Composite microbial inoculants regulate community assembly to reduce nitrogen loss during biodrying-composting process.

Composite microbial inoculants (CMI) can improve composting efficiency and reduce nitrogen loss, but their microbial mechanisms in the biodrying-composting (BC) process, particularly community assembly and functional regulation remain unclear. This study investigated the effects of a self-developed inoculant, WH2, against three commercial inoculants (VJ, VT1000, SKH) during the biodrying-composting (BC) process. While all inoculants improved community assembly and composting efficiency, WH2 demonstrated superior performance. It reduced total nitrogen loss by 19% compared to CK, surpassing VJ (0.9%) and VT1000 (6.7%), and was second only to SKH (19.3%). Meanwhile WH2 also achieved the highest moisture removal rate (60.57 %), volatile solids degradation rate (37.67 %) and seed germination index (110.77 %). Mechanistically, neutral community model (NCM) and null model analyses revealed that stochastic community processes were dominated in WH2, accounting for 80% of community assembly. Notably, WH2 uniquely and persistently enriched key genera (Lactobacillus, Bacillus and Saccharomonospora) critical for degrading complex organics at different stages. NCM and PICRUSt2 predicted that WH2 promoted the migration of nitrogen-cycling bacteria and was associated with a lower relative abundance of the nirS gene (3.82 %-87.18 %) than that in other groups, suggesting reduced denitrification potential. These results highlight WH2's enhanced community regulation potential, directly linking its ability to shape stochastic assembly and keystone taxa with improving BC efficiency and nitrogen retention, offering a promising microbial solution for kitchen waste utilization.