Effects of different cropping patterns on speciationdistribution of soil heavy metals Pb and Cd

To understand the moving-transiting rule of heavy metals in soil-plant systems under different croppingpatterns，a field experiment was carried out in Huizhou， to investigate the effects of different cropping patterns，including the paddy continuous cropping pattern，vegetable-paddy rotation pattern and vegetable rotation pattern，onthe speciation distribution of soil heavy metals Pb and Cd. The results showed that the four Pb distribution forms underthe three cropping patterns showed the order of reducible > residual > oxidizable > exchangeable，and the content ofreducible form was 37.26%-56.94%. The soil Cd was dominated by the high activity fractions with 46.70%-62.98% andin the order of exchangeable > reducible > residual > oxidizable. The total content of three effective Pb distribution formsincreased under the paddy continuous cropping patternand vegetable-paddy rotation pattern，while it decreasedunder the other cropping pattern. For Cd，the total contentof three effective distribution forms increased underthe vegetable-paddy rotation pattern，decreased underthe vegetable rotation pattern，and unchanged under thepaddy continuous cropping pattern. Additionally，Pb inthe reducible form could transform with the oxidizableand residual form，while the Cd distribution formscould also mutually transfer between the exchangeable form and reducible form. As the soil heavy metal pollution inagricultural fields has become increasingly severe，it is very significant for screening the suitable cropping patterns witheconomic and ecological benefits as well as repairing effects of heavy metal contamination.