The effect of phosphate application in Pb-contaminated soil on the oxidative stress of leaves, Pb accumulation in biomass of maize and Pb speciation in rhizosphere soil

Abstract

Reducing the bioavailability of Pb in soil is the key to alleviate the toxicity of Pb to plant. Maize were exposed to Pb 100 mg/kg of soil with three fertilizer levels of control (T1), nitrogen, phosphorus and potassium of 204 mg/kg (T2) and nitrogen, potassium of 204 mg/kg (T3). The phosphate supplement lead to the reduction by 24.92%, 29.73% and 25.31% respectively in activity of total superoxide dismutase (T-SOD), peroxidase (POD) and concentration of lipid peroxidation (MDA) in maize leaves, and reduced Pb accumulation in above- and below-ground biomass of maize by 39.20% and 37.58%. In T2 treatment group, the water soluble Pb, ionic fraction and carbonate fraction Pb in rhizosphere soil decreased by 37.57%, 36.36% and 43.24%, and organic fraction Pb and residual fraction Pb was the highest with the value of 11.67 and 18.57 mg/kg; the soil aluminum bound (Al-P) and iron bound phosphate (Fe-P) were the highest with 93.53 mg/kg and 230.32 mg/kg, indicating that the phosphate supplement increases the soil ionic P and transforms the chemically mobilized P (such as O-P, Ca-P) into the bioavailable P. Moreover, the soil organic fraction and residual fraction immobilized Pb was positively correlated with the bioavailable Al-P and Fe-P, indicating that the ionic fraction P (Al-P and Fe-P) react with Pb and produce residual P-Pb compounds. Therefore, phosphate supplement to Pb contaminated soil could transfer unstable fraction Pb into stable fraction Pb by P-induced Pb immobilization, reduce the bioavailability of Pb and alleviate the toxicity of heavy metal to plant.