Objective This study aims to investigate the composition and assembly processes of bacterial communities during the processes of organic matter accumulation in newly reclaimed farmland, in order to provide a microbiological basis for guiding the rational application of organic materials.

Method A field experiment was conducted in a newly reclaimed paddy field from a slag site. Six treatments were set up, with the sole application of chemical fertilizer as the control (CK). Based on CK, additional treatments included straw alone (R), straw with a commercial activator (RS), and three different amounts of natural humus material (RSM1, RSM2, and RSM3). Soil organic matter (SOM), soil nutrients and bacterial community structure were measured. Deterministic versus stochastic community assemblage processes were quantified using βNTI scores.

Result The application of natural humus material combined with straw and activator significantly increased contents of SOM, readily oxidizable organic carbon, dissolved organic carbon (DOC), and microbial biomass carbon (MBC) compared to the control, and were also significantly higher than treatments with straw combined with activator or not. The application of natural humus material combined with straw and activator also significantly changed bacterial community structure compared with other treatments. The molecular ecological network of bacteria was dominated by cooperative relationships, with keystone taxa mainly from Actinobacteria and α-Proteobacteria. βNTI analysis indicated that the assembly of bacterial communities was primarily driven by stochastic processes, specifically homogenizing dispersal, accounting for 68.8% of the variation. In addition, the contribution of stochastic processes increased with SOM content. The βNTI value was significantly positively correlated with changes in DOC content and significantly negatively correlated with changes in available phosphorus content.

Conclusion The application of natural humus material in combination with straw and activator can rapidly increase the SOM content in newly reclaimed paddy fields from slag sites. Stochastic processes dominate the assembly of bacterial communities during the SOM enhancement. The labile carbon fraction (DOC) and available phosphorus are the key drivers for influencing the assembly of bacterial communities.