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

Reducing the bioavailability of lead (Pb) in soil is the key to alleviating the toxicity of Pb to plants. The present study investigated the effects of phosphorus application on Pb speciation in soil and the oxidative stress of plant to Pb. Maize was planted in 100 mg/kg Pb soil with three fertilizer levels. The phosphate application led to reductions of 25%, 30% and 25% in the activity of total superoxide dismutase (T-SOD) and peroxidase (POD) and the concentration of lipid peroxidation (MDA) in maize leaves, and reduced Pb accumulation in the above- and belowground biomass of maize by 39% and 30%, comparing to the control. The water-soluble, ionic, and carbonate Pb fractions in rhizosphere soil decreased by 38%, 36% and 43%, respectively, and the organic and residual Pb fraction were increased with values of 11.7 ± 0.6 and 18.6 ± 0.4 mg/kg. The soil aluminum-bound (Al–P) and iron-bound phosphate (Fe–P) were highest, with values of 94 ± 8.2 and 230 ± 16.0 mg/kg, indicating that phosphate supplementation increased the soil ionic phosphorus and transformed chemically mobilized P (such as O–P and Ca–P) into bioavailable P. The phosphate supplementation of Pb-contaminated soil could transfer the unstable Pb fraction into the stable Pb fraction by P-induced Pb immobilization, could reduce the bioavailability of Pb and could alleviate the toxicity of heavy metals to plants.