Nitrogen addition enhances the bottom-up effects in the detrital food web

Bottom-up forces play a critical role in structuring communities and trophic interactions in detritus-based food webs, yet the impact of global change factors on these forces has been seldom explored. Here, we investigated the bottom-up effects of N-addition concentrations and methods (canopy and understory addition of N) on the trophic structure of detrital food webs comprising three trophic levels (saprotrophic fungi, Collembola and spiders) in a temperate and subtropical forest. At both forests, we found that high N concentrations significantly increased the concentration of litter N and saprotrophic fungi, and also increased Isotomidae collembolan density, but did not cascade up to the spider predator level. In contrast, the N addition methods had no significant impact on any component of trophic structure at either forest. In conclusion, N concentration significantly influenced trophic structure by bottom-up effects.