitrogen mineralization; (b) whether nitrogen mineralization is better predicted by the interaction of soil community and substrate quality than by each factor alone; (c) whether the role of the soil communities and their trophic interactions varies depending on the quality of the degrading substrate; and (d) whether some communities are better suited to degrade substrates of certain quality.

    Project objectives:

    1. Assess the response of the microbial community, nematodes and microarthropods to the following amendments over a growing season: (a) green manure from a leguminous alley cropping species (Amorpha fruticosa L., a native woody shrub.), (b) green manure from a leguminous non-woody species (Trifolium incarnatum L. or crimson clover) used as a winter cover crop, (c) green manure from cereal rye (Secale cereale L.), (d) wheat hay (Triticum aestivum L.), (e) pine needles (Pinus taeda L.), and an even mixture of these by weight.

      Investigate the effect of the complexity of the faunal community on nitrogen release into the soil and the interactive effects of the faunal community structure and the quality of organic amendments on nitrogen mineralization

      Using experimental and modeled data, study the effect of the observed responses of the soil community and their trophic interactions on nitrogen release into the soil.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.