Shrub encroachment induced by global change and human disturbance strongly affects ecosystem structure and function. In this study, we explore the degree to which invading leguminous shrubs affected neighboring grasses, including via the transfer of fixed nitrogen (N). We measured N concentrations and natural abundance (15)N of shoot tissues from three dominant grasses from different plant functional groups across seven distances along a local transect (up to 500 cm) to the leguminous shrub, Caragana microphylla. C. microphylla did transfer fixed N to neighboring grasses, but the amount and distance of N transferred were strongly species-specific. Shoot N concentrations decreased significantly with distance from C. microphylla, for a rhizomatous grass, Leymus chinensis, and a bunchgrass, Achnatherum sibiricum. However, N concentrations of another bunchgrass, Stipa grandis, were higher only directly underneath the shrub canopy. Shoot δ(15)N values of L. chinensis were enriched up to 500 cm from the shrub, but for S. grandis were enriched only below the shrub canopy. In contrast, δ(15)N of A. sibiricum did not change along the 500-cm transect. Our results indicated the rhizomatous grass transferred fixed N over long distances while bunchgrasses did not. The presence of C. microphylla increased the shoot biomass of L. chinensis but decreased that of S. grandis and A. sibiricum. These findings highlight the potential role of nutrient-acquisition strategies of neighboring grasses in moderating the interspecific variation of fixed N transfer from the leguminous shrub. Overall, leguminous shrubs have either positive or negative effects on the neighboring grasses and dramatically affect plant community composition and structure.

The existence of biogeographic patterns among most free-living microbial taxa has been well established, yet little is known about the underlying mechanisms that shape these patterns. Here, we examined soil bacterial β-diversity across different habitats in the drylands of northern China. We evaluated the relative importance of environmental factors versus geographic distance to a distance-decay relationship, which would be explained by the relative effect of basic ecological processes recognized as drivers of diversity patterns in macrobial theoretical models such as selection and dispersal. Although the similarity of bacterial communities significantly declined with increasing geographic distance, the distance-decay slope and the relative importance of factors driving distance-decay patterns varied across different habitats. A strong distance-decay relationship was observed in the alpine grassland, where the community similarity was influenced only by the environmental factors. In contrast, geographic distance was solely responsible for community similarity in the desert. Even the average compositional similarity among locations in the desert was distinctly lower compared with those in other habitats. We found no evidence that dispersal limitation strongly influenced the β-diversity of bacterial communities in the desert grassland and typical grassland. Together, our results provide robust evidence of habitat specificity for microbial diversity patterns and their underlying drivers. Our findings suggest that microorganisms also have multiple drivers of diversity patterns and some of which may be parallel to some fundamental processes for explaining biodiversity patterns in macroorganisms.