Effects of nitrogen addition on activities of soil nitrogen acquisition enzymes : A meta-analysis

It has been suggested that elevated nitrogen (N) deposition may increase soil N mineralization in N-limited ecosystems, but the underlying mechanisms have been not adequately explored. Soil N-acquisition enzymes play important roles on organic N mineralization. Thus, their responses to N deposition will be crucial for explaining the above phenomenon. Here, we conducted a meta-analysis from 64 studies to synthesize the responses of soil N-acquisition enzyme activities to N addition. Results showed that N addition significantly increased activities of N-acetylglucosaminidase and urease by 5.5% and 11.6%, respectively. However, N addition had negative or negligible effects on activities of protein-depolymerization enzymes, with no response for non-specific protease and leucine aminopeptidase but a significant decrease of 33.0% for glycine aminopeptidase. Because protein comprises more than 60% of the N in plant and microbial cells, and the protein depolymerization is an important rate-limiting step of organic N mineralization, the suppressed protein depolymerization indicates either that the changes to microbial activity may be not a dominant mechanism for the increased N mineralization in N-limited ecosystems with N addition, or that the increased N mineralization may be overvalued in the previous studies.