Chitosan-modified biochar and root exudates of rapes (Brassica napus L.) synergistically remediate cadmium-contaminated soil.

Cadmium (Cd) is a highly toxic heavy metal that inhibits the growth of rapes (Brassica napus L.) in contaminated soils. While chitosan-modified biochar (KC) shows potential to reduce Cd availability, its interaction with root exudates in regulating Cd speciation, soil physicochemical properties, and microbial community structure (in rhizosphere/non-rhizosphere soils) remains unclear. To elucidate KC's remediation mechanism for Cd-contaminated soils, we prepared KC by modifying biochar (derived from Firmiana simplex leaves) with chitosan, and conducted pot experiments with five treatments: uncontaminated control (CK), Cd (50 mg kg-1 Cd), chitosan + Cd (2 g kg-1 chitosan), biochar + Cd (10 g kg-1 biochar), and KC + Cd (10 g kg-1 KC). The results indicated that KC significantly enhanced rapes growth, with root length increasing by 213.78% relative to the Cd-only treatment. Additionally, KC altered the composition of root exudates by increasing microbial metabolic components such as tryptophan and humic acid, and elevated Cd concentrations within the soluble fraction of rapeseed subcellular structures, thereby mitigating Cd toxicity to organelles. In rhizosphere soils, KC treatment led to a 214.78% increase in soil organic matter (SOM) and a 160.19% enhancement in urease activity compared to the Cd treatment, thereby reducing exchangeable Cd content. Microbial community analysis revealed that KC, combined with root exudates, significantly increased the relative abundance of beneficial phyla (e.g., Proteobacteria, Actinobacteria), which mediate nutrient cycling and heavy metal resistance in rhizosphere soil. This study demonstrates that KC synergistically inhibits soil Cd bioavailability and rape Cd uptake by improving soil physicochemical properties (elevated SOM and urease activity) to stabilize Cd, regulating root exudate components to modulate Cd subcellular distribution in rapes, and enhancing beneficial rhizosphere microbes to optimize the growth environment. These findings confirm KC as an effective amendment for Cd-contaminated agricultural soils.