Pigeon Pea Biochar as a Soil Amendment to Repress Copper Mobility in Soil and Its Uptake by Spinach

A pot crop experiment was conducted to study the effect of biochar on Cu mobility in a soil-plant system. Pigeon pea biochar was prepared by slow pyrolysis at 300 °C. The experiment had three levels of Cu (0, 250, and 500 mg Cu kg-1 soil) and three levels of biochar (0, 2.5, and 5 g kg-1 soil), using spinach as the test crop. The dry matter yield of edible spinach leaf decreased by 16.7% and 27.9% at 250 and 500 mg Cu kg-1 soil concentration, respectively. The soil organic carbon (SOC) increased by 27.08% and 45.83% at 2.5 and 5 g kg-1 soil application of biochar, respectively. Cu mobility in soil was significantly reduced as a result of biochar application, as evident from the reduction in DTPA extractable Cu in soil, the transfer coefficient value (soil to plant), and the Cu concentration in the leaf and root. The increases in SOC and pH in the biochar amended soil affect copper dynamics because they control adsorption and precipitation on solid phase. Cu has higher affinity towards SOC and makes stable complexes, thereby decreasing the Cu mobility in soil. Adsorption and precipitation of heavy metals to solid phases and also increasing the negatively charged functional group due to increase in soil pH resulted in reduction of Cu mobility in soil