Mapping global nitrogen deposition impacts on soil respiration.

Soil respiration (R) is a key indicator of belowground biological activities of terrestrial ecosystems. Despite ongoing atmospheric nitrogen (N) deposition due to anthropogenic activities, it remains uncertain how R responds to globally varied atmospheric N deposition. Based on a meta-analysis of 340 simulated experimental nitrogen addition studies, we aimed to identify the key factors altering the responses of R to N deposition and extrapolate these results to the global mapping of R changes under N deposition. We found the overall experimental N addition effect on R was insignificant, but the responses of R significantly shifted from positive to negative with increasing accumulated N addition amount and lower soil pH, and the negative responses to increasing N amounts were significantly intensified in acid soils. Also, the response of heterotrophic respiration to N addition significantly increased with a lower N amount, and both responses of heterotrophic and autotrophic respiration were significantly more negative in soils with lower pH. Our mapping efforts showed that global R overall increased by 2.8 % in response to the accumulated N deposition from 2000 to 2020. Regions with combined characteristics of high accumulated N deposition amounts and low soil pH, including Eastern U.S., Europe, and Eastern Asia, were hotspots of R declines under current and future atmospheric N deposition. Our findings challenge the long-held notion that N deposition has universal negative impacts on R, and suggest the spatial heterogeneity in the impacts of N deposition on belowground activities and carbon release across the globe.