Organic-inorganic fertilization promotes paddy soil macroaggregate organic carbon accumulation associated with key bacterial populations in the subtropical area of China

Macroaggregate organic carbon (Macro-OC) accumulation in paddy soil is of great significance in promoting multiple agroecosystem services. However, the effects of different fertilization practices on Macro-OC accumulation in paddy soil at the regional scale have not been comprehensively investigated. Here, we selected four long-term fertilization field experiments at Taoyuan, Wangcheng, Jinxian, and Suzhou in the subtropical area of China to reveal the effects of inorganic fertilization and organic-inorganic fertilization on Macro-OC accumulation and the relationships with important microbial traits in paddy soil. The results showed that long-term fertilization (particularly organic-inorganic fertilization) significantly increased the content of Macro-OC, which mainly consisted of particulate organic carbon (POC). Organic-inorganic fertilization decreased the percentage of O-alkyl C but increased the percentages of alkyl C, aromatic C, and phenolic C. Organic-inorganic fertilization promoted the abundance of bacterial cellulose-degrading gene retrieved from macroaggregate. At the order level, Anaerolineales, Bacillales, and Clostridiales were identified as keystone bacterial taxa in macroaggregate, and significantly correlated with the physical fraction and chemical structure of Macro-OC. Structural equation modeling revealed that fertilization-induced changes in soil pH and C:N ratio affected the richness of Anaerolineales, Bacillales, and Clostridiales, which was strongly associated with the increase of percentages of aromatic C and phenolic C and further facilitated Macro-OC accumulation. Together, these results suggested that organic-inorganic fertilization promotes Macro-OC accumulation associated with key bacterial populations in paddy soil. The results provide an important basis for boosting soil carbon accrual in the subtropical rice-growing areas.