Fractionation of lead in paddy soils and its bioavailability to rice plants

The impact of heavy metal pollution on growth and water quality has become a public concern. Fractionation of soil lead (Pb) is an important approach of chemical characterization and can provide useful information on its bioavailability. A greenhouse study was undertaken to estimate the availability of Pb fractions to rice in two paddy soils of Inceptisol (Fluventic Umbrepts) and Ultisol (Plinthudic Aquults). Lead fractionation was performed by using a sequential extraction procedure (SEP) and single extraction with 1 mol NH4Ac L− 1. The increased Pb loading (0–1200 mg kg− 1) in two paddy soils had variable effects on plant dry biomass, and Pb concentration in rice plants. Plant dry biomass was higher in Ultisol than Inceptisol, and grain yield in both soils decreased with increasing Pb loadings. The concentrations of Pb in root, leaf, stalk, and grain of rice increased significantly with increasing levels of Pb in both soils, and this increase was more pronounced in Ultisol than Inceptisol. Lead accumulated mainly in the roots and the increase in Pb concentrations due to external Pb input for different plant parts was in the order of root > stalk > leaf > grain.Soil NH4OAc extractable Pb was higher in Ultisol than Inceptisol and was better correlated with root Pb than grain or straw Pb, indicating slow translocation of Pb from root to shoot, especially to grain. The dominant fractions of Pb in Ultisol were residual and exchangeable fractions, whereas in Inceptisol Pb was mainly bound to residual and Fe/Mn oxides fractions. Lead was more mobile in the Ultisol than in the Inceptisol soil. External Pb loading decreased soil pH and increased Pb bioavailability in soil, thus resulting in increased Pb uptake and accumulation in the edible parts of rice. These results indicate that plant availability of externally loaded Pb is related to Pb transformation and fractionation in soils, which are affected by basic soil properties such as clay and oxide content and composition, pH, and organic matter. External Pb loading increases Pb uptake by rice plant, but most of the absorbed Pb is accumulated in the root with a minimal increase in grain Pb. This information merits attention in the management of Pb-polluted soils.