Optimum organic fertilization enhances rice productivity and ecological multifunctionality via regulating soil microbial diversity in a double rice cropping system

Context: Double rice cropping system are crucial for sustainable food and security and agricultural ecosystem balance in South China. However, intensive chemical fertilization has reduced rice productivity, and soil and ecosystem degradation. Objective: To develop a suitable organic fertilization scheme for double-rice cropping systems and explain its association with soil quality index (SQI), microbial diversity and ecological multifunctionality (EMF). Methods: A 4-years field trial was conducted to examine the effects of four different organic materials (bio-organic fertilizer, OF; decomposed straw and manure, ST; biochar, BC and soil conditioner as silicon calcium magnesium fertilizer, SC) combined with chemical fertilizers (NPK) on rice yield, soil microbial abundance and diversity, SQI and EMF. Results: Compared with NPK, NPK.OF and NPK.ST in early rice, and NPK.OF and NPK.BC in late rice both resulted in higher rice yield and SQI by enhancing soil microbes, soil dissolved organic carbon (DOC), and carbon (C)- and nitrogen (N)-cycle enzyme activities. Additionally, optimum organic fertilization increased soil bacterial abundance in early rice and that of fungi in late rice. Proteobacteria, Acidobacteriota, Bacteroidota and Nitrospirota were the dominant microbial groups in both rice seasons. Specifically, NPK.OF and NPK.ST increased the abundance of Bacteroidetes and Proteobacteria, but suppressed that of Acidobacteria and Nitrosospira in early rice. Conversely, the fungal community showed no significant changes with organic fertilization in late rice. Microbial phylogenetic diversity (PD) of bacteria and fungi showed positive linear relationships with soil EMF in both rice seasons. Heatmap analysis indicated that Proteobacteria and Nitrospirae can serve as bioindicators of soil EMF in response to organic fertilization in early rice. This was related to the soil indices of bacterial PD, and DOC. Random forest and structural equation model analyses revealed that soil bacterial PD, DOC, and N-functional enzymes were the primary drivers and predictors of EMF. Conclusions: Soil bacterial diversity and its interactions with soil properties played an important role in determining rice productivity and EMF. Suitable fertilization management in the region include NPK.OF and NPK.ST for early rice, and NPK.OF and NPK.BC for late rice.