Study On Pytoavailability Of Cadmium In Three Purplish Paddy Soils

To find out the laws of cadmium migration and transformation in purple soil-rice system and their influence factors, an investigation on the characteristics of cadmium adsorption and desorption, the transformation of cadmium in three purplish paddy soils including an acidic purplish paddy soil(APPS), a neutral purplish paddy soil(NPPS) and a calcareous purplish paddy soil(CPPS) was conducted by a batch experiment and pot culture experiment. The results can be summarized as follws:The adsorption of cadmium in three paddy soils increased with the increasing of initial concentrations of cadmium in solution, with the highest adsorption ability being observed in the CPPS. The amount of cadmium adsorbed by NPPS was slightly higher than that of APPS. The adsorption data of the soils could be well described with the Langmuir equation and the Freundlich equation.The desorption of cadmium from those soils increased with the increasing of cadmium adsorbed by soils. Among them, the desorption rate of cadmium by CPPS was the lowest and the percentages of cadmium desorbed from APPS and NPPS were higher than 50%.There were close relationships between adsorption-desorption and soil physicochemical property. Soil pH played an important role in controlling the behavior of cadmium adsorption and desorption in soils.With the increase of the pH,the cadmium adsorbed amount increased rapidly and the desorption rate decreased.The influence of SOM content on adsorptive capacity and fixed of cadmium was obvious because the adsorption decreased and desorption increased when the SOM had been eliminated.The calcium carbonate also made contributions of different degrees to adsorption and stabilization of cadmium.The sorption of cadmium was a spontaneous and endothermic process and the sorption increased with the rise of temperature.The mechanism of cadmium in three soils was physical adsorption and it could be concluded that CPPS had the strongest ability to fix cadmium from theΔG values.The results showed that CPPS had the strongest ability to fix cadmium, thus had greater buffer capacity to the exogenous input cadmium. And there was high environmental risk as exogenous cadmium was input into APPS and NPPS.Soil properties such as soil pH, SOM, calcium carbonate, temperature, played an important role in controlling the behavior of cadmium adsorption and desorption in soils and soil pH was crucial.The environmental risk of cadmium in three soils varied with seasons,and the risk was lower in summer than in winter.The results from pot experiments can be summarized as follows:(1) After flooded, the pH values of three soils were closer to neutral, and with the rice growth, the contents of amorphous iron and manganese oxide in general, were on the rise. Rhizospheric soil pH and amorphous iron-manganese oxide content were both higher than the rhizosphere. Compared to the continuous flooding treatment, drainage at tillering periods could reduce the pH and amorphous iron-manganese oxide content of rhizosphere soil.(2) Adding cadmium could result in a good distinct decrease of exchangable cadmium and available cadmium contents of three soils. Along with rice growth, part of exchangable cadmium transformed to other forms, iron manganese oxide and organic bound fraction cadmium of rhizosphere soil increased.This was the most important reason of the decline of available cadmium. The exchangable cadmium and available cadmium contents of CPPS were the lowest of three soils and it showed that the activity of cadmium of CPPS was lower than the other soils. Compared to the continuous flooding treatment, drainage at tillering periods could curb the increase of organic bound fraction cadmium、carbonate bound fraction cadmium and residual cadmium, so that the bioavailability of Cd was improved. The available cadmium contents of rhizosphere were lower than no-rhizosphere because of the raise of rhizosphere pH and the organic matter.(3) Cadmium had no significant effect on the growth, and drainage at tillering periods could get higher dry matter of rice than continuous flooding treatment. The yield of rice in CPPS was higher than the other soils because of the high organic matter content and CEC.(4) Addition of cadmium significantly increased the uptake and accumulation of cadmium in rice plant. The Cd contents of rice grown on CPPS were obviously lower than those on the other two paddy soils, suggesting that the phytoavailability and potential ecological risk of Cd in CPPS were lower than the other two soils. The Cd contents of various organs in different growing periods had a strong positive relationship with mobile Cd of soils, but had no obvious relationship with the iron manganese oxide plaque formation on the root surface. Compared to the continuous flooding treatment, drainage at tillering periods leaded to uptake more cadmium, especially increased environmental risk of cadmium pollution in APPS and NPPS.