Mitigation of mercury accumulation in rice using rice hull-derived biochar as soil amendment: A field investigation

Effect of application of 24 t ha(-1) and 72 t ha(-1 )rice hull-derived biochar (RHB) on total Hg (THg) and methyl-Hg (MeHg) immobilization and their accumulations by rice plants were studied in a field experiment (Wanshan Hg mine, China). The addition of two doses of RHB significantly increased the biomass of rice plants, and decreased the MeHg concentration in the pore water, as compared to the control. The RHB promoted the partitioning of pore water MeHg to the soil solid phase throughout rice growing season, and pore water THg partitioning only at rice filling stage. Mercury methylation potential was weakly affected by the RHB addition to the soil. Mercury might be immobilized through binding of thiols (e.g., cysteine) presented in the RHB or in the soil induced by RHB addition. Biochar addition decreased MeHg and THg contents in the tissues of rice plants, particularly in the polished rice. We attributed the reduction of THg in the rice to the bio-dilution effect, and of MeHg content in the rice to the decreased MeHg availability in the soil by RHB addition. Results suggest that RHB might be suitable for managing Hg transfer in soil-rice plants at Hg contaminated mining regions in China and beyond.