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.

This dataset originates from a long‐term nitrogen (N) enrichment experiment conducted in coastal poplar (Populus deltoides cv. “I-35”) plantations at the Dongtai Forest Farm, Jiangsu Province, eastern China (32°52′ N, 120°49′ E). The region has a temperate monsoon climate with a mean annual temperature of 14.9 °C and precipitation of ~1,050 mm.

The experiment was established in May 2012 in two stand ages (13- and 17-year-old plantations at the start of the experiment). A randomized block design included five N enrichment levels (0, 5, 10, 15, and 30 g N m⁻² yr⁻¹ as NH₄NO₃) with four replicate blocks per level. Each treatment subplot measured 25 × 30 m with 10 m buffers, and blocks were separated by ≥500 m. Nitrogen solutions were applied six times annually during the growing season (May–October) to simulate chronic atmospheric deposition; control plots received equal volumes of water only.

Isopod sampling was conducted across eight seasonal time points from August 2020 to June 2022 using pitfall traps (Ø7 cm, 14 cm depth) filled with saturated NaCl solution. Six traps per plot were deployed for seven days per sampling event. All terrestrial isopods (adults and juveniles) were counted and oven-dried (60 °C) to determine active density (individuals m⁻² day⁻¹) and dry biomass (g m⁻² day⁻¹).

Understory vegetation was surveyed concurrently in five 2 × 2 m quadrats per plot to quantify species richness, functional group cover (herbs, grasses, legumes), and biomass (oven-dried at 60 °C). Poplar leaf litterfall was collected in six 0.5 × 0.5 m traps per plot; forest floor detritus was collected from five 0.5 × 0.5 m frames per plot. All plant and detritus samples were dried to constant weight.

For C:N stoichiometry, isopod tissue (composite of 20 individuals per plot), poplar litter, understory vegetation, and detritus were ground and analyzed for total carbon and nitrogen using an Elementar Vario EL III elemental analyzer.

The accompanying R scripts include code for data cleaning, generalized linear mixed models (GLMMs), and piecewise structural equation modeling (SEM) used to evaluate the effects of N enrichment, stand age, and season on isopod density/biomass, plant diversity, and C:N ratios.