Contrasting impacts of nitrogen enrichment on soil nematode diversity in natural and managed ecosystems

Abstract Soil nematode communities are increasingly subjected to pressures from multiple global change drivers, such as nitrogen (N) enrichment and land management practices. Although the critical role of N inputs in regulating soil nematode communities has been well studied, the contrasting responses of soil nematode diversity to N enrichment in natural versus managed ecosystems remain poorly understood. To address this knowledge gap, we conduct a global meta‐analysis using 3323 paired observations from 173 publications to quantify the impacts of mineral N inputs on the richness and abundance of soil nematode diversity across natural ecosystems (e.g. unmanaged grasslands and forests) and managed ecosystems (e.g. croplands). N enrichment significantly reduced the richness and abundance of soil nematode communities in natural ecosystems, primarily driven by the prohibiting effects of N enrichment‐induced soil ammonium toxicity and soil acidification on the abundances of plant‐feeding, fungal‐feeding and omnivorous‐carnivorous nematodes. In contrast, while N enrichment reduced the taxon richness of soil nematodes in managed ecosystems, it did not diminish their total abundance. This discrepancy may be explained by the increased soil microbial biomass under N enrichment, which favoured the dominance of bacterial‐feeding nematodes. These nematodes thrived at the expense of other trophic guilds with low resource competitiveness and high N sensitivities, leading to a loss of species diversity but maintaining overall community abundance. Furthermore, the responses of soil nematode richness and abundance to N enrichment in managed ecosystems were not regulated by N addition regimes and climate factors. This suggests that management practices may override the constraints imposed by climate change on nematode diversity. Synthesis and application. Our findings demonstrate that N enrichment exerts a greater negative impact on soil nematode diversity in natural ecosystems compared with managed cropping systems, which arises from the distinct responses of different soil nematode trophic guilds to management practices and environmental changes. Understanding the mechanisms underlying the contrasting effects of N enrichment on soil nematode diversity in natural versus managed ecosystems is critical for enhancing the ecological resilience of soil food webs and sustaining soil biodiversity in the face of global change.