| According the climatic features and cultural systems of Guangdong province, vegetable-rice-vegetable rotation system was firstly proposed as a new planting pattern by the Guangdong Academy of Agricultural Sciences. This planting pattern would make full use of the time difference between the climate, soil and other resource utilization of different crops, and alleviate the soil ecological environment problems caused by continuous cropping. Present studies on the vegetable-rice-vegetable rotation system were devoted mainly to soil fertility, reduce the effect of plant diseases and insect pests, prevent and control continuous cropping of secondary salinization, increase economic benefits. Few studies were about the content and bioavailability changes of soil heavy metals as well as heavy metals migration and accumulation in the soil-crop system under the vegetable-rice-vegetable rotation system. We carried out a field experiment on a seriously heavy metal polluted area in Machong town, Dongguan. It was aimed to investigate the form distribution of heavy metals (Cd, Pb and As) in soil and their cumulative characteristics in crops (vegetables and rice) under the vegetable-rice-vegetable rotation system. And then, the main factors which affected the heavy metal absorption in rice and vegetables would were also studied.The conclusions were as follows:(1) During the experimental process, total As content of top soil under the vegetablecontinuous cropping treatment was decreased significantly. Total Pb, Cd in top soil under the rice continuous cropping treatment was also decreased significantly. Total Pb in underlying soil under the vegetable continuous cropping treatment was decreased comparing with before planting. But the total Cd and As contents in the top soil were higher than that in the underlying soil.(2) The order of plant Pb absorptive capabilityas:rice husk> rice>flowering Chinese cabbage> eggplant> cucumber; the order of Cd as: rice> rice husk> eggplant>flowering Chinese cabbage> cucumber; the order of As as:rice husk>flowering Chinese cabbage> rice> cucumber> eggplant. In Dongguan, Cd and Pb in rice existed excessive risk s, especially for Cd.(3) The rice continuous cropping treatment could significantly increase the content of oxidation state Pb of the tillage layer and underlying soil. The combined form contents of Pb in the upper and lower soil were reducible> oxidizable> residue> exchangeable. The exchangeable Pb content in the top soil was significantly lower than that in the underlying soil, but the reducible Pb content was significantly higher than that in the underlying soil.(4) The rice continuous cropping treatments could significantly reduce the Cd contents of exchangeable, oxidable and residual state in the arable layer soil. The rotation system of rice and vegetable processing could significantly reduce the residual Cd content in the underlying soil. The rice continuous cropping treatment could significantly reduce residual Cd content in the underlying soil. The sequence of Cd contents for each species in upper and lower soil was exchangeable> reducible> oxidizable>residue. The Cd contents of the four combined forms in the tillage layer soil were higher than those in the underlying soil.(5) Vegetable continuous cropping treatment could significantly improve the reduced As content in the tillage layer soil. The vegetable-rice-vegetable rotation could make the As content of oxidation state in tillage layer soil decrease significantly while residual state increased significantly. Vegetable continuous cropping treatment could significantly increase exchangeable and reducible As content in the subsurface soil, but reduce oxidation state content. The rotation system of rice and vegetable made residual As content in the undersurface soil increase significantly. Rice continuous cropping made the exchangeable As in the underlying soil decreased significantly while the reduced state As increase significantly. The As contents in the upper and lower soil were ordered as residual> reducible> exchangeable> oxidizable. The exchangeable, reducible, oxidizable As contents in the tillage layer soil were higher than those in the undersurface soil, but the residual state As was lower than that in the undersurface soil.(6) Rice continuous cropping treatment could significantly reduce available Cd content in the soil, which was increased under the vegetable continuous cropping treatment. The vegetable-rice-vegetable rotation system could significantly reduce the bioavailability of As in soil.(7) The exchangeable and reducible Pb in the tillage layerand underlying soil could be transformed into each other; the reduced state could be conversed with oxidation state; and oxidable and residual state could be transformed into each other,the reduced state and residual state could be transformed into each other:The reducible Cd in the tillage layer transformed into oxidable and residual state,the reducible Cd in the underlying soil could be transformed into exchangeable and residual state:The residual state As could be transformed into exchangeable and reducible in the tillage layer and underlying soil.(8) The pH and organic matter contents of the tillage layersoils were significantly higher than that of the underlying soils. The vegetable-rice-vegetable rotation system could obviously improve soil pH, and help to reduce the bioavailability of heavy metals... |
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