Linking soil bacterial community assembly with the composition of organic carbon during forest succession

Changes in the composition of organic carbon lead to shifts in the selective environment, which in turn contribute to changes in the composition and assembly process of soil bacterial communities at different successional stages. Deterministic and stochastic processes are expected to clarify bacterial community assembly process during long-term ecosystem recovery. Therefore, by studying the bacterial community of the organic horizon and mineral horizon during forest succession, the main role of organic carbon composition in determining soil bacterial community structure and assembly process during forest succession in the arid regions of Loess Plateau, China was revealed. Stochastic assembly was predominant in the organic horizon (75.0%) and mineral horizon (71.6%), and its relative contribution of stochastic assembly was similar in two horizons. In addition, network analyses demonstrated that bacterial nodes (as individual taxa) and connectivity between them was higher in the organic horizon (nodes: 297 and edges: 5242) than mineral horizon (nodes: 251 and edges: 1694). During forest succession, the bacterial community gradually changed to a K-strategy, and the degree of bacterial network connection cross the organic horizon and mineral horizon gradually decreased. The taxon- organic carbon composition cooccurrence network aided in identifying the relationship between the composition of organic carbon and the bacterial community, and ploysaccharide, methyl and aliphatic carbon were the main reason for the differences in the bacterial community. Meanwhile, the relationship between the composition of organic carbon in bacterial assembly was different in the organic horizon and mineral horizon: ploysaccharide, methyl, aliphatic, aromatics and carboxylic carbon were the main factors driving bacterial assembly in the organic horizon, whereas the interaction of them with bacterial traits jointly participated the assembly process in the mineral horizon. Our results highlight the importance of resource availability, particularly the composition of organic carbon, in determining how the microbial community composition assembles during forest succession. • Bacterial network connectivity decreased with forest succession. • Stochastic processes dominated microbial community assembly in the organic horizon. • The relative influence of deterministic processes increased in the mineral soil layer. • Organic carbon structures were essential drivers of organic versus mineral soil layer bacterial communities.