Combined organic amendments reduce Cd accumulation in double-cropping rice by restructuring soil bacterial communities.

Cadmium (Cd) contamination in paddy soils poses a serious threat to rice safety and human health. The combined application of milk vetch, rice straw, and sesbania biochar (MRFB) has shown promise in mitigating Cd pollution in paddy fields; however, the associated bacterial mechanisms remain insufficiently understood. Based on a long-term field experiment, this study systematically assessed the effects of MRFB on soil physicochemical properties, Cd bioavailability, and bacterial community structure, and their subsequent influence on Cd accumulation in a double-cropping rice system. The results demonstrated that the MRFB application significantly increased soil pH, nutrient availability (ammonium nitrogen, nitrate nitrogen, available phosphorus, and available potassium), and soil enzyme activities (urease, acid phosphatase, and catalase) compared with conventional fertilization (CF) in double-cropping rice fields. Meanwhile, MRFB treatment significantly (P < 0.05) reduced soil available Cd and rice grain Cd concentrations by 67.19% and 52.51%, respectively, in the early rice season, and by 55.66% and 57.50% in the late rice season. High-throughput sequencing revealed that MRFB reshaped the soil bacterial community and enriched key taxa including Thiobacillus and SC-I-84, which exhibited significant negative correlations with soil available Cd (P < 0.05). Network analysis further indicated that MRFB enhanced the complexity and stability of soil bacterial interaction networks. Random forest and partial least squares path modeling identified Thiobacillus as a key functional genus and confirmed that soil nutrient status was the primary driver suppressing Cd uptake by rice. Overall, the co-application of milk vetch, rice straw, and sesbania biochar reduced Cd accumulation in rice grains by improving soil properties, modulating bacterial community structure, and enhancing bacterially mediated Cd immobilization. Moreover, Cd immobilization efficiency was maintained across both rice seasons through enhanced bacterial network stability, providing a theoretical basis for the sustainable remediation of Cd-contaminated paddy soils.