Effects of long-term N addition and seasonal changes on soil fauna community structure in a boreal forest

Soil fauna, as a key component of belowground ecosystems, are crucial to soil organic matter decomposition and trophic cycling. Community structure shifts serve as sensitive bioindicators of environmental change. As a primary driver of global change, nitrogen (N) deposition profoundly influences the structure and functional stability of terrestrial ecosystems. However, existing studies have predominantly focused on vegetation and microbial responses in mid- and low-latitude regions, leaving the influences of N deposition on soil fauna in cold-temperate forests understudied. Here, we conducted a 12-year field experiment of N addition (launched in 2011) with four application levels: control (CK, 0 g N m −2 yr −1 ), low nitrogen (LN, 2.5 g N m −2 yr −1 ), medium nitrogen (MN, 5.0 g N m −2 yr −1 ), and high nitrogen (HN, 7.5 g N m −2 yr −1 ). Soil fauna and environmental data were collected in 2023 to examine the response patterns of boreal forest soil fauna communities to N gradients and seasonal dynamics. We found that LN treatment significantly increased the abundance and richness of soil fauna, while HN treatment suppressed community development in the sampling year. Detritivorous and predatory groups were particularly susceptible to HN treatment, whereas herbivorous and omnivorous groups showed higher abundance under LN and MN treatments. Seasonal variations revealed that soil fauna community indices reached their peak in September (autumn) and reached their minimum in May (spring), with the interactive effect of N addition and season influencing the distribution pattern of detritivorous groups. Further analysis indicated that soil water content (SWC), plant community composition, and microbial biomass carbon (MBC) were the core drivers of community changes. This study suggests that moderate N input may enhance community structural stability by optimizing microhabitats, while excessive N addition appears to induce soil acidification, inhibit microbial activity, and alter plant communities, which is consistent with diminished soil fauna abundance and decreased functional group diversity observed in the sampling year. The findings provide a scientific basis for formulating management strategies to mitigate the impacts of increasingly severe N deposition in boreal forest ecosystems.