Data from: Nitrogen enrichment promotes isopods through reduced carbon and nitrogen stoichiometric mismatch with understory 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 detritivore 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 two 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 yr⁻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, understory plants, and
detritus. Nitrogen enrichment also reduced the understory plant diversity
and altered understory 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 understory
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 understory rather than
the overstory. Our results suggest that long-term nitrogen enrichment
affects soil detritivores through seasonally dependent pathways mediated
by the understory plant community. This reveals a previously unrecognized
mechanism through which nitrogen deposition influences soil food web
structure.