Straw Returning Strategies and Decomposing Inoculant Addition Shape Microbial Communities and Functional Potential in Black Soil

Straw returning methods and decomposing inoculants can jointly affect soil microbial communities and their functional potential. However, the differences in microbial responses under different straw returning methods remain unclear. In this study, a 180-day incubation experiment was conducted to compare the effects of straw mulching, shallow tillage, and deep tillage, with or without decomposing inoculant addition, on soil properties, microbial communities, CAZy families, and KEGG pathways. The results showed that straw mulching maintained the highest soil moisture content, reaching 19.31%. Under shallow tillage with inoculant addition, soil total organic carbon reached 27.31 g/kg, representing an 18.58% higher than in CK. This treatment also showed relatively high microbial community richness and diversity, with enrichment of Candidatus_Nitrosocosmicus, Clostridium, and Sphingobacterium. Under deep tillage with inoculant addition, soil total nitrogen reached the highest value of 1.227 g/kg, and Limnobacter, Candidatus_Nitrosotalea, and Sanguibacter were enriched. Additionally, inoculant addition was associated primarily with changes in GT and CBM families and amino acid biosynthesis-related pathways under shallow tillage, but with changes in CE and GH families and nitrogen metabolism-related pathways under deep tillage. Soil moisture was significantly associated with community composition, while total nitrogen and available nitrogen were significantly associated with KEGG functional profiles. Overall, the effects of decomposing inoculant addition on soil nutrients, microbial communities, and functional profiles varied across straw returning methods. Soil moisture and nitrogen availability emerged as important factors associated with these microbial and functional differences.