Heterogeneous distribution may substantially decrease initial decomposition, long‐term microbial growth and N‐immobilization from high C‐to‐N ratio resources

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Abstract
Summary: The effect of heterogeneous resource distribution in soil has seldom been studied under controlled laboratory conditions, even though this type of distribution is the rule rather than the exception in both agricultural and undisturbed soil systems. We use distribution trials to test the effect of stratified distribution of chopped maize, sheep faeces derived from maize, and chopped rape stems compared with even distribution. In all treatments, CO2 mineralization decreased initially in the stratified experiments and in the rape treatment less (39 versus 43%) of the added carbon was mineralized even after 202 days of incubation at 15°C. In both maize and rape residue treatments, we observed much less immobilization of Nmin and less microbial growth in the stratified experiments. The data set for rape was sufficiently detailed to allow a model interpretation using a spreadsheet version of the soil organic matter module from DAISY, a soil‐plant‐atmosphere system model. This indicated that the observed differences between the stratified and evenly distributed experiments could be largely understood by assuming diffusion limitation of nitrate from the bulk soil to the residuesphere. The residuesphere is the part of soil that is immediately affected by the decomposition of residue, and was assumed to be included in the first 10 mm of soil surrounding the residues in the stratified experiments at all times. Only a very small part of the variation in CO2 respiration, Nmin and soil microbial biomass N (SMB‐N) could be explained by additionally assuming less substrate utilization efficiency of the less decomposable fraction of rape in the stratified experiment. We observed greater N mineralization in the evenly distributed experiment with faeces, and smaller concentrations of soil microbial biomass nitrogen extracted by chloroform fumigation. We assume that most of the additional SMB‐N determined in the faeces treatments was from microorganisms present in the faeces at the time they were added to soil, and that these were less susceptible to microbial predation in the stratified than in the distributed experiment. We conclude that the spatial distribution of decomposing litters in soil significantly affects the C and N dynamics. Diffusion limitation of available N in the active zone of decomposition was the main causal agent, which induced a decrease in microbial growth and substrate utilisation.

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