Nitrogen deposition stimulates decomposition via changes in the structure and function of litter food webs

Nitrogen (N) deposition poses a threat to terrestrial biodiversity and ecosystem functioning globally. However, little is known concerning how the structure and function of litter fauna communities will respond in this context. Here, a gradient of N deposition (0, 20, and 40 kg N ha−1 yr−1) was simulated in a subtropical forest of southwestern China, to assess the potential effects of increased N deposition on the trophic structure and functioning of fauna communities in decomposing leaf litters of three main subtropical plant functional groups covering a total of 18 species: six evergreen broadleaf, six deciduous broadleaf, and six coniferous trees. We found that N addition shifted the trophic structure of fauna communities in decomposing litter, and different fauna feeding guilds showed distinct response patterns. Specifically, N addition increased the abundance of predators, decreased the abundance of omnivores, while detritivores were less affected. Compared to deciduous broadleaf and coniferous litters, evergreen broadleaf litter had less complex structure of fauna communities, but it decomposed slower. Further, structural equation modeling (SEM) showed that N addition increased litter decomposition in part via changing the trophic structure of fauna communities, with a positive correlation between predators and detritivores being associated with higher decomposition. As omnivores often exhibit top-down pressure on all other trophic guilds in (sub-)tropics, our observed shifts in trophic structure and litter decomposition might be partly due to the N addition-induced declines in omnivorous ant population. Future studies should investigate the role of bottom-up and top-down forces between detritivores, predators, and omnivores in driving litter decomposition. Collectively, our findings suggest that (i) N deposition consistently shifts the trophic structure of fauna communities across litter types, and (ii) such a shift further translate into changes in the functioning of subtropical forest ecosystems.