Long‐term nitrogen input alters plant and soil bacterial, but not fungal beta diversity in a semiarid grassland

Anthropogenic nitrogen (N) input is known to alter plant and microbial α‐diversity, but how N enrichment influences β‐diversity of plant and microbial communities remains poorly understood. Using a long‐term multilevel N addition experiment in a temperate steppe, we show that plant, soil bacterial and fungal communities exhibited different responses in their β‐diversity to N input. Plant β‐diversity decreased linearly as N addition increased, as a result of increased directional environmental filtering, where soil environmental properties largely explained variation in plant β‐diversity. Soil bacterial β‐diversity first increased then decreased with increasing N input, which was best explained by corresponding changes in soil environmental heterogeneity. Soil fungal β‐diversity, however, remained largely unchanged across the N gradient, with plant β‐diversity, soil environmental properties, and heterogeneity together explaining an insignificant fraction of variation in fungal β‐diversity, reflecting the importance of stochastic community assembly. Our study demonstrates the divergent effect of N enrichment on the assembly of plant, soil bacterial and fungal communities, emphasizing the need to examine closely associated fundamental components (i.e., plants and microorganisms) of ecosystems to gain a more complete understanding of ecological consequences of anthropogenic N enrichment. Increasing nitrogen (N) deposition is an important component of global environmental change. Our work provides novel empirical evidence that N deposition differentially influences the assembly trajectories of communities of plants and their co‐occurring soil microbes. With increasing N input, plant communities are increasingly governed by deterministic processes and fungal communities remain largely regulated by stochastic processes, whereas the importance of stochastic processes first increases then decreases for bacterial communities. These contrasting patterns indicate differences in the assembly mechanism of plant and soil microbial communities, with important implications for understanding aboveground–belowground interactions under anthropogenic N deposition.