| 其他摘要 | As urbanization proceeds with speed in China, food production zones along suburb areas have been increasingly created and developed. However, quick urbanization has already given rise to a variety of serious environmental issues, which include soil, atmosphere, surface and ground water pollution. In addition, great quantities of agrochemicals are required for intensive farming, and heavy metals may accumulated in soils, be taken up by crops, and be passed on in the food chain, and hence result in serious impact on human health. Therefore, it is of much importance to understand bioavailability and other biogeochemical behaviors of heavy metals in agricultural soils and associated vegetable system to prevent heavy metal pollution of vegetable, and to promote green agriculture production and protect human health.
The urban fringe area of Guiyang was chosen for the research, which is the most typical karst area in the world, situated in the southwest of China. Three major vegetable-production sites were selected. The samples of soil and vegetable were analyzed for their soil physical properties, concentrations of heavy metals in both of soils and vegetables, speciation of heavy metals in the vegetable soils and profiles, and the isotopic composition of Pb in soil, different parts of vegetable, and rain water. By using the theories and methods of trace element geochemistry, isotope geochemistry, as well as soil science, we discuss the key controlling factors of heavy metals speciation, sources, migration, and transformation in the soil-vegetable system.
Concentrations of heavy metals in vegetable soils were significantly correlated with soil types：the vegetable soil of yellow soil type was assessed at moderate pollution level, while those of calcareous soil type at light pollution level. The fraction bound to residual of heavy metals in vegetable soil is the main fraction, the next is the the fraction bound to organic matter, the fraction bound to Fe oxides，and the acid-extractable fraction, repectively. In the soil profile, the content of most heavy metals in the vegetable topsoil is higher than that of background. The main fraction of heavy metals is also the fraction bound to residual in both vegetable and background profile. The content of bioavailable fraction heavy metals in vegetable profile are higher than those of background profile. It is shows that anthropogenic causes has increased the activity and bioavailability of heavy metals.
The edible part of sweet potato has higher concentration factor of heavy metals than those of other vegetable in the studied sites, which means that it is much sensitive to heavy metals in the soil. In the soil-vegetable system, concentrations of heavy metals decrease in the order of soil > root > stem > fruit. The interface between soil-vegetable roots was most active place for heavy metal transportation from soil to vegetable. The transfer factors ( TF0 ) of heavy metal between soil-vegetable root interface decrease in the order of Cd>>Cu, Zn>Pb, and the TF0 of most heavy metals in yellow soil type are higher than those of calcareous soil type. Soil pH was negatively correlated with TF0 of Cd, and was positively correlated with other elements in soil-vegetable system. Speciation of heavy metals was remarkably correlated with the main components of soil and physicochemical properties. However, the correlations between heavy metal speciations of soil and vegetable root are not significant. The plastic mulching decreased soil pH from 4.5 to 3.8, changed the soil physicochemical properties, and affected the distribution characteristic of heavy metal speciation in soil and the distribution of heavy metals in vegetable, according to our experimental studies. This technique may increase the risk of heavy metal in the agricultural system. Lead isotope studies show that the soil lead was mostly derived from parent rock weathering, and that vegetable had slightly lower radiogenic lead than soil. Lead isotope composition of soils and vegetables form a line in the 208Pb/206Pb vs. 207Pb/206Pb diagram, showing a typical two-end-member mixing system between rainwater and the natural Pb component. Using binary mixing equation, the contribution pocentage of atmospheric Pb was increased from 22.3%~61.3% to 54.6%~84.03% from root to stem and leaf of vegetable, suggesting that the heavy metal pollution of vegetable by atmospheric deposits can not be neglected. |
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