Background: Anthropogenic activities have increased the inputs of atmospheric reactive nitrogen (N) into terrestrial ecosystems, affecting soil carbon stability and microbial communities. Previous studies have primarily examined the effects of nitrogen deposition on microbial taxonomy, enzymatic activities, and functional processes. Here, we examined various functional traits and interactions of soil microbial communities in a Mediterranean-type grassland administrated with 14 years of 7 g m-2 year-1 of N amendment, based on estimated atmospheric N deposition within areas in California, USA by the end of the 21st century.

Results: Soil microbial communities were significantly altered by N deposition. Consistent with higher aboveground plant biomass and litter, fast-growing bacteria, based on abundance-weighted average rRNA operon copy number, were favored in N deposited soils. The relative abundances of genes associated with labile carbon (C) degradation (e.g., amyA and cda) were also increased. In contrast, the relative abundances of functional genes associated with the degradation of more recalcitrant C (e.g., mannanase and chitinase) were either unchanged or decreased. Compared with the ambient control, N deposition significantly reduced network complexity, such as average degree and connectedness. The network of N deposited samples contained only genes associated with C degradation, suggesting that C degradation genes became more intensely connected under N deposition.

Conclusions: We propose a conceptual model to summarize the mechanisms of how changes in above and below-ground ecosystems by long-term N deposition collectively lead to more soil C accumulation.  [link to publication]