Biotic and abiotic drivers of microbial carbon and nitrogen cycling in soils at the continental scale

The pools of C and N in soil organic matter (SOM) contain 80% of the C and 90% of the N in the terrestrial biosphere, and microorganisms in the soil catalyze the turnover and release of nutrients tied up in SOM. As microbes decompose SOM they respire, producing CO2, while waste nitrogen is released as NH4+. Microbes limited by N will immobilize both NH4+ and NO3- , but recent research indicates that the soil itself can chemically incorporate both NH4+ and NO3 -. Given that NO3- is typically thought of as being unreactive, this pathway is particularly intriguing and could be a mechanism by which soils might retain a highly mobile form of N, and maintain N limitation to microbes and plants.;Our goal was to determine the drivers of microbial respiration, N mineralization, and abiotic NO3- incorporation. We used a comparative approach, sampling soils at the continental scale to maximize the range of soil and site characteristics. To study microbial C and N turnover, we conducted 50 day incubations of 84 soils and measured microbial respiration (the rate of accumulation of CO2) and net N mineralization (the rate of accumulation of extractable NH4+ and NO3-). We then used structural equation modeling (SEM) to develop quantitative models showing which soil and site characteristics are drivers of microbial respiration and net N mineralization. To study abiotic NO3- incorporation we added a range of concentrations of NO3 - to a subset of 44 autoclave-sterilized soils from the larger collection. After 24 hours we quantified the amount of NO3- that was lost from solution.;We found that net N mineralization was generally near zero, indicating that production closely matched consumption of NH4+ and NO3- across most soils in our study. As such, we were unable to model net N mineralization. Microbial respiration was driven by climate, soil C concentration, microbial biomass, and clay content, with these factors explaining 79% of the variance in microbial respiration. There was no evidence of abiotic NO3- incorporation in any of the soils we examined, though high iron in the soil extracts gave the appearance of abiotic NO3- incorporation when we analyzed our samples with a common method of NO3 - analysis which suffers from iron interference. We hypothesized that the one study that has documented this process in sterile soils may have suffered from this same artifact.