Integrated application of Bacillus vireti and sawdust biochar enhances soybean productivity and mitigates arsenic uptake

This study evaluated the integrated effects of Bacillus vireti inoculation and sawdust biochar (SB) on soybean growth, productivity, seed nutritional quality, and arsenic (As) mitigation over two cropping seasons in As-contaminated soils of the Mekong Delta. Molecular analysis confirmed Bacillus vireti as an efficient nitrogen-fixing endophyte with high nitrogenase activity and strong biochemical adaptability. Field results demonstrated that both Bacillus vireti and SB significantly improved agronomic traits including plant height, branch number, chlorophyll content, and biomass. These enhancements were more pronounced in the second cropping season due to cumulative improvements in soil physicochemical conditions. Yield attributes also responded positively, with pod number, filled pod weight, seed weight, and overall yield increasing under both factors. The highest productivity was achieved with 12 t ha -1 SB combined with Bacillus vireti , reaching 3.54 t ha -1 in the second season. Seed nutritional quality improved similarly; lipid and protein contents increased across all treated conditions, with maximum values observed under the highest SB rate. A key finding is the marked reduction in As accumulation. Inoculation reduced seed As concentration by 60–85%, while SB particularly at 12 t ha -1 lowered As in seeds to 14.7 μg kg -1 in crop 1 and to undetectable levels in crop 2. The combined treatment showed the strongest mitigation effect, suggesting a synergistic mechanism involving biochar-driven immobilization and microbial transformation of As. Overall, the results highlight the potential of integrating Bacillus vireti and SB as a sustainable approach to enhance soybean yield and safety in metal-affected agricultural systems.