Divergent biogeography and assembly mechanisms of nematodes and arthropods in rice–wheat agroecosystems

Soil fauna play essential roles in regulating soil biogeochemical processes, yet their regional-scale biogeography and community assembly in intensively managed agroecosystems remain poorly understood. In particular, it is unclear whether dominant faunal groups exhibit shared or divergent assembly rules when exposed to the same environmental gradients. Here, we conducted a regional-scale survey across 230 rice–wheat rotation fields in eastern China to examine the spatial patterns, environmental drivers, and assembly mechanisms structuring the two dominant components of the soil faunal community, nematodes and arthropods. Faunal richness showed pronounced spatial structuring, but with contrasting taxon-specific patterns: nematodes exhibited a unimodal richness peak at the regional center, whereas arthropod richness was organized into spatially fragmented hotspots. While soil pH consistently shaped community composition across both groups, richness was regulated by distinct environmental constraints. Nematode richness reflected the combined influence of nitrogen availability and total potassium-associated abiotic stress, whereas arthropod richness was primarily constrained by pH-defined habitat conditions. Both communities displayed significant distance-decay relationships; however, nematodes combined stronger spatial turnover with a higher contribution of stochastic assembly, whereas arthropod communities were dominated by deterministic processes. These contrasting assembly patterns indicate that organismal traits modulate how soil chemical environments are translated into faunal community structure: Larger body size corresponds to stronger deterministic assembly, as microhabitat dependence and dispersal limitation enhance stochasticity in nematodes, whereas greater mobility and trophic integration strengthen deterministic filtering in arthropods. These findings reveal fundamentally divergent biogeographic patterns and assembly mechanisms between nematodes and arthropods, showing that soil-faunal diversity in agroecosystems is organized by lineage-specific assembly rules rather than universal environmental controls.