Limiting the mobility and phytoavailability of cadmium in paddy-upland soils after the application of various biochar fractions and proportions

This study delves into the pivotal role of biochar particle size and addition ratio in mitigating cadmium (Cd) contamination in paddy-upland soils, with a specific focus on curbing Cd uptake by Chinese cabbage (Brassica chinensis L.) during the pivotal transition from paddy to upland cultivation. Through a meticulously designed laboratory experiment, we uncovered that biochar's adsorption capacity for soil Cd is markedly lower than its maximum adsorptive capacity as determined by adsorption kinetics, potentially due to competitive adsorption by other cationic species present in the soil. This insight underscores the critical need to optimize biochar application rates in Cd-contaminated soils to effectively modulate Cd phytoavailability. Furthermore, our findings demonstrate that biochar not only effectively converts non-residual soil Cd fractions into a more stable "residual Cd" form but also diminishes the uptake of Cd by Chinese cabbage from the soil. Notably, the application of fine biochar (Φ₃ < 0.25 mm) at a 10 % application rate was particularly efficacious, reducing soil non-residual Cd concentration by 28.2 %, equivalent to approximately 1.79 mg/kg. This research introduces a novel perspective on biochar-mediated remediation of soil Cd contamination, emphasizing the critical influence of particle size and addition ratio on regulating Cd phytoavailability and enhancing food safety in agricultural soils under paddy-upland rotation cultivation affected by Cd contamination.