Perennial Legumes Project
Yolima Carrillo
Bogotá, Colombia, 1977
B.S. National University of Colombia, 2000
Ph.D. University of Georgia, 2007
Currently working as a postdoctoral fellow
Belowground effects of aboveground pruning of the hedgerow species Amorpha fruticosa L.
For this project we use false indigo (Amorpha fruticosa L.) a native leguminous species with potential to be used in agroforestry systems to increase soil fertility and to increase the biological diversity and complexity of agricultural systems. Part of the management strategy for this species involves periodic pruning. The goal of the project is to assess whether mechanical pruning of leaves has an impact in the carbon and nitrogen concentrations in soil through affecting the roots' dynamics or affecting exudation of carbon and nitrogen compounds from roots.
To answer this question we are conducting an experiment with potted plants and we are using stable isotopes (13C and 15N) to track carbon and nitrogen after the pruning.
Photo at right: Stable isotopes of carbon and nitrogen are applied in a spray solution to the leaves of Amorpha fruticosa.
PAST PROJECTS:
Most agricultural practices aiming at enhancing economic and environmental sustainability involve, at some point, the use of organic soil amendments such as crop residues or green manures. Soil organic amendments can have an important role not only in building soil organic matter and in conservation of soil and water, but also in supplying nutrients to subsequent crops in rotations and to simultaneous crops. Whenever the use of inorganic fertilizers is being reduced in favor of organic materials, the availability of nutrients in soil depends increasingly on the natural processes of decomposition and mineralization. The soil biota plays an important role in regulating organic matter decomposition and nutrient mineralization in agroecosystems.
In agroecosystems, for instance, practices such as soil amendment with crop residues or green manures can affect population size and dynamics of organisms in soil food webs and as a consequence affect nutrient mineralization. In turn, the choice of the amendments and the rate and timing of application determine the quality and quantity of resources that are made available to the soil community. Thus, in agroecosystems the interplay of plant input quality and the soil community is of special importance and can be a determining factor in the functioning and sustainability of an agricultural strategy.
Between 2003 and 2006 we conducted a series of laboratory and field experiments to study the interactions between the soil biotic community and the chemical quality of plant litter used as an organic amendment. Below are the titles of these projects and a summary including the approach and the most important findings. For more information contact Yolima Carrillo (yolcarri@uga.edu).
Chemical quality of litter as a driver of detrital community assemblage in mineral soil
We studied the short term effect of surface application of five plant materials and a mixture on the microbial community, nematodes and microarthropods in the mineral soil. We identified some patterns of response to litter quality parameters by microbial groups. Fungi and Gram-negative bacteria responded positively to high nutrient content in litter and negatively to %C, C/N and %lignin. Gram-positive bacteria and actinomycetes were stimulated by high %C and % lignin. C/N was the main driver of the fungi-to-bacteria ratio. We also found that different quality variables were related to the responses of the soil community at different times. Early during decomposition those responses were mostly related with nutrient content while later %C, lignin and lignin-to-N had the greatest influence. Microbial feeding and omnivorous nematodes showed a response to substrate type that was correlated with the response of the bacteria. No pattern of response by microarthropods was observed. By affecting populations in the mineral soil at different trophic levels, litter quality thus has the potential to influence processes in the mineral soil that are driven by trophic interactions such as nutrient mineralization and soil organic matter decomposition.
Effect of plant litter quality on the ability of soil to process fresh substrates in the presence of different soil fauna
Our objective was to test if the changes brought about in soil by the quality of litter could influence the ability of soil to mineralize and decompose newly added substrates and materials already present in the mineral soil and if the composition of fauna would affect this ability. We set up a laboratory experiment in which the soil biotic and chemical environment was manipulated by exposing it to two plant litters of contrasting qualities for 16 months. We then monitored nitrogen and carbon mineralization and litter mass loss after removing the old litter and (a) adding fresh litter of the same species with which the soil had been pretreated, (b) adding fresh litter of the second species and (c) leaving soil bare. We found that the preexisting conditions in soil affected the evolution of CO2 from fresh litter and overrode the effect of litter quality on nitrogen mineralization in the first week of decomposition. Also by modifying the soil environment litter quality affected the mass loss pattern of one of the litter materials. We found indications that the composition of fauna affected the influence of litter quality on nitrogen mineralization, but the effects on mass loss and carbon mineralization were not clear. Our results suggest that the effects of plant litter quality on soil function are not limited to determining its own decomposition and mineralization rate and can generate biotic and abiotic conditions that affect the soil’s ability to process freshly added substrates. Our study also contributed some evidence that the composition of soil fauna can change the extent of the influence of litter quality on soil properties that can affect its function.
Modeling the effect of the interaction of soil community structure and plant litter quality on c and n mineralization
In this paper we used a soil food web modeling approach to simulate carbon and nitrogen mineralization from surface applied litter of differing chemical qualities and from mineral soil based on observed population sizes and the feeding interactions among the members of the soil food web. To calibrate and validate the model we assessed the abundances of soil bacteria, fungi, protozoa, nematodes and microarthropods and measured nitrogen mineralization over six months of decomposition. We then used the model to investigate interactions between litter quality and soil community structure. Our simulations suggested that (a) soil food webs generated by plant litter of different chemical qualities have differential abilities to mineralize litter and soil organic matter, (b) soil communities are better able to mineralize substrates of quality that is similar to the one of the litter in which they had developed, and (c) the structure of the soil community is of less importance in determining mineralization rates when the degrading substrate is of intermediate quality.
Mediation by soil fauna of the effect of litter quality on nitrogen mineralization
In this paper we explore one potential way in which fauna could affect nitrogen mineralization: by mediating the control that the quality of litter exerts on the structure of the micro-food web. In a factorial arrangement, we exposed soil to surface-applied litter 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 litter decomposition under both scenarios after 21, 91 and 165 days. We also estimated the abundances of microarthropods and nematode trophic groups in soil. We found some evidence that the composition of soil fauna affected the control of nitrogen mineralization by the chemical quality of plant litter. Specifically, we found that the influence of the chemical composition of litter was enhanced when the larger members of the soil fauna were not excluded from the system. Our results suggested that the presence of fauna affected nitrogen mineralization by affecting the abundance of the consumer level of the micro-food web, the nematodes in particular. Our study also indicated that that the presence of fauna also accentuated the response of the microbial biomass to the quality of litter and therefore supported the hypothesis that the effect of fauna on mineralization can at least in part be due to its effects on the structure of the micro-food web.
