Cd immobilization and enhanced plant photosynthesis in wetlands by sulfur-modified biochar converted from Spartina alterniflora

Background: Converting the invasive species Spartina alterniflora into sulfur-modified biochar (SBC) offers a dual solution for managing biological invasions and remediating contaminated wetlands. This study investigated the efficacy and underlying mechanisms of SBC in remediating Cd-contaminated sediments and promoting native plant growth. Results: Mechanistically, sulfur modification introduced functional groups that significantly enhanced Cd adsorption and immobilization compared to unmodified biochar. In pot experiments, SBC application raised sediment pH and facilitated the transformation of Cd from the bioavailable acid-soluble fraction (decreased from 55.5 to 46.9%) to the stable residual fraction, thereby reducing ecological toxicity. Consequently, SBC alleviated Cd stress in plants by restricting Cd uptake and translocation (reduced bioaccumulation factor (BCF) and translocation factor (TF)). This reduction in tissue Cd burden mitigated oxidative damage (lower malondialdehyde and proline) and protected the photosynthetic apparatus, as evidenced by increased quantum yield and photochemical quenching, particularly under high Cd stress. Conclusions: These findings confirm that SBC functions through a “chemical immobilization–physiological protection” pathway, providing a sustainable strategy for coastal wetland restoration.