Heavy Metal Bio-availability And Its Affecting Factors In Soil-plant System

A heavy metal is bioavailable if it is in a chemical form that organisms can absorb readily and if, once absorbed, it affects the life cycle of the organisms. The bioavailability of an element is affected by the environment and organism itself, involving physical, chemical and biological processes. Studies on the bioavailability of heavy metals in soil-plant system can directly indicate pollution degree of heavy metals in the soil and the corrsponding hazards to ecosystem and human health, and thus provide theoretical bases for soil remediation and local agricultural production.A sewage-irrigated area along the Liangshui River and the Shijingshan industrial area of Beijing were selected as the experimental plots in the present study. The studies consisted of continuous position sampling in the fields, soil and plant analyses in the laboratory and pot experiments. The sources, distribution and bioavailability of heavy metals (including main affecting factors and the mechanisms) were systematically studied based on total concentrations and speciation of heavy metals, and their correlations with soil basic physico-chemical properties, under compound pollution, and with spiking of Si, respectively. Main contents are as follows:1) Individual pollution index, Nemerow synthetical pollution index, geo-accumulation index and principal component analysis were adopted for soil environmental quality assessement based on total concentrations of heavy metals in the soil. Results showed that soils were slightly contaminated by Cd, Cu and Pb, with pollution indices of 19(3⁴) and a randomized complete block design were conducted to systematically study the effects of silicon addition on physiological and biochemical characteristics and heavy metal uptake of ryegrass under the compound pollution of Cd, Zn and Pb. Obvious dose-effect was found for Zn, Cd and Pb, and Si addition can significantly reduce heavy metal bioavailability. Soil pH increased 0.6 unit after Si addition. Exchangeable fraction of Zn, Cd and Pb reduced in the order of Cd>Zn>Pb with the increasing Si addition. Exchangeable fraction can better indicate heavy metal bioavailability in the soil-plant system, though total concentration is still an important factor. Soil pH, Si addition levels and their interaction significantly inhibited uptake of heavy metals by ryegrass. Zinc, Cd and Pb uptake in alkaline and neutral soil, and Pb uptake in acid soil were significantly reduced by Si addition, but no significant reduction of Zn and Cd was found in acid soil, which may be due to the interaction of soil pH and Si.