Effects Of Straw Returning On Cd Activity In Soil And Accumulation In Rice

Nowadays,the soil environmental quality of arable land in China is not optimistic.Cadmium（Cd）pollution is the first inorganic pollutant.Straw is the main part of Cd enrichment in crop,however,Cd-contaminated straw returning to the filed will release the Cd during the decomposition process,causing secondary pollution of Cd.Therefore,this study analyzed the Cd content and Cd forms change under different straw returning mode.In addition,the study analyzed the effect of straw decomposition products on physicochemical properties,microbial activity,Cd content of subsequent crops and four Cd fractions in paddy soil under the process of straw decomposition.The main research results were as follows:（1）After straw returned to the filed,the soil p H increased and soil Eh decreased.The content of organic matter,dissolved organic carbon,ammonium nitrogen and nitrate nitrogen in soil also increased.Furthermore,the activities of acid phosphatase,urease,cellulase,invertase and catalase increased after straw returning.Such results indicated that straw returning can provided substrate for microbial growth and development and enhance soil microbial activity.Straw returning to the filed is an effective measure to increase soil fertility.（2）In pot experiment,total Cd content in soil increased after straw returning and decreased after straw leaving.It indicated that straw returning would increase the Cd content in the soil and be absorbed by the subsequent crops.Meanwhile,DTPA-extractable Cd content in soil decreased during the hole pot experiment period.The content of DTPA-extractable Cd in the all-remove straw treatment decreased from 0.58mg/kg to 0.26mg/kg,while the content in all-return straw treatment decreased from 0.58mg/kg to 0.31mg/kg.Indicating that the return of straw would increase the content of available Cd.In addition,the contents of acid-soluble Cd and reducible Cd were higher in all straw returning and half returning modes than the all straw leaving mode,which further proved the risk of straw returning to soil environment.In the soil culture experiment,the residual Cd content in straw continued to decrease,from 0.33mg/kg to 0.24mg/kg,while the total Cd content in the soil continued to increase,from 0.19mg/kg to 0.27mg/kg,within 90 days,which further proved the above conclusion.（3）In order to further study the effect of different straw returning mode on the accumulation of Cd content in the subsequent crops.The Cd content of second-year rice in the pot experiment was detected,and it was found that the content of Cd of subsequent crops in the all-returning group was significantly higher than the all-removal group.This results further proved that returning Cd-contaminated straw to field will cause secondary Cd pollution in soil and accumulation of Cd in subsequent crops.（4）In the experiment of flooded soil adding straw dissolved organic matter（DOMs）,it was found that the Cd content in surface water and DTPA-extractable Cd in soil decreased over time.However,when there is no DOMs addition,illumination will increase the content of Cd in surface water,while there is no change in dark group,indicating that illumination would affect the activity of Cd in flooded soil.After illumination of Cd S-MMT material,it was found that Cd²⁺in Cd S will be continuously released,yet it will not be released in darkness,which is due to the photogenerated holes generated after illumination of Cd S.After Cd S-MMT was added to simulated flooded soil and illumination,the content of acid-soluble Cd and reducible Cd increased,and reduced the content of Oxidizable Cd（mainly sulfide binding state）,indicating that illumination increased the dissolution of Cd²⁺from Cd S in soil and increased environmental risk.