Nitrogen deposition differentially affects soil gross nitrogen transformations in organic and mineral horizons

Reactive nitrogen (N) input can profoundly alter soil N transformations and long-term productivity of forest ecosystems. However, critical knowledge gaps exist in our understanding of N deposition effects on internal soil N cycling in forest ecosystems. It is well established that N addition enhances soil N availability based on traditional net mineralization rate assays. Yet, experimental additions of inorganic N to soils broadly show a suppression of microbial activity and protein depolymerization. Here we show, from a global meta-analysis of N-15-labelled studies that gross N transformation rates in forest soil organic and mineral horizons differentially respond to N addition. In carbon (C)-rich organic horizons, N addition significantly enhanced soil gross rates of N mineralization, nitrification and microbial NO3- immobilization rates, but decreased gross microbial NH4+ immobilization rates. In C-poor mineral soils, in contrast, N addition did not change gross N transformation rates except for increasing gross nitrification rates. An initial soil C/N threshold of approx. 14.6, above which N addition enhanced gross N mineralization rates, could explain why gross N mineralization was increased by N deposition in organic horizons alone. Enhancement of gross N mineralization by N deposition was also largely attributed to enhanced N mineralization activity per unit microbial biomass. Our results indicate that the net effect of N input on forest soil gross N transformations are highly stratified by soil C distribution along the soil profile, and thus challenge the perception that N availability ubiquitously limits N mineralization. These findings suggest that these differences should be integrated into models to better predict forest ecosystem N cycle and C sequestration potential under future N deposition scenarios.