Soil Food Web Management to Enhance Nitrogen Availability

Howard Ferris

Conversion of nitrogen (N) from incorporated crop residues and manure to available mineral forms requires decomposition and mineralization processes mediated by bacteria, fungi and other components of the soil food web. When carbon is abundant, increases in microbial biomass may result in immobilization of N. In that case, mineralization may be increased by organisms that graze on the primary decomposers.

In laboratory studies, bacterial-feeding nematodes mineralized between 1.2 and 5.8 ng N per individual per day, depending on body size, while fungal-feeding nematodes mineralized up to 3.3 ng N per individual per day, depending on host status of the fungal substrate. Since bacterial-feeding nematodes have a higher C:N ratio (* 6:1) than their substrate (* 4:1), considerable mineralization of N is associated with their metabolic activity. In consuming sufficient bacteria to provide the C necessary for their body structure and respiration, nematodes assimilate excess N. The excess N is excreted as ammonia. Fungal-feeding nematodes have a body C:N ratio closer to that of their substrate, so the N associated with C used in body structure is not in excess. However, the N associated with C that is respired exceeds body needs and is excreted.

Field observations in the organic and low-input tomato plots of the UC Davis Sustainable Agriculture Farming Systems (SAFS) project indicated that population levels of microbivorous and fungivorous nematodes were low early in the growing season. In those plots, which either do not receive mineral fertilizers (organic) or receive them only when necessary (low-input), symptoms of N deficiency are sometimes seen during the early vegetative growth phase. The goal of this study was to manage the soil food web to enhance population levels of microbivorous and fungivorous nematodes in the spring so that their feeding would release N immobilized in the primary decomposer biomass.

To avoid delaying planting of tomatoes until nematode populations responded to spring soil temperatures, we attempted to increase numbers of microbivorous nematodes and other microbial grazers the previous fall. Soil temperature conditions are adequate for nematode population increase in the late summer and early fall in northern California, but moisture is a constraining factor. The moisture constraint to soil biological activity was relieved by irrigation and a carbon source was provided in the form of a late summer cover crop. Abundance of microbivorous nematodes, and presumably other organisms at similar trophic levels, increased with the enhancement of biological activity the previous fall. Soil N level increased in the spring with nematode abundance. Subsequent tomato yields also increased when N levels were limiting.

A convenient biomarker system is necessary to provide a basis for management and monitoring of the soil food web. From the standpoint of soil fertility, greatest amounts of mineralization probably occur at lower trophic interchanges in the food web, where biomass of organisms is greatest. Members of functional guilds of soil nematodes, classified at the family level, respond similarly to food web enrichment and to environmental perturbation and recovery. Indices derived through nematode faunal analysis provide bioindicators for condition of the soil food web and disturbance of the soil environment. Resolution is enhanced by a weighting system for the indicator importance of the presence and abundance of each functional guild in relation to food web enrichment and structure. Graphical representations of food web structure, based on nematode faunal analyses, allow diagnostic interpretation of its condition. Simple ratios of the weighted abundance of representatives of specific functional guilds provide useful indicators of food web structure and enrichment, and of the nature of decomposition channels. These analyses can be used as a basis for management decisions and as a means of monitoring management success.

Our results confirm that an abundance of nematodes representing opportunist bacterial- and fungal-feeding guilds indicates a biologically active soil. When such guilds predominate at planting time, N provided by incorporation of organic material will not remain bound in the primary decomposer biomass but will be mineralized through the grazing activities of the nematodes.

Howard Ferris
Department of Nematology
University of California
Davis, CA 95616
Tel: (530) 752-8432
Hferris@ucdavis.edu

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