Responses of Soil Nitrogen Pools to Litter Input Under Nitrogen Addition: A Meta-Analysis

Litter input essentially regulates soil nitrogen (N) and phosphorus (P) pools in natural ecosystems, but uncertainties remain about how this regulation is affected by increasing N deposition. Here, we synthesized 1,263 observations from 34 field studies with paired treatments to elucidate how N addition affects the responses of soil N and P pools to litter input. Our results showed that the combination of litter input and N addition significantly enhanced soil N pools, with increases in total N (13.7%), microbial biomass N (38.7%), ammonium N (37.7%), and nitrate N (79.0%), but slightly affected soil P pools. Strikingly, soil N pools in forests displayed more positive responses to combined treatment than those in grasslands. Furthermore, experimental design and mean annual precipitation were critical controllers in regulating the responses of soil N pools to combined litter input and N addition. The present results underscore the importance of combined litter input and N addition in promoting soil N pools, thereby providing essential insights into how N addition can change nutrient cycling within natural ecosystems. Litter input plays a primary role in driving nitrogen and phosphorus cycles in terrestrial ecosystems. Anthropogenic nitrogen input appears to increase litter nitrogen and phosphorus content and affects the quantity and quality of litter input. However, how nitrogen input affects soil nitrogen and phosphorus pools when combined with litter input remains unclear. By analyzing 1,263 observations from 34 field studies, we found that the combination of litter and nitrogen input significantly increased soil nitrogen pools, with only minimal impact on soil phosphorus pools. More positive responses of soil N pools to combined treatment were found in forests rather than in grasslands. The study highlights the importance of considering both litter and nitrogen input to soil nutrient cycling in natural ecosystems. A meta-analysis of 34 studies reveals how nitrogen addition affects the responses of soil nitrogen and phosphorus pools to litter inputCombined litter input and nitrogen addition significantly increased soil nitrogen poolsSoil nitrogen pools in forests responded more positively to combined litter input and nitrogen addition than those in grasslands