Effect of Potassium Cations on Cadmium Bioavailability and Its Mechanisms in Lateritic Red Soils

Based on the importance of co-existingcations in soils that may change bioavailability of cadmium,the effect of applied different rates of K fertilizers on bioavailability of cadmium was investigated.Pot and isothermal adsorption experiments were explored to investigate the effect of the K fertilizers applied onto Cd-contaminated lateritic red soil on rape biomass,Cd accumulation,K and Cd concentrations in soil solution as well as adsorption of cadmium.The results show that,compared with the control,the rape dry mass increased to a great extent when grown on soils spiked with either low or high concentrations of K regardless of Cd levels,with the average increases of dry mass for two seasons being 16.6% for low level of Cd and 17.7% for high level of Cd.At either low or high levels of Cd,with the increase of K application,K concentration in soil solution increased,and K concentration in rape organs increased as well.At 195 mg/kg of K applied to the soil,the Cd concentration in soil solution reached up to the maximum,being by 2.80% and 23.7% for low and high levels of Cd,respectively,compared with the control.but Cd concentration in rape organs increased on average by 10% and 45.9%,respectively.Coexistence of K and Cd cations in soils may change the adsorption ability of Cd significantly.The total adsorption amount of Cd in K system decreased by 31.3% compared with the Na applied system.When potassium co-existed with cadmium,soil adsorption on Cd significantly affects Cd uptake by rape,and could be a key controlling factor that influences cadmium bioavailability.