Global Soil Gross Nitrogen Transformation Under Increasing Nitrogen Deposition

Soil nitrogen (N) transformations, which regulate soil N availability, are important for ecosystem productivity and other functions. Nitrogen deposition influences soil N transformations, but it remains unclear how soil N transformations, especially microbial N immobilization, change with different N deposition rates. In this study, we compiled a global data set of gross N mineralization, nitrification, and microbial mineral N immobilization rates with different N addition treatments across terrestrial ecosystems to reveal their responses to different N deposition rates. The results showed that gross N mineralization and gross nitrification rates were stimulated by both low and high N additions, but the increments slightly decreased with increasing N addition rate. However, microbial ammonium (NH4+) and nitrate (NO3-) immobilization rates responded differently under low versus high N additions. They changed from being inhibited (NH4+ immobilization) or not affected (NO3- immobilization) under low N addition to being stimulated under high N addition (>55 kg N ha(-1) yr(-1)), performing a linear increase along with the increasing N addition rate. This challenges the traditional perception that N enrichment limits microbial N utilization. Our results suggest that the effects of N deposition on gross N transformations are highly dependent on N deposition rate. These findings indicate that biogeochemical models should take into account the differential responses of gross N transformations to different N addition rates to better predict ecosystem N cycling in the context of increasing N deposition.