Seasonal Variations Of Heavy Metal Species In Soil And Their Response To Variations Of DOM

As the important pollutants in soil, heavy metals and their environmental behavior have become one of the research emphases in environmental science. Biotoxicity of heavy metal not only depends on the total concentrations, but more on their chemical forms. So a lot of researches have been done about heavy metal forms extraction, distribution, transformation and influencing factors. However, because soil environment is an open system, its environmental conditions will change at any time, and so do heavy metal species in soil. Therefore, studying environmental effects of heavy metals can not overlook transformation direction and time course, but the current research in this area is also less. Moreover, organic materials are widely used in agricultural production, but studies have found that organic materials will bring a lot of dissolved organic matter (DOM) into the soil. DOM, a very active component in terrestrial ecosystem, has conspicuous influences on the adsorption and desorption, migration and transformation and biotoxicity of heavy metal.Based on above reasons, the main cultivate soils of Chongqing, purple soil and yellow soil, were regarded as objects of investigation and research in this study. The seasonal variation regularity of species of Cd, main contaminative heavy metal in Chongqing, and Cu, greatly influenced by organic matter, were studied by field experiment. The regularity of growth and decline of DOM and its influences on species variation and plant absorption of Cd, Cu and Hg were studied by pot experiment. The aim is to offer scientific basis for risk assessment and prediction of soil environment pollution in Chongqing and more rational application of organic materials.The results of field experiment showed that the species of Cd and Cu presented certain regularities with the change of time. EXC (exchangeable form), CAR (carbonate bound form) and OM (organic matter form) fluctuated obviously, but Fe-Mn (Fe-Mn oxide bound form) and RES (residual form) tended to smooth. Because environmental factors changed more obvirously in spring and summer than in autumn and winter, the range of variation of every form was relatively large in spring and summer. Fertilization especially application of organic fertilizer had influences on transformation of species, particularly on EXC, CAR and OM.The results of pot experiment showed that the species of heavy metal presented certain transformation regularities after adding exotic heavy metal. For Cd, transformation tended to the species of high bioavailablity at first. EXC in purple soil and CAR and Fe-Mn in yellow soil increased significantly. But with time going, OM and RES, which are more stable species, gradually increased in the two soils. For Cu, EXC-Cu increased considerably initially, but also transformed to OM and RES at last in the two soils. For Hg, there was a tendency of transformation from water-soluble, acid-soluble and hydrogen peroxide-soluble forms to alkali-soluble and residual forms. This study also indicated that planting and DOM additions had influences on transformation direction and process, but there were obvious differences among different forms in different soil.DOM concentration of soil increase sharply after adding exotic DOM, but it gradually declined as the result of microbial degradation. The reduction rate of DOM was more than 50% during the whole trial period, especially in the treatments which were added the DOM extracted from rice straw. Besides, planting can increase DOM concentration of soil in a certain extent.The variations of heavy metal species in soil in response to the variations of DOM concentration was further studied by correlation analysis. The results showed that effect of DOM on available heavy metal was the most obvious. In purple soil, DOM was significant positive correlation with EXC-Cd and signifiant negative correlation with water-soluble Hg. In yellow soil, DOM was extra significant negative correlation with EXC-Cd and EXC-Cu and extra significant positive correlation with water-soulable Hg. Effect of DOM on OM was also noticeable. DOM was positive correlation with OM of every studied heavy metal, so DOM promoted transformation from other species to OM.There was relativity between plant absorption and heavy metal species in soil. EXC-Cd and EXC-Cu were significant positive correlation with corresponding concentration in plant, so EXC was the direct availability of plant. CAR also had a centain contribution to plant absorption. However, RES was negative correlation with plant absorption. The higher the RES content was, the lower the available heavy metal in soil and plant content were.Different heavy metal had its own transfer characteristic in lettuce. Comparing Cd with Cu, propotion of heavy metal content in each part of lettuce to total absorption was in order of Cu＞Cd of root, Cd＞Cu of stem and Cd＞Cu of leaf. In above-ground part of lettuce, the accumulation of Cd achieved 60%, but less than 30% of Cu. It is obvious that transfer ability of Cd was better than Cu, so pollution risk of Cd was larger than Cu. The absorption amount in different part of lettuce would be changed by adding exotic DOM, which had few effects on heavy metals mobility in lettuce.