Distribution and plant availability of heavy metals in different particle-size fractions of soil

The distribution of heavy metals and their availability to plants were studied with respect to the particle-size fractions of soil. Soil samples with a range of chemical and physical properties were collected from 10 different rural regions of China. Extractable heavy metals (Ni, Co, Cu and Pb) in the soils using different extractants are compared with the metal contents in winter wheat (Triticum aestivum L.) and alfalfa (Medicago sativa L.) grown on the soils in a greenhouse study. Correlation analysis showed that 0.1 M HCl gave the best estimate of plant-available Ni and Co, while DTPA was most suitable for Cu and Pb. Seven of the soils were partitioned into five particle-size fractions: coarse sand (> 500 μn), medium sand (125–500 μm), tine sand (50–125 μm), silt (2–50 μm), and clay (<2 μm). The metals were characteristically enriched in the clay and one of the sand fractions. Extraction studies on each size fraction indicated that fine sand gave a large amount of extractable Ni, Cu, and Pb and medium sand gave high extractable contents of Co. The extractable amounts of Co and Pb were also high in the clay fraction. With regard to the relative contribution of different size fractions, silt was found to be the major fraction responsible for metal availability, primarily due to its abundance in all the soil samples. For Ni, Cu, and Pb, clay and fine sand fractions also had a significant influence on metal availability. For Co, after the silt fraction, the dominant fraction responsible for its availability was clay followed by the medium sand fraction. From the soil textural data and extraction data of each size fraction, it was possible to infer the relative importance of the fractions with respect to their contribution to metal availability.