Microbial and Faunal Interactions and Effects on Litter Nitrogen and Decomposition in Agroecosystems

Abstract

We conducted field experiments to test the general hypothesis that the composition of decomposer communities and their trophic interactions can influence patterns of plant litter decomposition and nitrogen dynamics in ecosystems. Conventional (CT) and no—tillage (NT) agroecosystems were used to test this idea because of their structural simplicity and known differences in their functional properties. Biocides were applied to experimentally exclude bacteria, saprophytic fungi, and microarthropods in field exclosures. Abundances of decomposer organisms (bacteria, fungi, protozoa, nematodes, microarthropods), decomposition rates, and nitrogen fluxes were quantified in surface and buried litterbags (Secale cereale litter) placed in both NT and CT systems. Measurements of in situ soil respiration rates were made concurrently. The abundance and biomass of all microbial and faunal groups were greater on buried than surface litter. The mesofauna contributed more to the total heterotrophic C in buried litter from CT (6—22%) than in surface litter from NT (0.4—11%). Buried litter decay rates (1.4—1.7%/d) were ≈2.5 times faster than rates for surface litter (0.5—0.7%/d). Ratios of fungal to bacterial biomass and fungivore to bacterivore biomass on NT surface litter generally increased over the study period resulting in ratios that were 2.7 and 2.2 times greater, respectively, than those of CT buried litter by the end of the summer. The exclusion experiments showed that fungi had a somewhat greater influence on the decomposition of surface litter from NT while bacteria were more important in the decomposition of buried litter from CT. The fungicide and bactericide reduced decomposition rates of NT surface litter by 36 and 25% of controls, respectively, while in CT buried litter they were reduced by 21 and 35% of controls, respectively. Microarthropods were more important in mobilizing surface litter nitrogen by grazing on fungi than in contributing to litter mass loss. Where fungivorous microarthropods were experimentally excluded, there was less than a 5% reduction in mass loss from litter of both NT and CT, but fungi–fungivore interactions were important in regulating litter N dynamics in NT surface litter. As fungal densities increased following the exclusion of microarthropods on NT surface litter, there was 25% greater N retention as compared to the control after 56 d of decay. Saprophytic fungi were responsible for as much as 86% of the net N immobilized (1.81 g/m2) in surface litter by the end of the study when densities of fungivorous microarthropods were low. Although bacteria were important in regulating buried litter decomposition rates and the population dynamics of bacterivorous fauna, their influence on buried litter N dynamics remains less clear. The larger microbial biomass and greater contribution of a bacterivorous fauna on buried litter is consistent with the greater carbon losses and lower carbon assimilation in CT than NT agroecosystems. In summary, our results suggest that litter placement can strongly i...

Legume, cropping intensity, and N-fertilization effects on soil attributes and processes from an eight-year-old semiarid wheat system

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Soil and crop residue CO2-C emission under tillage systems in sugarcane-producing areas of southern Brazil

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“…In accordance with these results, Videla et al (1996), Ferrari et al (1997) and Henriksen and Breland (1999) found that biomass was the only parameter that did not reflect the changes induced by nitrogen fertilization in the mineralization of organic matter. However, Beare et al (1992) proposed that fumigation with chloroform was not the most suitable approach to use when the soils had an important contribution from organic wastes as occurs in f ield crops under zero tillage. Therefore, factors other than N were probably responsible of the evolution of the soil biomass such as the availability of phosphorus or other nutrient elements and also the methodology used.…”