Molecular Insights Into PGPR-Biochar–Mediated Arsenic Detoxification and Nutritional Enhancement in Rice Rhizospheres

Arsenic (As) contamination in paddy soils threatens rice productivity and food safety through enhanced As(III) mobility under anaerobic conditions. This study evaluated the synergistic effects of rice biochar (RBC) and plant growth-promoting rhizobacteria (PGPR) on arsenic bioavailability and rice physiological responses under controlled and field conditions. Rice straw biochar pyrolyzed at 400 °C exhibited high porosity and surface functionality, while Metabacillus indicus AMB4 demonstrated arsenic tolerance (≤3500 mg kg ^-1 ) and multiple PGPR traits (IAA: 45.2 μg mL ^-1 , phosphate solubilization index: 2.8, ACC deaminase: 18.7 nmol α-ketobutyrate mg ^-1 h ^-1 ). Combined RBC+PGPR treatment reduced soil arsenic bioavailability by 85% (p<0.001) and plant arsenic accumulation by 78% compared to controls, while significantly enhancing antioxidant enzyme activities (CAT: 4.2-fold, SOD: 3.7-fold, APX: 3.1-fold increase). Metagenomics revealed restructured microbial communities with increased Proteobacteria abundance and upregulated arsenic detoxification genes (arsC, arsB, aioA) coupled with enhanced nutrient cycling pathways (nifH, phoD, soxA). Network analysis demonstrated functional integration between As-transformation and nutrient metabolism, with coordinated expression of 47 arsenic-responsive and 126 nutrient-cycling genes. Field trials confirmed superior plant performance with RBC+PGPR treatment yielding 56% more tillers and 73% more panicles than controls, while maintaining enhanced soil nutrient status. This integrated biochar-PGPR system provides a mechanistically robust approach for simultaneous arsenic remediation and crop productivity enhancement in contaminated rice ecosystems, demonstrating practical applicability for sustainable paddy agriculture.