Anthropogenic nitrogen deposition decouples relationships with decomposing microbes, altering SOM molecular composition, but not molecular complexity or diversity

Soil organic matter (SOM) consists of different biochemical constituents that exist across a spectrum of chemical complexity from simple to complex, and the relative abundance of these potential substrates for microbial metabolism can have consequences for the fate of soil C, and consequently the pace of climate change. However, current understanding of the mechanistic controls on SOM biochemistry and decay, and how these may be affected by climate change, is still incomplete. As such, the primary objectives of this study are to assess (1) the trajectory of organic matter decomposition and stabilization from undecayed plant root litter to mineral SOM and (2) how these processes are altered by anthropogenic nitrogen (N) deposition by using high-resolution SOM biochemical and microbiome datasets and a long-term field experiment in the Upper Great Lakes region of North America. N deposition did not significantly alter the biochemical composition of SOM in the undecomposed and 1-year decomposed samples but did alter mineral SOM biochemistry. This shift was attributed to changes in the relative abundance of specific biochemical compounds rather than changes in more broadly defined compound classes, such as lipids, for example. A reduced association between SOM biochemistry and microbial decomposer communities under N deposition was also observed. Taken together, atmospheric N deposition altered the composition of mineral SOM and decoupled the relationship between microbial composition and SOM biochemistry, likely contributing to the previously observed reduction in the decomposition of SOM while not significantly altering SOM’s overall biochemistry.