Modeling the effects of increasing anthropogenic nitrogen on soil microbial decomposition

Background/Question/Methods

Ecosystems worldwide are receiving increasing amounts of reactive nitrogen (N) through anthropogenic activities. Exogenous N can have large consequences on microbially-mediated processes such as alterations to soil carbon (C) cycling rates. In particular, a growing body of literature has shown, in mineral soils, N amendments induce a marked decrease in soil respiration rates and microbial biomass. The decrease in soil decomposition rates is a seemingly counterintuitive response, as N is often considered a limiting resource in ecosystems and aboveground primary productivity generally increases with N amendments. In conjunction with suppressed decomposition rates, our work also demonstrates that N fertilization consistently shifts soil bacterial community composition and the metabolic capabilities of the community. Based on these results, we hypothesized that N additions favor communities that are more ‘copiotrophic’ and less capable of decomposing recalcitrant C pools, thereby lowering decomposition rates. Given this hypothesis we constructed a novel theoretical model in which we replicated the functional attributes of two distinct microbial pools (copiotrophs and oligotrophs) to control the decay of C. Within the decomposition model, each microbial pool is restricted by ecological traits (growth rate, substrate use efficiency, and C:N ratio), which contribute to the effect N additions have on decomposition.

Results/Conclusions

We examine whether the inclusion of functionally dissimilar microbial biomass pools can better predict soil process responses to N than previously proposed one-pool models. We contrast our results against competing (though not mutually exclusive) hypotheses, including N-mining and enzyme inhibition. The model supports our hypothesis that N favors more 'copiotrophic' communities, shifting the physiology of the community so more labile C pools are preferentially consumed over recalcitrant C pools, resulting in a decrease in soil respiration and microbial biomass. While, previously work has restricted decomposition models to first-order kinetics, assuming the size of the C pool controls decomposition rates, this work demonstrates that directly linking the functional microbial community can better predict soil decomposition responses. Microbes are critical to the cycling of soil nutrients and understanding their response to exogenous N will help to better predict the consequences on ecosystem health and the global carbon (C) budget.