Deactivation Effects Of Amendments On Cadmium-Contaminated Purple Soil From Croplands Of Chongqing

Cadmium（Cd）is a non-essentially toxic and carcinogenic trace metal to the human body,the Cd pollution of land has become a serious global issue with the impact of human activity（such as sewage irrigation,fertilization,mining,smelting and fuel combustion,etc.）,which can occur as a food contaminant or a global pollution.Long-term Cd occupational exposure contributes to the developing of lung cancer,and high Cd exposure may induce kidney and bone damages and hematuria,which poses a serious threat to human health.In situ immobilization,which involves applying additives to soil for the purpose of reducing the mobility and bio-availability of heavy metals in soil,has represented an effective,inexpensive,and environment compatible remediation way for plants safety regulation and soil.Because the farmland in China is vast and widely distributed,the farmland soil in different area has different physicochemical properties.Currently,the remediation for heavy metal are mainly pay attention to acid soil in south China.However,the remediation technology for medium-alkaline soil,especially purple soil cadmium pollution is relatively insufficient,and the relevant mechanism needs further analysis.Therefore,we investigated five amendments including organic（OA,such as XZ）,inorganic（IA,such as FS,LIME）and mixed form（OIA,such as FY,PX）to reduce the availability of soil Cd and the uptake Cd of plants.Firstly,optimized appropriate amendments for purple soil region through assessing soil pH and Cd fractions in soil cultivate experiment,and verify selected amendments through assessing these indicators of plants（biomass,Cd content）and soil（Cd fractions,pH,enzyme activities）in the greenhouse and the field experiment.At the same time,the effects of cadmium deposition on soil and plants were studied by simulating the cadmium deposition in pot experiment to verify whether the passivation effect of passivating agent was good and lasting under the condition of cadmium deposition.The results were as follows:（1）Effects on soil pH and Cd fractionsIn soil cultivate experiment,the soil exchangeable Cd（EX-Cd）concentration was significantly reduced by 12.10¹3.59%under the five amendments（FY,XZ,PX,FS,LIME）for 20 days（P<0.05）,among them,PX has the largest reduction.As for Cd sequential fractions,FY,XZ,PX and FS decreased the concentration of EAC-Cd and increased the concentration of DAC-Cd,among them,PX had the lowest EAC-Cd(4.04mg·kg^-1)and the highest DAC-Cd(1.60 mg·kg^-1),but LIME increased the EAC-Cd concentration and decreased the DAC-Cd concentration.Compared with CK,all amendments significantly reduced EX-Cd concentration,LIME significantly increased CB-Cd concentration,PX,FS and LIME significantly increased OX-Cd concentration,XZ,PX,FS significantly increased OM-Cd concentration,FY,XZ and PX significantly increased Res-Cd concentration（P<0.05）.Based on these findings,PX was selected to apply in both a greenhouse pot and a field experiment.In the pot experiment,EAC-Cd,EX-Cd,CB-Cd concentration of PX treatment was significantly reduced by 11.2%,5.2%,3.5%and OX-Cd and OM-Cd concentration was significantly increased by20.0%,1.25 times in the mustard season,respectively（P<0.05）.In LIME treatment only OX-Cd concentration was significantly increased by 15.0%,and there was no significant difference in other forms（P<0.05）;EAC-Cd,EX-Cd and CB-Cd concentration of PX treatment was significantly reduced by 53.4%and 46.1%,14.8%and DAC-Cd and OM-Cd concentration was significantly increased by 50.4%,1.25times in the corn season,respectively（P<0.05）.EX-Cd concentration of LIME treatment was significantly reduced by 19.8%and OX-Cd and OM-Cd concentration was significantly increased by 7 times and 2 times,respectively（P<0.05）.In the field experiment,EAC-Cd concentration of PX treatment was significantly reduced by 11.2%and DAC-Cd,OX-Cd,OM-Cd and Res-Cd concentrations was significantly increased by 12.9%,12.1%,21.7%,12.5%in the mustard season,respectively（P<0.05）.EAC-Cd of concentration PX treatment was significantly reduced by 29.9%and OM-Cd and Res-Cd concentration was significantly increased by 4 times and 38.0%in the corn season,respectively（P<0.05）.（2）Effects on plant biomass and plant cadmium contentIn the pot experiment,PX and LIME treatment had no significant effect on the biomass of mustard,but the biomass of corn was decreased（P<0.05）.The cadmium content of the mustard in PX and LIME treatment was significantly reduced by 47.4%and 32.0%,respectively（P<0.05）.The cadmium content in stalks and leaves of maize in PX treatment significantly reduced by 38.8%and 63.2%,respectively（P<0.05）.In the field experiment,the biomass of mustard in PX treatment was increased by 25%（P<0.05）,while the application of LIME inhibited the growth of plants.PX and LIME treatment had no significant effect on the biomass of maize（P<0.05）;The cadmium content of mustard in PX and LIME treatment was significantly reduced by 33.0%and16.0%,respectively（P<0.05）.The cadmium content of maize in PX treatment was decreased and the cadmium content of ear of corn did not exceed the national standard food contaminant limit（GB 2762-2017）.At the same time,the order of the cadmium content of corn plants is:leaf>bract>stalk>ear.Therefore,the results indicate that the biomass of the plants in PX treatment was significantly increased and significantly reduced the cadmium content of plant.Pearsons correlation analysis showed that the cadmium content of plants was positively correlated with easily-absorbed form of Cd（EX-Cd,CB-Cd）in soil and negatively correlated with hardly-absorbed form of Cd（OX-Cd,OM-Cd,Res-Cd）in soil.（3）Effect on soil enzyme activityIn the mustard season,catalase,urease,and sucrase activities PX treatment significantly increased by 18.1%⁵2.2%,61.9%¹4.3 times in pot and field experiments,respectively（P<0.05）;while catalase and urease activities in LIME treatment was increased,but sucrase activities was significantly decreased by 42.4%in pot experiment（P<0.05）.In the corn season,catalase,urease,and sucrase activities in PX treatment was significantly increased by 35.2%⁵6.6%,1.5%²5.8%,respectively（P<0.05）.Although urease activities in LIME treatment was increased,sucrase activities in the pot experiment and catalase activities in the field experiment were significantly reduced by 78.5%and 4.5%,respectively（P<0.05）.It shows that PX is mor ecologically friendly compared with LIME and it can remediate the ecological environment of microorganisms.（4）the effects on soil and plants under the Cd depositionIn the pot experiment,the soil high-risk Cd（EX-Cd、CB-Cd）was increased under simulating the atmospheric Cd deposition and the plant biomass was reduced and the uptake of Cd for plants was increased.PX treatment reduced the easily-absorbed form of crops in two seasons（EAC-Cd）and significantly increased easily-absorbed form of crops（DAC-Cd）and reduced the Cd content of mustard and corn,so PX was still determined to be an effective soil amendment for Cd-contaminated soil exposed to atmospheric Cd.In summary,the findings above demonstrated that PX not only significantly reduced the bioavailability of Cd,improved performance of plants,but also effectively increased biological activities for remediation of the Cd-contaminated purple compared with the control and lime treatments.Moreover,under the atmospheric Cd deposition,it still can effectively reduce the Cd bioavailability in the soil and increase the biomass of the plant and reduce the absorption of cadmium by the plant and promote soil ecological health.Therefore,PX can be used as an environmentally safe and effective amendment for the remediation of the purple soil contaminated with Cd.