Canopy nitrogen addition affects ground‐dwelling spider assemblages and trophic position

Abstract

Abstract While it is clear that nitrogen (N) deposition impacts the plant diversity of major terrestrial ecosystems, how ground‐dwelling fauna respond to N deposition is less understood. Here, we use a novel experimental methodology of canopy addition of N (CAN) to simulate realistic N deposition and contrast this method to standard understory addition of N (UAN) in a temperate forest and a subtropical forest. We investigated the shifts in community composition and trophic position for ground‐dwelling spiders under different N deposition treatments. We found that the CAN deposition treatment increased the diversity of sit‐and‐wait spiders and the abundance of actively hunting spiders at the subtropical forest, but N treatments had no effect on spider abundance or diversity at the temperate forest. In contrast, UAN treatments had no significant impact on spider communities compared to control treatment. Furthermore, N treatments had significant effects on spider assemblage at subtropical forest. Overall, spiders in the CAN treatment had a higher trophic position as indicated by δ15N isotopic signatures at both forest types. In conclusion, our study suggests that the effects of N deposition on ground‐dwelling spider assemblages depend on N addition approaches. Furthermore, our results suggest that CAN is a more realistic method for understanding the effects of atmospheric N deposition on soil arthropods in forest ecosystems.