Crop-specific mechanistic shifts of lignin biochar in reducing cadmium accumulation over two continuous cropping

While biochar is recognized for immobilizing cadmium (Cd) in soil, its long-term effectiveness and specific mechanisms for immobilizing Cd in sequential monocultures-particularly the differences between flooded and upland systems-remain largely unexplored. This study systematically investigated the persistence and mechanistic shifts of lignin biochar (LBC) in reducing Cd bioavailability and accumulation in two contrasting crops (rice and pak choi) over two continuous cropping. Results demonstrated that LBC application persistently suppressed Cd accumulation in edible parts, even in the second season, with reduction rates of 21.01%-47.21% in rice grains and 24.42%-55.40% in pak choi leaves. The mechanisms, however, revealed significant crop and season specificity. For rice, the primary mechanism shifted from a synergistic effect of soil immobilization (significantly increasing the OM-Cd and OX-Cd fractions) and root retention in the first season, to the formation of an internal "stem-leaf translocation barrier" that effectively hindered Cd transfer to the grains in the second season. In contrast, soil immobilization and root interception remained the consistent mechanism for pak choi across two seasons. A health risk assessment confirmed the practical effectiveness, showing LBC significantly reduced the target hazard quotient (THQ), with a maximum decrease of 67.8%. This study underscores LBC's potential as a sustainable and effective strategy for ensuring food safety in Cd-contaminated soils.