Effects of nitrogen application on the decomposition of fine roots in temperate forests: a meta-analysis

Purpose Fine root decomposition plays an essential role in the nutrient cycle and energy transfer in terrestrial ecosystems, and changes in decomposition induced by nitrogen (N) deposition have become a global concern. However, patterns of fine root decomposition with N application are still scattered, and the dominant factors regulating decomposition are still controversial. Here, we aimed to explore general patterns and key drivers of decomposition in temperate forests with N application. Methods From 20 studies, we synthesized 123 records of fine root decomposition in temperate forests where N was applied. We explored the overall effect of decomposition with N application and the variation in decomposition among N application rates, N forms, fertilization condition of root growth and decomposition (FF, from fertilized to fertilized conditions, and UFF, from unfertilized to fertilized conditions), tree functional types and soil depth. The dominant factors of decomposition were identified using regression. Results Our results showed that N application decreased fine root decomposition. Specifically, decomposition decreased at the application rate of 100-150 kg N ha(-1) yr(-1), under NH4NO3 application, in broadleaf trees and in deep layers, attributable to the inhibited microbial enzyme activity. Decomposition decreased in FF, likely resulting from home-field advantage (HFA) effects. Multiple regressions showed that initial lignin content was the most important factor determining decomposition. Conclusion Our results suggested that inhibited microbial enzymes were associated with decreased decomposition under N application in temperate forests. Additionally, our results confirmed the importance of initial root traits, such as lignin, in regulating decomposition.