Effects Of Organic Acids On Cd Migration And Accumulation In Soil-Rice Systems

Organic acids are important component of dissolved organic matter in soil.They can affect the bioavailability of Cd by participating in the precipitation dissolution,adsorption desorption and coordination complexation of Cd in soil.However,due to the complexity of farmland soil environment and the diversity of soil types,there are few reports about the quantitative relationship between soil organic acids and Cd bioavailability,and the differences of soil types and properties on the impact of them are not clear.Therefore,to explore the impact of organic acids on Cd transport in different parent material development soil rice system,and to study the impact of soil organic acids and heavy metals.The response mechanism is very important for the prevention and control of cultivated land heavy metal pollution.Two kinds of paddy soils with typical parent material development in South China(the yellow clayey soil with shale parent material development and the red and yellow soil with quaternary red soil parent material development)were selected as the research objects.By adding different concentrations of organic acids to different Cd contents of soil.Combined with indoor soil culture without crops,the changes of soil Cd2+concentration and Cd occurrence patterns after organic acid adding in advance were analyzed through the rice pot experiment,the relationship between Cd content in different parts of rice,organic acid and soil physical and chemical properties was analyzed,the effect of organic acid on Cd migration and accumulation in soil rice system was discussed,and the effect of organic acid on Cd bioavailability in different soil was clarified.The main results are as follows:(1)The results showed that there was a significant positive correlation between the concentration of Cd2+in soil solution and the amount of exogenous organic acid added,which indicated that organic acid promoted the transfer of Cd2+from solid phase to liquid phase.The concentration range of Cd2+in the red and yellow soil is 0.24-43.46μg·L-1,the mean is 22.48 μg·L-1,and the yellow clayey soil is 0.17-53.03 μg·L-1 μg·L-1,the mean is 28.07 μp.g·L-1.The removal of organic matter reduced the adsorption capacity of soil to Cd2+:after the removal of organic matter,the concentration of Cd2+in soil solution of the red and yellow soil is 0.33-47.11μg·L-1,the mean is 24.46 μg·L-1.the yellow clayey soil is 0.09-54.67 μg·L-1,the mean is 29.35 μg·L-1.The results showed that the adsorption capacity of red and yellow soil was stronger than that of yellow clayey soil.Organic acids influenced the slope and intercept of the linear fitting formula between solution Cd2+concentration and Cd level.The results showed that there was a significant correlation between the amount of organic acids and the amount of organic acids in the soil.(2)The results showed that the initial pH of soil solution decreased with the increase of organic acid content,and tended to be neutral with the extension of time.Organic acid decreased the pH of soil in yellow clayey soil,but had little effect on the content of soil organic matter.In the early stage of organic acid addition,the content of Cd extracted from weak acid in the two soils was higher than that of CK,and decreased in the later stage.The dynamic changes of Cd content in reducible state and in oxidizable state were significantly affected by the content of organic acid,and the content of Cd in oxidizable state in yellow clayey soil reached the maximum value at the 40th day,about 0.8 mg-kg-1.At the beginning of applying organic acid,the content of Cd in soil residue increased and was higher than CK at the 30th day.(3)The results of pot experiment showed that the application of organic acids significantly increased the aboveground biomass of rice in red and yellow soil.In red yellow soil and yellow clayey soil,the order of Cd content in each part of rice was root>stem leaf≈ brown rice>husk,and increased with the increase of exogenous Cd level.There was a significant positive correlation and a significant negative correlation between the amount of organic acids added and the Cd content in all parts of rice.Adding low molecular organic acids enhanced the Cd availability in soil,but also had the potential to weaken the Cd bioavailability under the specific Cd/organic acid ratio.Under different Cd levels,the Cd content in different parts of rice was significantly correlated with the Cd extracted from soil HCI,which indicated that the Cd extracted from HCl could be used to evaluate the bioavailability of Cd in soil.At the same level of Cd,there was a significant negative correlation between Cd content in all parts of rice and pH of soil solution,a negative correlation with organic matter content,and a significant or extremely significant correlation with Cd extracted from HCl,which indicated that organic acids could affect the effectiveness of Cd by affecting the physical and chemical properties of soil.In conclusion,there are differences in the effects of exogenous organic acids on the physical and chemical properties of soils,the concentration of Cd2+in soil solution,and the occurrence of Cd in soil.Therefore,there are also differences in the effects of organic acids on the bioavailability of Cd in rice grown in different soils and on the migration and transport of Cd in different parts of rice.Therefore,when evaluating the effects of organic acids on the bioavailability of Cd or using exogenous organic acids to control the effectiveness of Cd,we should According to the different soil types and Cd pollution levels.