Global Patterns and Drivers of Litter Decomposition Under Nitrogen Enrichment: A Meta-Analysis

Nitrogen (N) enrichment has substantially altered patterns of terrestrial litter decomposition, with positive, neutral, and negative effects. However, the general response patterns and drivers of litter decomposition to N enrichment rates are poorly understood, and how litter decomposition has changed under the N enrichment rate, especially in different ecosystems, still requires further study. We reviewed 118 published papers dealing with litter mass remaining after N enrichment to assess the influences of various environmental and experimental factors on the relationships between N enrichment and litter decomposition in grasslands, forests, and wetland ecosystems. The results indicated that N enrichment had an insignificant effect on litter decomposition globally. However, the effects varied greatly among ecosystem types, with an increase in litter decomposition of 3.91% in grasslands and 1.82% in wetlands and a decrease of 1.23% in forests. When forests were subdivided into plantations, primary, and secondary forests, the results showed that N enrichment significantly slowed litter decomposition rate by 2.96% in plantations but had no significant influence in primary and secondary forests. However, litter decomposition was significantly influenced by the level of N addition in plantations and secondary forests, with an increase in litter mass loss at low N addition (50 kg N ha(-1) year(-1)) and a decrease in litter mass loss at high N addition (>50 kg N ha(-1) year(-1)). The magnitude and direction of the N effect are affected by experimental and environmental factors. Specifically, mixed N enrichment (for example, urea and glycine) exerted a stronger effect on litter decomposition compared with an N fertilizer alone. Our findings indicated the different effects of N on litter decomposition in forests and grasslands and knowledge which will greatly advance our ability to accurately evaluate and predict global C cycling under increased N deposition, which should improve future models of global biogeochemical cycling.