Different bacterial co-occurrence patterns and community assembly between rhizosphere and bulk soils under N addition in the plant–soil system

AimsExtensive studies have demonstrated a significant impact of nitrogen (N) addition on microbial community diversity, composition, and functions in various soils. However, microbial co-occurrence patterns and assembly processes under enhanced N levels is less known, especially with respect to the rhizosphere where microbes are closely linked with plant growth.MethodsIn this study, we performed a pot experiment using N addition gradient (0, 2.5, 5, 7.5, 10, and 15 g N·m−2·year−1) and amplicon sequencing and metabolomics to investigate the effect of short-term N addition (2 years) on the co-occurrence patterns and assembly process of bacterial communities as well as their potential drivers in the rhizosphere.ResultsShort-term N addition significantly altered bacterial community diversity and composition in the rhizosphere soil but not in the bulk soil; i.e., the rhizosphere bacterial diversity increased with N addition but decreased under high N input (>5 g N·m−2·year−1). The rhizosphere co-occurrence networks showed a more complex topology than the bulk soil, and this complexity could be strengthened by the quantity of N added. The N-induced establishment of the bacterial community in the rhizosphere was mainly dominated by deterministic processes with an increase in the variable selection in response to the increase in the N-addition rate, while the bacterial community in the bulk soil exhibited a transition from stochastic to deterministic processes. N addition stimulated the release of amino acids and short-chain organic acids into the rhizosphere, and these changes in root exudates and the soil NH4+-N level were mainly responsible for the changes in the structure of the rhizosphere bacterial community and co-occurrence networks of keystone species.ConclusionsOur results demonstrated the differences in the co-occurrence patterns and assembly processes of bacterial communities between rhizosphere and bulk soils under N addition, which were closely associated with root exudates. These findings improve our understanding of plant–microbial interactions.