Effects of nitrogen enrichment on soil enzyme activities in grassland ecosystems in China: A multilevel meta-analysis

Nitrogen (N) enrichment has resulted in widespread alteration of grassland ecosystem processes and functions mainly through disturbance in soil enzyme activities. However, we lack a comprehensive understanding of how N deposition affects specific key soil enzymes that mediate plant-soil feedback of grassland. Here, with a meta-analysis on 1 446 cases from field observations in China, we show that N deposition differently affects soil enzymes associated with soil biochemical processes. Specifically, N-promoted C, N, and P-acquiring hydrolase activities significantly increased by 8.73%, 7.67%, and 8.69%, respectively, related to an increase in microbial-specific enzyme secretion. The increased relative N availability and soil acidification were two potential mechanisms accounting for the changes in soil enzyme activities with N enrichment. The mixed N addition in combination of NH₄NO₃ and urea showed greater stimulation effect on soil enzyme activities. However, the high rate and long-term N addition tended to weaken the positive responses of soil C-, N- and P-acquiring hydrolase activities to N enrichment. Spatially increased mean annual precipitation and temperature primarily promoted the positive effects of N enrichment on N- and P-acquiring hydrolase activities, and the stimulation of C- and N-acquiring hydrolase activities by N enrichment was intensified with the increase in soil depth. Finally, multimodal inference showed that grassland type was the most important regulator of responses of microbial C, N, and P-acquiring hydrolase activities to N enrichment. This meta-analysis provides a comprehensive insight into understanding the key role of N enrichment in shaping soil enzyme activities of grassland ecosystems.