Effect Of Exogenous Organic Matter On Migration And Transformation Of Heavy Metal Cadmium In Rice

Cadmium is the most serious heavy metal element in the farmland soil pollution.The potential threat of cadmium pollution to the earth ecosystem,agricultural production and human health can not be ignored.As a large grain crop with strong cadmium accumulation ability,how to reduce the accumulation of cadmium in rice has become a major issue of common concern to the global environmental workers.In this study,vermicompost was selected as the source of organic matter in pot experiment to study the effect of vermicompost on the bioavailability of cadmium and the accumulation of cadmium in rice under the two water management conditions of flooding irrigation and wetting irrigation.On this basis,the threshold and mechanism of vermicompost were further explored,compare of the effects of Medicago sativa and vermicompost on the accumulation of cadmium in rice at the same organic matter addition level.The main results are as follows:(1)The pH value and Eh value of soil were significantly reduced by exogenous organic matter under the two water management modes of wet irrigation and flood irrigation.The pH value and Eh value of soil under wet irrigation were higher than that under flood irrigation during the whole growth period of rice.The application of organic matter in the two water management conditions sigIificantly increased the content of soil Fe2+,and the content of soil Fe2+under flood irrigation was higher than that wet irrigation.In addition to the maturity stage of rice,organic matter can reduce the DTPA-Cd content of soil under two water management modes,and the DTPA-Cd content of soil under flood irrigation is significantly lower than that wet irrigation,so does the effect on the proportion of environmental active Cd in soil.The Cd accumulation of rice under flood irrigation without exogenous organic matter was lower than that under wet irrigation,and the Cd content of rice in flood irrigation group was significantly lower than that under flood irrigation group,lower than the national food safety standard limit(0.2 mg·kg-1).There was a significant negative correlation(P<0.01)between DTPA-Cd content and Fe2+ content in rice soil.Under the condition of long-term flood irrigation,organic matter had a better effect on the reduction of Cd accumulation in rice.(2)In the whole growth period of rice,the content of soil Eh and exchangeable and reducible Cd in vermicompost decreased significantly.There was a significant positive correlation between environmental active Cd and soil eh(P<0.05).Vermicompost inhibited the transport of Cd from root to stem,but promoted the transport of Cd from stem to leaf and brown rice.The organic matter of vermicompost increased the activities of peroxidase(POD)and catalase(CAT)enzymes in the roots and leaves of rice,improved the ability of rice to alleviate Cd toxicity,decreased the malondialdehyde(MDA)content in rice plants,and significantly increased the indexes of rice economic characters.The organic matter of vermicompost can significantly reduce the Cd content in rice under the designed addition amount,which can reach the national food hygiene standard(0.2 mg·kg-1)when the vermicompost addition amount is higher than 16.3 gC·kg-1 soil,and there is no significant difference after the application amount of organic matter is increased.(3)In this experiment design,two different sources of organic matter significantly reduced the soil Eh value,and the effect of Medicago sativa was more significant under the same organic matter content.Both vermicompost and Medicago sativa organic matter can promote the transformation from exchangeable fraction to reducible fraction,and Medicago sativa can significantly increase the proportion of Cd in soil residue state under 5.74 gC·kg-1 soil addition.Medicago sativa significantly increased the panicle length,panicle number,seed number and 200 grain weight of rice.The effect of vermicompost organic matter on 200 grain weight was better than that of Medicago sativa.Both organic matter significantly reduced the Cd content of rice root,stem,leaf and brown rice.Under the same organic matter content,Medicago sativa was better than vermicompost.In this experiment,the content of Cd in brown rice of Medicago sativa treatment group reached the national food safety standard(0.2 mg·kg-1)under the designed adding amount,and the content of Cd in brown rice of vermicompost treatment group decreased significantly with the increase of adding amount,but it did not reach the national food safety standard.The effect of Medicago sativa on Cd accumulation in rice was better than that of vermicompost.