Effect of rice husk biochar on lettuce Cd uptake and soil fertility

Biochar application has become a promising technology for remediation of soil heavy metal pollution due to potential beneficial effects on contaminated soils. Previous studies have largely dealt with the properties of biochar, the improvement of soil fertility as well as the process of remediation of heavy metal polluted soils. Rice husk biochar has also been reported to be used in the solidification of Cd and Pb in solid wastes and the adsorption of Cd in waste waters. However, little has been documented on the changes in forms of soil Cd, Cd content in vegetables and soil nutrient availability after the application of rice husk biochar in Cd contaminated soil. Thus two successive lettuces (Lactuca sativa L.) crops were planted in a pot experiment with Cd of 4.564 mg(CdSO₄)·kg ^-1(air-dried soil) to explore the effects of different doses of rice husk biochar on lettuce shoot and root Cd concentration, soil nutrient and Cd forms. Biochar dosage levels used in the study were 5 g kg^-1, 10 g kg^-1, 15 g kg^-1, 20 g kg^-1 and 25 g kg^-1, with none application of rice husk biochar as the control. The results showed that compared with the control, rice husk biochar reduced Cd concentration in the shoots and roots of the two lettuce crops. 25 g kg 1 rice husk biochar showed the best effects, respectively decreased Cd concentration by 19.6% and 45.8% in shoots, and by 36.8% and 28.0% in roots, for two crops. At this dose, soil pH and soil available P, available K and organic matter contents increased by a maximum of 4.0%, 19.6%, 44.0% and 48.5%, respectively. Also soil alkali-hydrolyzable N content significantly decreased by 19.7%. Rice husk biochar also changed the forms and availability of soil Cd. With increasing dose of rice husk biochar, the concentrations of ammonium acetate extractable Cd and weak acid soluble Cd in the soil decreased by 17.9% and 10.4%, while oxidisable Cd concentration decreased with a residual Cd concentration increase of 17.6%. It was apparent that enhancement of soil pH, reduction of soil available Cd content and increase in residual Cd content were the main processes by which rice husk biochar reduced Cd content in lettuce. Therefore rice husk biochar could be used as an effective element for soil amendment that inhibit Cd uptake by vegetables in Cd contaminated soil.