Nitrogen enrichment promotes isopods through reduced carbon and nitrogen stoichiometric mismatch with understorey plants

As key detritivores and fungal grazers, terrestrial isopods (Isopoda: Oniscidea) play crucial roles in mediating ecosystem processes. Although nitrogen enrichment represents a major global change driver known to modify soil food webs, its long‐term effects on the abundance of these keystone detritivores remain largely unknown. For this study, we conducted a 10‐year nitrogen enrichment experiment to monitor the active density and biomass of terrestrial isopods across eight seasons over 2 years during 2020 to 2022 in 13‐ and 17‐year‐old poplar plantations, respectively. Our results revealed that nitrogen enrichment increased the abundance and biomass of terrestrial isopods. For nitrogen enrichment levels of 5, 10, 15 and 30 g N m −2 year −1 , the corresponding increases in isopods active density were estimated to be 11.6%, 24.5%, 38.9% and 92.9% higher, respectively, relative to the ambient N level. Furthermore, nitrogen enrichment did not alter the carbon to nitrogen ratio (C:N) of the body of isopods, but reduced the C:N of poplar leaf litter, understorey plants and detritus. Nitrogen enrichment also reduced the understorey plant diversity and altered understorey plant species composition. The structural equation models revealed that the responses of isopod abundance to nitrogen enrichment were always inversely related to the C:N of the understorey community, while showing no correlation with poplar leaf litter quality. This study reveals a new mechanism by which nitrogen enrichment promotes soil invertebrates by affecting the quality of the understorey rather than the overstory. Our results suggest that long‐term nitrogen enrichment affects soil detritivores through seasonally dependent pathways mediated by the understorey plant community. This reveals a previously unrecognized mechanism through which nitrogen deposition influences soil food web structure. Read the free Plain Language Summary for this article on the Journal blog.