Effects of biochar on soil properties, heavy metal availability and uptake, and growth of summer squash grown in metal-contaminated soil

Pollution of agricultural soil with heavy metals is a serious hazard to the environment and human health. The application of biochar can be a cost-effective and environmentally friendly way to reduce heavy metal bioavailability in contaminated soils and thus their uptake by plants. A pot experiment was conducted to evaluate the effect of three types of biochar (mango, Casuarina and Salix as feedstocks) at two application rates (2% and 4% w/w) on soil characteristics, heavy metal availability in soil and bioaccumulation in plants, as well as the growth of summer squash grown in highly polluted soil. The results indicated that the highest dry weight of root and shoot was recorded at the lowest application rate of Casuarina biochar followed by mango and Salix biochars. Compared to untreated soil, the adding of Casuarina biochar at 4% decreased the concentrations of Cd, Co, Cr, Cu, Ni, Pb and Zn by 25.7%, 52.1%, 12.1%, 32.3%, 31.0%, 85.0% and 25.2% in the root, and by 37.2%, 66.9%, 24.3%, 40.2%, 42.0%, 89.2% and 35.5% in the shoot, respectively. Casuarina biochar at 4% treatment was found to be the most effective in reducing the uptake of heavy metals by roots and shoots. Also, this treatment had the highest significant reductions in the availability of heavy metals over the untreated soil. The reductions in bioconcentration factor and translocation factor achieved with biochar application varied significantly for different types and rates of biochars. All biochar applications increased significantly soil pH, organic matter and electrical conductivity compared to untreated soil. It can be concluded that Casuarina biochar has great potential as a soil amendment for increasing the heavy metal immobilization, reducing the availability and plant uptake of heavy metals, and enhancing the growth of summer squash plants.