Accumulation and distribution of zinc in rice plants (Oryza sativaL.) growing in zinc contaminated paddy soils with biochar

At low concentrations zinc (Zn) is essential for plants and animals, but it becomes toxic when it exceeds a critical level. This study aimed to explore the effects of biochar on the remediation of Zn-polluted soil and the distribution of Zn in rice plants using pot experiments. Various amounts of biochar (0% (control), 2.5%, and 5% w/w) derived from rice straw were mixed with Zn-contaminated soil collected from farmland near a mine. The results showed that biochar enhanced the biomass of rice tissues and significantly reduced the concentration of Zn in brown rice. These results could be attributed to the combination of several effects: (1) Biochar increased the soil pH and organic carbon content, resulting in a reduction in the concentration of soil available Zn and transforming forms of immobilized Zn from acid soluble to reducible. (2) Biochar addition decreased the amorphous iron and complex iron concentrations in the soil solution and reduced the formation of root Fe plaques as well as the storage of Zn in iron plaques, thus diminishing the probability of Zn transfer from root surfaces to tissues. (3) Biochar played a key role in abating the toxic effect of Zn in rice plants as it decreased the transfer of Zn from roots to leaves and promoted Zn storage in the vacuoles of root cells. These results suggest that the application of biochar to Zn-contaminated soil is effective in reducing Zn accumulation in rice grains and can enhance rice production.