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

Aims Extensive 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. Methods In this study, we performed a pot experiment using N addition gradient (0, 2.5, 5, 7.5, 10, and 15 g N center dot m(-2)center dot 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. Results Short-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 center dot m(-2)center dot 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. Conclusions Our 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.

Automated summary

Our study investigated the effect of short-term nitrogen (N) addition on the co-occurrence patterns and assembly processes of bacterial communities in the rhizosphere. We found that N addition significantly altered bacterial community diversity and composition in the rhizosphere soil, and the co-occurrence networks in the rhizosphere were more complex compared to the bulk soil. The establishment of the bacterial community in the rhizosphere was mainly influenced by deterministic processes, while the bulk soil showed a transition from stochastic to deterministic processes. N addition also affected the release of root exudates and soil NH4+-N levels, which played a role in shaping the structure of the rhizosphere bacterial community and co-occurrence networks.