Microbiome-mediated effects of Bacillus subtilis inoculants on rice straw decomposition, residue chemistry, and nutrient release in double-cropping paddy fields

Returning rice straw to fields is key for sustainable agriculture, yet its slow decomposition conflicts with tight schedules in double-cropping rice systems. Whether laboratory-effective microbial inoculants can consistently enhance straw decomposition and nutrient turnover in the field remains unclear. We conducted a two-season field experiment using nylon-mesh bags that prevented direct mixing of straw from soil-applied inoculants (laboratory-developed decomposing inoculant, LABIN; conventional commercial decomposing inoculant, COMIN, both Bacillus subtilis-based), thereby evaluating their primarily indirect effects via stimulation of the indigenous soil microbiome. Neither inoculant significantly increased net dry matter loss in either season ( p >0.05). However, LABIN exerted profound effects on nutrient release dynamics, it markedly elevated phosphorus release in the late-season period and persistently drove greater lignin degradation as well as unique structural alterations within straw residues, which was verified via Fourier transform infrared spectroscopy (FTIR). LEfSe analysis identified enrichment of putative decomposer taxa such as Vicinamibacter and Candida tropicalis , without affecting alpha diversity. Collectively, these results indicate that LABIN did not accelerate overall mass loss under conditions of limited direct inoculant–straw contact. Nevertheless, it altered straw residue chemistry and nutrient release, likely through selective shifts in the soil microbial community. This finding provides proof-of-concept evidence for a primarily indirect, microbiome-mediated mode of action of LABIN in paddy systems.