Effects of modified sawdust and rice husk biochar on cadmium forms in soil and cadmium uptake by rice

Objective The contamination of farmland by cadmium（Cd）poses a serious threat to human health.This study aims to assess the effects of different modified biochars on the morphological changes of Cd in Cd-contaminated farmland in southern China and its enrichment characteristics in rice，thus providing a scientific foundation for the safe utilization of such soils. Method Wood chips and rice husks were used to produce wood chip biochar（WCBC）and rice husk biochar（RHBC）.With the modification by KOH and K2SiO3，four biochars were obtained，namely KOH-modified wood chip biochar（WCBC-KOH），rice husk biochar（RHBC-KOH），K2SiO3-modified wood chip biochar（WCBC-K2SiO3）and rice husk biochar（RHBC-K2SiO3）.A pot experiment was conducted to investigated the effects of different biochars and addition rates（0.25%，0.5%，1%），including soil chemical properties（pH and available silicon），Cd forms，rice biomass，and Cd accumulation and silicon contents in various rice tissues. Result All six types of biochar increased the pH and available silicon content of paddy soil，with higher addition rates resulting in higher increase.The addition of biochar reduced the exchangeable Cd content in the soil，while increasing the residual Cd content.This resulted in a significant reduction in the bioavailability and mobility of Cd in the soil，subsequently decreasing the uptake of Cd by rice.Additionally，the addition of biochar enhanced the aboveground biomass and yield of rice，while significantly increasing the silicon content in various rice tissues.Correlation analysis indicates a highly significant negative correlation between soil pH and available silicon content with exchangeable Cd and Cd content in rice tissues.Soil pH had a greater effect on Cd reduction than that of available silicon in the soil. Conclusion All six types of biochar are effective in transforming Cd in the soil into less available forms for plants，while promoting rice growth and increasing rice yield.The effect of biochars modified with KOH and K2SiO3 is better than unmodified biochar.