Microbial driven variation in carbon cycling during root litter decomposition

Soils store more carbon than plants and the atmosphere combined, and the ultimate fate of this carbon is tied to subsurface microbial communities. Yet, our fragmented understanding of how subsurface communities influence carbon cycling prohibits accurate estimates of the fate of carbon produced on the surface and future carbon storage. The conditions in the subsurface differ from the surface conditions in key factors such as temperature, humidity, nutrient availability and consequently microbial community composition. These all might vary somewhat independently for the surface because of the buffering effects of soil, but they are also influenced by surface processes such as precipitation, litter decomposition and nitrogen deposition. Because of the tight link between nitrogen and carbon cycles, nitrogen deposition has potential to drastically alter subsurface microbial composition and function. In this study, we used a common garden microcosm experiment to assess the range of subsurface microbial driven variation in carbon and nitrogen pools during root litter decomposition under fertilized (NH4NO3) and unfertilized conditions. We demonstrate that subsurface microbial community composition can create large variation in dissolved organic carbon (DOC, ~ 6x) and total nitrogen (TN, ~7x), and two fold variation in CO2. Nitrogen addition altered the balance of DOC, TN, and CO2 in ways that were specific to the origin of microbial inoculum. These communities differed drastically in their ability to use the added nitrogen, which when consumed, was related to higher levels of CO2 and lower levels of DOC compared to communities from the same origin without added N. Thus, we conclude that differences in subsurface microbial community composition and their response to nitrogen deposition could have far-reaching impacts on ecosystem function and feedbacks to global nutrient cycles.