Soil Organic Carbon and pH Interactions Mediate N Impacts on Nematode Abundance and Composition

Human activities have dramatically increased nitrogen (N) inputs to terrestrial ecosystems, with cascading effects on soil biodiversity and function. Soil nematodes, the most abundant animals on Earth, play critical roles in nutrient cycling and ecosystem health. Although numerous studies have examined N effects on soil nematodes, crucial knowledge gaps remain regarding how these varying responses depend on fundamental soil properties like organic carbon (C) content and soil acidity, particularly across soils with contrasting baseline properties under identical fertilization regimes. Through a long-term field experiment with soils differing in organic C contents, here we show that baseline soil conditions strongly affect the effects of N enrichment on soil nematode communities. Specifically, N addition increased nematode abundance in C-rich soils, while significantly reducing it in C-poor soils through inducing soil acidification. Soil pH emerged as a critical filter regulating nematode responses to N enrichment, determining which trophic groups thrived or declined and thereby reshaping soil C cycling pathways. Results from a global meta-analysis further reveal that nematode abundance exhibited a hump-shaped relationship with soil pH, peaking at 5.9-6.0, while increasing monotonically with SOC. Together, our results demonstrate that local soil properties mediate N impacts on soil food webs more strongly than N input alone. By linking N inputs with nematode community shifts through measurable soil properties, our work provides a framework for predicting how global changes might alter belowground ecosystems. This knowledge is crucial for developing sustainable agricultural practices that maintain soil biodiversity while meeting crop production demands.