Exogenous Bacillus drives organic matter transformation in wheat straw solid-state fermentation: Roles of DOM dynamics and microbial networks

Solid-state fermentation (SSF) of wheat straw is a sustainable approach to valorize lignocellulosic biomass, yet the molecular ecological mechanisms of organic matter (OM) transformation mediated by exogenous Bacillus remain unclear. This study investigated the dynamics of physicochemical/biological indicators, dissolved organic matter (DOM) fluorescent components and microbial communities during wheat straw SSF inoculated with three pure Bacillus strains ( B. subtilis , B. velezensis , B. stercoris ) and their composite agent (MB), using EEM-PARAFAC, high-throughput sequencing, correlation network analysis and structural equation modeling (SEM). The results showed that the inoculation of Bacillus promoted the conversion of labile low-molecular-weight (LMW) DOM to recalcitrant high-molecular-weight (HMW) humic-like DOM, thereby increasing the degree of humification. These changes contributed to the improvement of the germination index (GI). Among the tested Bacillus strains, B. velezensis exhibited the best comprehensive performance, achieving 26.3% OM degradation, 49.96% humic acid-like component C5, and a GI of 133%. Compared with the single-strain treatments, MB simultaneously enhanced the complexity of both bacterial and fungal co-occurrence networks. SEM confirmed microbial diversity directly drove directional DOM transformation by modulating biological indicators and environmental factors. This study uncovers the differential regulatory mechanism of exogenous Bacillus on microbial communities and DOM transformation, providing a theoretical and technical basis for designing high-efficiency microbial agents for lignocellulosic biomass valorization.