Responses and Controlling Factors of the Litter Decay Rate to Nitrogen Addition Across Global Forests: A Meta-Analysis.

Plant litter decay is an essential process for recycling C and nutrients in natural ecosystems. However, the impacts of N addition on litter decay are not well understood in global forest ecosystems. Therefore, a meta-analysis was used to examine how N addition affects the litter decay rate through three kinds of litter decay traits (i.e., common litter trait (litter collected from control plot is decomposed in N addition plots); common site trait (litter collected from N addition plots is decomposed in control plot); and in situ trait (litter collected from control and N addition plots is decomposed in situ)), based on 1145 field observations from 166 published studies on global forests. Nitrogen addition significantly reduced the litter decay rate by 3.92% across the three kinds of decay traits. However, there were different responses of the litter decay rate to N addition among the decay traits. The N addition notably inhibited litter decay by 5.99% for the common litter trait but consistently promoted it by 8.37% and 7.48% for common soil and situ traits, respectively. The magnitude and direction of such effects varied with the N addition amount, form and duration. The effect size of the litter decay rate due to N addition was negatively related to the initial N concentration and C:N ratio for the common litter trait. The N concentration in litter was raised by N addition, resulting in an increase in the litter decay rate for the common situ trait. For the situ trait, N addition increased N concentration and reduced C:N and lignin/N in litter, resulting in an increase in the decay rate, and the responses of the litter decay rate to N addition were also influenced by the humidity index. Overall, our results showed that the responses of the litter decay rate to N addition were different among the three kinds of decay traits and were controlled by environmental and experimental factors. These findings help us to better understand the effects of N addition on biogeochemical cycling in global forest ecosystems.