| Soil pollution from heavy metal caused by high intensity of human activities such as “three wastes” emissions of industrial; sewage irrigation, mining etc. were exacerbated. Especially the pollution of cadmium exceeded cannot be ignored. However, cadmium toxicity was recognized to be not only related to the total amount of metals, but also depended on its bioavailability mostly. Six paddy soils sampled from around steel plant(steel plant), smelter(smelter), thermal power plant(thermal power plant), Dabaoshan copper-iron mine waste water downstream(Dabaoshan copper-iron mine), of Shaoguan, around lead-zinc mine(lead-zinc mine) of Lechang, and Zhongshan delta deposite were investigated. The affecting factors of cadmium bioavailability were examined in this study. Incubation experiments were conducted to investigate the dynamic of cadmium fractions and its influencing factors in 6 soils under different water management methods(flooding and moist). Pot experiments and root box experiments were conducted to investigate the uptake and distribution of Cd in rice plants grown in 6 soils under different water management methods. The aim of this study is to reveal the bioavailability of Cd in soils and its mechanisms, and supply scientific basis for safe production of rice cultivated in Guangdong Province. The major results are shown as follows:(1) The soils were contaminated with Cu, Zn, Pb, As, Hg, Cr and Cd. Among the 6 soils, smelter soils was mostly contaminated, followed by steel plant > thermal power plant > lead-zinc mine > Dabaoshan copper-iron mine > delta deposite. Cd was dominated mainly by the fraction of acid extractable Cd. The RAC showed that soils around steel plant, around smelter and around thermal power plant posed a high ecological risk, whereas soils of Lechang, Shangba and Zhongshan posed very high ecological risk. The total concentration of Cd was the main effect factor of its form distribution. Soil p H, clay, Mn AAO also related with the distribution of Cd. Concentrations of residual fraction were not affected by soil properties.(2) According to the adsorption kinetics of 6 soils, the adsorption rate was divided into a fast initial specific adsorption and a slow non-specific adsorption followed, and the inflection point was about 30-60 min. The order of Cd adsorption rate onto 6 soils was: lead-zinc mine > delta deposite > steel plant > thermal power plant > smelter > Dabaoshan copper-iron mine. The results from non-linear fitting method showed that pseudo-second order equation was more adaptable than pseudo-first order, Elovich and intraparticle diffusion model to fit the adsorption kinetics. According to the adsorption isotherms, Cd adsorption of each soil increased with the increasing of equilibrating solute concentration, and both Langmuir and Freundlich models were successfully fitted the adsorption isotherm. The order of Cd maximum adsorption amount onto 6 soils from Langmuir model was: lead-zinc mine > delta deposite > steel plant > thermal power plant > smelter > Dabaoshan copper-iron mine and the desorption order was opposite. The conclusion can be given that Cd was easier to be adsorbed by lead-zinc mine soil and more difficult to be desorbed. The findings of stepwise multiple regression analysis between coefficients of equations and soil properties were indicated that p H and soil organic matter were the most important soil characteristics in controlling Cd adsorption and desorption behavior in 6 soils. The higher adsorption of Cd in lead-zinc mine soil resulted from the higher p H and soil organic matter.(3) Flooding water management not only increased the Aci-Cd, decreased the Red-Cd and Org-Cd, but also increased the available Cd and decreased soil p H compared with moist. The PCA results that the Aci-Cd concentration correlated positively with the available Cd and did negatively with the Red-Cd and Org-Cd concentration, respectively, indicating that the Cd transformation from the acid extractable fraction to the reducible and oxidizable fraction was the main reason for the available Cd decrease after flooding of 6 soils.(4) The available Cd content of 6 soils reduced 40%-55% and 70%-80% under flooding after tillers and whole growth season flooding mode compaired with routine water management. The Cd content of brown rice were dramatically reduced under flooding after tillers and whole growth season flooding mode compaired with routine water management. At the same time, take flooded irrigation methods reduced the transfer of Cd from root to other parts, because flooding irrigation made the Cd mainly concentrate on the roots. We can take the method of flooding after tillers to reduce the Cd absorption and accumulation in rice production from the view of food safety and save water. We can even take the whole growth season flooding to reduce the Cd absorption and accumulation if there is a rich water quantity.(5) The results of rhizobox experiment showed that Cd concentration of rhizosphere were significantly higher than that of non-rhizosphere soil except for delta deposite under routine water management. There is no significant difference between Cd concentrations of rhizosphere and non- rhizosphere under flooding water management. The available Cd content of rhizosphere and non- rhizosphere reduced 60%-90% and 40%-50% under flooding mode compaired with routine water management.(6) Fe contents in the iron plaque of rice root in continue flooded condition all the time were significantly higher than those in routine water management, however, Cd contents in the iron plaque of rice root, rice shoot and rice root in flooded condition were significantly lower than those in normal water management. The above indicated that Cd contents in the root iron plaque, root and shoot of rice were decreased by increased Fe contents in the iron plaque of rice roots grown in different polluted soils. |
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