Spatial patterns and ecological drivers of soil nematode β-diversity in natural grasslands vary among vegetation types and trophic position

Understanding biogeographic patterns of community assemblages is a core objective in ecology, but for soil communities these patterns are poorly understood. To understand the spatial patterns and underlying mechanisms of beta-diversity in soil communities, we investigated the beta-diversity of soil nematode communities along a 3,200-km transect across semi-arid and arid grasslands. Spatial turnover and nested-resultant are the two fundamental components of beta-diversity, which have been attributed to various processes of community assembly. We calculated the spatial turnover and nested-resultant components of soil nematode beta-diversity based on the beta-partitioning framework. Distance matrices for the dissimilarity of soil nematode communities were computed using the 'Sorensen' method. We fitted negative exponential models to compare the distance decay patterns in nematode community similarity with geographic distance and plant community distance in three vegetation types (desert, desert steppe and typical steppe) and along the whole transect. Variation partitioning was used to distinguish the contribution of geographic distance and environmental variables to beta-diversity and the partitioned components. Geographic distance and environmental filtering jointly drove the beta-diversity patterns of nematode community, but environmental filtering explained more of the variation in beta-diversity in the desert and typical steppe, whereas geographic distance was important in the desert steppe. Nematode community assembly was explained more by the spatial turnover component than by the nested-resultant component. For nematode feeding groups, the beta-diversity in different vegetation types increased with geographic distance and plant community distance, but the nested-resultant component of bacterial feeders in the desert ecosystem decreased with geographic distance and plant community distance. Our findings show that spatial variation in soil nematode communities is regulated by environmental processes at the vegetation type scale, while spatial processes mainly work on the regional scale, and emphasize that the spatial patterns and drivers of nematode beta-diversity differ among trophic levels. Our study provides insight into the ecological processes that maintain soil biodiversity and biogeographic patterns of soil community assemblage at large spatial scales.