Nitrogen addition stimulates litter decomposition rate: From the perspective of the combined effect of soil environment and litter quality

Despite the essential role of litter decomposition in carbon (C) and nutrient cycle in terrestrial ecosystems, some uncertainties remain about how this fundamental process is affected by increasing nitrogen (N) deposition. Based on a large dataset comprising 1108 observations from 162 studies, we conducted a meta-analysis to explore the effect of N addition on litter decomposition rate under three kinds of litter decomposition experiments (i.e., common litter experiment (litter collected from control plot is decomposed in N addition plots); common site experiment (litter collected from N addition plots is decomposed in control plot); in situ experiment (litter collected from control and N addition plots is decomposed in situ)). In general, N addition significantly decreased litter decomposition rate by 2.3% across the three kinds of litter decomposition experiments. However, litter decomposition rate responded differently to N addition among different kinds of litter decomposition experiments. N addition significantly decreased litter decomposition rate by 5.1% in common litter experiment. In contrast, N addition significantly increased litter decomposition rate by 9.2% and 10.3% in common site and in situ experiments, respectively. The response of litter decomposition rate to N addition was positively correlated with initial N and phosphorous (P) concentrations, but negatively correlated with initial C:N and lignin:N ratios of plant litter in common litter experiment. For common site and in situ experiments, the N-induced increase in litter decomposition rate was attributed to the increased N and P concentrations and decreased C:N and lignin: N ratios of plant litter under N addition. Collectively, our results suggest that common litter experiment might underestimate the positive effect of N addition on litter decomposition. By contrast, the overall stimulatory effect of N addition on litter decomposition rate under in situ experiment should be more realistic, and its adoption could improve the prediction of ecosystem consequences of increased anthropogenic N deposition.