Different Bacillus sp. play different roles on humic acid during lignocellulosic biomass composting

The yield of rice straw is increasing year by year, and composting is a common and effective treatment. The genus Bacillus has been widely used in rice straw composting because of its good ability to degrade lignocellulose. In addition, inoculation with Bacillus had an effect on the composting humification degree. However, the route, object, and extent of influence of different Bacillus sp. on the humification degree have not been clarified. In this study, four treatments were carried out for a 75-day experiment, including inoculated with Bacillus siamensis (H1) composting, inoculated with Bacillus halophilus (H2) composting, inoculated with Bacillus parahemolyticus (S1) composting, and composting without inoculation (CK) as a control. The effects of inoculation on precursors and humification indexes were investigated by chemical methods. Also, microbial changes were investigated by high-throughput sequencing. Finally, structural equation models (SEMs) were used to reveal the influence route of Bacillus sp. The humic acid (HA) concentration of CK compost, H1 compost, H2 compost and S1 compost increased by 39.88%, 43.39%, 57.42% and 51.83%, respectively. Network analysis showed differences in core microorganisms and microorganisms related to Bacillus among treatments. The SEMs indicated that H2 and S1 could not only directly affect humification ratio (HR), but also promote humification through their effects on bacterial communities. And H1 could only further affect HR by affecting the microorganisms involved. Collectively, inoculation with Bacillus could increase the HA content. Moreover, the influence routes of H2 and S1 are different from those of H1, with the former having two and the latter only one. Therefore, this study provided a theoretical basis for revealing the influence routes of different Bacillus sp. in the humification process. Meanwhile, it also alleviates the environmental pollution caused by the burning of straw waste, which is of great significance to environmental protection.