Immobilization and bioavailability of heavy metals in greenhouse soils amended with rice straw-derived biochar

Pollution of greenhouse soils and crops by heavy metals as a result of annual application of large amounts of pesticides and fertilizers is of great concerns. We investigated the effect of biochar (BC) as a potential immobilizing medium for cadmium (Cd) by assessing Cd bioavailability, fractions, and uptake of metals by lettuce plants in metal-contaminated greenhouse soils. Experiments were conducted in two typical vegetable greenhouses displaying different levels of heavy metal pollution (GH1, lightly heavy-metal-polluted greenhouse; GH2, heavily heavy-metal-polluted greenhouse), in Wuhan City suburbs, Hubei Province, China. For each greenhouse site three replicates of each of three levels of BC (B0, 0 t ha−1; B10, 10 t ha−1; B20, 20 t ha−1) were arranged in a randomized complete block design. The application of BC increased soil pH and soil organic matter and changed Cd fractions in the soil. The exchangeable Cd fraction decreased and there was an increase in the oxide- and organic-bound and residual fractions with the application of BC, implying that BC application could immobilize the Cd in greenhouse soil. The rates of Cd immobilization in soil increased with the amount of BC applied. The exchangeable Cd fractions in soils amended with B10 decreased by 45–62% and in soils amended with B20 by 66–89% compared to the control. The addition of BC reduced the concentration of Cd in lettuce shoots in the lightly heavy-metal-polluted soils, but in the heavily heavy-metal-contaminated soils it had either no effect, or promoted Cd absorption by lettuce shoots. Total Cd measured in lettuce tissues did not decrease with the addition of BC when compared to the control, as there was increased accumulations of lettuce biomass with biochar application.