Body size determines soil multitrophic diversity along a litter stoichiometry gradient in agroecosystems

Abstract Plant litter retention is increasingly recognized as an effective strategy for restoring soil biodiversity and related functions in agroecosystems. However, how litter stoichiometry (e.g. carbon‐to‐nitrogen, C:N) influences soil biodiversity across multiple trophic groups and their assembly mechanisms remains poorly understood. To inform crop residue management for biodiversity conservation, we investigated the diversity and assembly processes of soil bacteria, fungi and nematodes (i.e. bacterivores, fungivores and predators) in a field experiment where six types of plant litter were manipulated to create a broad gradient of litter C:N stoichiometry. We particularly focused on community β‐diversity, which is important in capturing the variability in species composition among different locations. The results revealed that with increasing litter C:N stoichiometry, the β‐diversity of microbivores declined and that of predators slightly increased, contrasting with the fluctuating patterns in microbial β‐diversity, thereby leading to greater community homogenization at higher trophic groups with larger body sizes. The community assembly of soil microorganisms (bacteria, fungi) was mostly influenced by stochastic processes, whereas that of soil nematodes (bacterivores, fungivores and predators) was more deterministic, with soil nutrients and habitat (i.e. soil moisture and aggregates) serving as key environmental drivers. The importance of selection processes relative to stochastic processes increased with organism body size and became more pronounced with increasing litter C:N ratio. Synthesis and applications . Our results suggest that litter C:N ratio exerts contrasting effects on community assembly across multiple trophic groups of different body sizes, emphasizing the need to incorporate life‐history traits into biodiversity conservation strategies. These findings provide a robust applied field test of ecological theory linking litter stoichiometry to biodiversity conservation and management.