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.