Effect Of Two Passivating Agents On Cadmium Enrichment In Amaranthus

In recent years,the industries of mining,smelting,and electroplating and so on have been rapidly developed in our country,which results in a severe problem of heavy metal pollution in soils,especially cadmium(Cd)pollution.Therefore,the remediation of Cd-contaminated soils and the reduction of Cd accumulation in crops become particularly important to ensure the food safety of agricultural products and to protect human health.Oyster shell powder,a kind of natural,abundant and cheap new passivating agent,has the characteristics of large specific surface area,plentiful nanopores,and great capacity of Cd sorption.Similarly,biochar,which is produced from various straw sources,is also a new passivating agent with large specific surface area,stable properties,carbon-rich porous,and oxygen-containing functional groups.Biochar can reduce the mobility and availability of Cd in soils through coprecipitation,adsorption,complexation,and ion exchange reactions with Cd ions.Currently,the application of biochar alone or in combination with other passivating agents has been reported in many previous studies,but the remediation effect of biochar and oyster shell powder is still unclear.Therefore,this study selected a calcareous soil with two levels of cadmium pollution(low level-0.3 mg/kg,high level-2 mg/kg)as the object,used amaranth as the test crop,and employed the indoor soil incubation combined with pot experiment,aiming to understand the remediation effect of oyster shell powder combined with biochar,and their applied proportions on the Cd-polluted soil in karst area and their influence on Cd uptake by plant,There were a total of 6treatments: no oyster shell powder and biochar(CK),5% oyster shell powder(A),5%biochar(B),2.5% biochar +2.5% oyster shell powder(C),2% biochar +3 oyster shell powder(D),and 3% biochar +2% oyster shell powder(E).The available Cd(DTPA-Cd),Cd fractions,enzyme activities,physiological status of amaranth(plant height,malondialdehyde,chlorophyll,enzyme activities)and Cd uptake were systematically studied.The results showed that:(1)The addition of oyster shell powder and/or biochar in callitic soil could significantly reduce the content of available Cd(DTPA-Cd)in the soil,ranging from15.5%-44.1%(P<0.05),and DTPA-Cd in low Cd soil decreased from 15.5%-43.6%(P<0.05).DTPA-Cd in high Cd soil decreased in a range of 16.1%-44.1%(P< 0.05),and there was no significant difference in DTPA-Cd content between indoor soil culture experiment and pot experiment(P>0.05).Under different soil cadmium pollution levels,5% oyster shell powder had the best effect,ranging from 21.8% to44.1%(P<0.05).In the pot experiment,the DTPA-Cd content in rhizosphere soil was also significantly reduced by single or combined application of biochar and oyster shell powder,ranging from 12.0% to 39.4%(P<0.05).Among different treatments,the Cd content in DTPA-extracted soil was most significantly reduced by 5% oyster shell powder.(2)Adding oyster shell powder and/or biochar to the calcareous soil can significantly reduce the proportion of ion exchange cadmium and carbonate binding Cd in soil(9.6%-42.3% and 5.5%-42.2%,respectively).At the same time,the proportions of Fe-Mn oxide binding cadmium(1.6-22.7%),organic matter and sulphide binding Cd(1.1%-24.2%)and residual Cd(0.2%-40.3%)were increased.Among them,5% oyster shell powder treatment had the greatest effect on soil Cd fractions,followed by 5% biochar.There was no significant difference between the indoor soil culture experiment and the pot experiment(P > 0.05).(3)The application of oyster shell powder and biochar alone or in combination with different Cd pollution levels increased the soil p H value by 0.1～0.3 units.In addition,single or combined application of oyster shell powder and biochar also changed soil enzyme activities.Urease activity in high Cd soil was significantly increased by treatments A,B and C,and the effect of C treatment was the most significant,up to158.6%.Catalase activity was significantly increased by B and C treatments,up to21.8% and 18.6%,respectively.The activity of sucrase increased by 99.7%,51.4%and 42.2% under A,B and D treatments,but decreased by 26.1%-63% under C and E treatments.(4)In the pot experiment,the order of Cd content in all parts of amaranth was leaves > roots > stems,and the cadmium content in all parts of roots,stems and leaves increased with the increase of Cd pollution level.The Cd content in the root,stem and leaves of amaranth decreased by 20.5%-52.6%,20.4%-50.3% and 26.4%-52.9%(P<0.05),respectively,after oyster shell powder and biochar application alone or combined application.In the soil with low level of Cd,the Cd content in the roots,stems and leaves of amaranth did not exceed the maximum limit(Cd≤0.2mg/kg)stipulated by the national standard for safe consumption(GB2762-2017)after applying 5% oyster shell powder alone.(5)Single or combined application of oyster shell powder and biochar had significant effects on the activities of catalase(CAT),peroxidase(POD)and superoxide dismutase(SOD)in roots,stems and leaves of amaranth.In low cadmium soil,treatments A,B,D and E significantly decreased the activities of CAT and increased the activities of POD and SOD in leaves.Treatments B,C and D reduced the activities of CAT and POD,and increased the activities of SOD.C,D and E treatments reduced the activities of CAT and POD.In high cadmium soil,treatments A,B,C and D increased the activities of CAT,POD and SOD in leaves.Treatments A and C reduced POD activity,while treatments B,D and E increased CAT activity and decreased SOD activity.Treatments A and D increased the activities of CAT,POD and SOD in roots,while treatments B,C and E reduced the activities of CAT and increased the activities of POD and SOD in roots.(6)The application of biochar and/or oyster shell powder in soil with different Cd pollution levels decreased the malondialdehyde content of amaranth from 5.0% to87.1%(P<0.05),and the lowest MDA content was 2.64mmol/g FW under E treatment.The content of malondialdehyde increased with the increase of Cd concentration under A,B and C treatments.The contents of chlorophyll a,chlorophyll b and chlorophyll a+b were not significantly affected by different treatments(P>0.05).(7)The concentration of Cd in amaranth shoot was positively correlated with Cd content in amaranth root,malondialdehyde,CAT activity in leaves,SOD activity in leaves,CAT activity in stems,POD activity in stems,available Cd content in soil,Cd exchangable state in soil,carbonate binding state,iron and manganese oxide binding state,organic matter and sulfide binding state.It was negatively correlated with leaf POD activity,leaf chlorophyll content,root SOD activity and soil phosphatase activity.These results indicated that single or combined application of oyster shell powder and biochar could affect the uptake of Cd by amaranth mainly by affecting soil available Cd content,Cd fractions,phosphatase activity and the physiological status of plants.In conclusion,in a calcareous soil with different levels of Cd pollution,single application or combined application of oyster shell powder and biochar can significantly reduce the bioavailability of soil Cd i,and then effectively reduce the accumulation of Cd in amaranth.The single application of 5% oyster shell powder has the best effect.Therefore,oyster shell powder can be used in the remediation of calcareous Cd-contaminated soil in Guangxi,however,it is not recommended to plant amaranth in calcareous soil contaminated by Cd.