The effect of biochar nanoparticles on rice plant growth and the uptake of heavy metals: Implications for agronomic benefits and potential risk.

The interaction between biochar nanoparticles (nano-BC) and plant roots in the rhizosphere is largely unknown, although it is crucial for understanding the role of BC in plant growth and bioavailability of pollutants. The effect of nano-BC produced at a series of temperatures (300-600 °C) on alleviating the phytotoxicity of Cd2+ to rice plants was investigated from the aspects of biochemical changes and Cd uptake in this study. The kinetics of Cd2+ fluxes in different root zones in the presence of nano-BC were also measured using a scanning ion-selective electrode technique. We found that the high-temperature nano-BC could more significantly alleviate the phytotoxicity of Cd2+ than the low-temperature and bulk BCs as reflected by the higher increased biomass, root vitality, chlorophyll content, and decreased MDA content as well as relative electrical conductivity of rice plants, which is due to the high adsorption affinity of nano-BC for Cd2+. Also, for the first time we demonstrated that nano-BC could differentially affect the net flux of Cd2+ in different zones of the root tips. However, nano-BC (especially that produced at higher temperatures) more significantly increased the contents of antioxidative enzyme activities (e. g., SOD, POD, and CAT) and soluble protein than the treatment only with Cd2+ (5.0 mg/L), indicating that nano-BC could induce oxidative stress in the rice plants. These results indicate that nano-BC could greatly reduce the uptake and phytotoxicity of Cd2+, but its potential risk should not be overlooked during the environmental and agricultural applications of biochar.