Abstract: Global warming and altered precipitation regimes may profoundly affect soil nitrogen (N) transformations. However, a comprehensive understanding of how soil N cycling responds to such climatic changes remains lacking, with few syntheses of field-based observations. Here, a meta-analysis was conducted using 755 paired data points from field observations worldwide to explore the effects of warming and altered precipitation on soil N transformation rates and to assess possible drivers of these effects. Warming positively affected the soil N mineralization and nitrification rates (+21.8% and +20.9%), but had no effect on the microbial immobilization rate. Similarly, increased precipitation accelerated soil N mineralization and nitrification (+10.2% and +9.4%), but did not alter microbial immobilization. In contrast, decreased precipitation did not affect any of the three N transformation rates. Moreover, warming effects on the N mineralization rate were mainly driven by the variations in soil moisture and soil total N content, while effects on the nitrification rate were regulated by changes in ammonia-oxidizing bacterial abundance. In addition, the effects of increased precipitation on the N mineralization rate were largely dependent on changes in soil moisture and experimental manipulation characteristics, while effects on the nitrification rate were shaped by mean annual precipitation, soil pH, ecosystem types and treatment duration. Overall, increased temperature and precipitation accelerated soil N cycling and increased soil N availability, but decreased precipitation did not. These findings may improve predictions of biogeochemical cycling under future climate change scenarios.

Key words: warming, altered precipitation, nitrogen cycling, nitrogen mineralization, nitrification, microbial immobilization