| Cd contamination in farmland soil poses serious threats to food safety and human health,which has aroused worldwide public concerns.In recent years,soil Cd contamination is increasing due to atmospheric deposition,sewage irrigation and livestock manure application in northern China,a main wheat producing area.Application of amendments to reduce soil Cd activity is an effective strategy to alleviate Cd toxicity risk to humans through the food chain.At present,most amendments decrease the soil Cd activity mainly through the regulation of p H,which is not suitable for the remediation of weakly alkaline soil in northern China.Therefore,it is highly necessary to develop efficient amendments for the remediation of alkaline Cd-contaminated farmland soil to ensure food security and human health.Traditional amendments such as raw biochar and clay minerals generally have poor performance in the remediation of heavy metal contaminated weakly alkaline soil.Therefore,modification of traditional amendments to enhance their remediation effect on heavy metal-contaminated soil has become a hot spot of research in recent years.Although their contents in soil are generally not high,iron and manganese oxides have high adsorption and oxidation activities.Further improvement of their environmental function to remediate heavy metal contaminated soil has attracted extensive attention.Manganese oxide is a kind of mineral with high adsorption capacities for heavy metals.Modification of traditional amendments with manganese oxide minerals can significantly improve the remediation efficiency of heavy metal contaminated soil.In this study,the effect of manganese oxide on Cd immobilization was clarified through wheat field plot and large-scale field experiments.Then,the remediation effect and mechanism of manganese oxide-modified amendments（zeolite-supported manganese oxide and Mn O2-modified bamboo charcoal）on Cd-contaminated weakly alkaline upland farmland soil were investigated through wheat pot experiment.The main results are as follows.1.Combined amendments of biochar,zeolite and humus showed good performance in the remediation of Cd-contaminated weakly alkaline upland soil.The combined amendment significantly decreased the content of soil available Cd（Ca Cl2-Cd）by 76.5%–84.8%,which could be ascribed to the conversion of soil Cd from acid soluble fraction to reducible fraction under the treatment.The contribution of single components to Cd immobilization in the combined amendments follows the order of humus>biochar>zeolite.The Cd content in wheat grains,straw and roots could be significantly reduced by59.3%,45.0%and 68.2%,respectively,and that in wheat grains was below the national food safety standards of China(0.1 mg kg–1,GB2762–2017).Moreover,the combined amendment also exhibited high stability in weakly alkaline upland soils.The Cd content in wheat grains decreased from 0.12 mg kg–1 to 0.04–0.07 mg kg–1 in the second successive year,indicating that the function of soil iron and manganese oxides in Cd stabilization could be enhanced through the compound amendments,which may help realize safe production of wheat in the Cd contaminated weakly alkaline soil.2.Zeolite-supported Mn oxides could well remediate moderately Cd-contaminated(1.96±0.13 mg kg–1)weakly alkaline upland soil,and significantly reduce Cd accumulation in wheat.The decrease in soil Cd activity could be attributed to the conversion of exchangeable Cd fraction to more stable Fe-Mn oxide-bound Cd fraction.In addition,the loading amount of manganese oxide also exhibited an important influence on the stabilization of soil Cd.Under the optimal conditions,the soil available Cd content decreased from 1.06 mg kg–1 to 0.59 mg kg–1.The proportion of exchangeable Cd fraction in total soil Cd decreased from 49.5%to 16.0%,and that of Fe-Mn oxide-bound Cd fraction increased from 26.6%to 63.4%.The Cd content in wheat grains decreased from 1.20 mg kg–1 to 0.42 mg kg–1.Wheat pot experiments demonstrated that zeolite-supported manganese oxide has a high chemical stability and stable Cd immobilization performance in two successive years.3.Mn O2-modified bamboo charcoal exhibited excellent immobilization performance on slightly Cd-contaminated(1.39±0.02 mg kg–1)weakly alkaline upland soil,and significantly decreased Cd accumulation in wheat.This compound amendment could significantly decrease the bioavailability of soil Cd and wheat Cd accumulation by transforming acid soluble Cd fraction to more stable reducible Cd fraction.Mn O2-modified bamboo charcoal significantly increased the content of free Mn O2 in soil silt sized-aggregates,which could promote Cd stabilization in soil silt sized-aggregates.The correlation analysis results revealed that free Mn O2 in soil aggregates had significant negative correlations with the content of Cd in different parts of wheat and available Cd in soil,further indicating that Mn O2-modified bamboo charcoal can decrease the bioavailability of soil Cd and Cd accumulation in wheat by increasing the content of soil free Mn O2.Under the optimal conditions,the content of soil available Cd was decreased by 44.7%,and that in wheat grain,straw and root was reduced by 66.4%,38.4%and 34.8%,respectively.4.In the further pot experiments of wheat with slightly Cd contaminated soil(1.11±0.02mg kg–1),Mn O2-modified bamboo charcoal significantly improved the soil quality,as well as obviously increased the soil enzyme activity,available potassium and organic matter content.Moreover,soil enzyme activity had a great inhibitory effect on wheat Cd accumulation,and soil available Mn content,available potassium content,cation exchange capacity and soil p H showed significant negative correlations with soil available Cd content and the Cd content in wheat grains,straw and roots.These results further indicated that Mn O2-modified bamboo charcoal can improve soil enzyme activity and soil properties,which may contribute to the decrease in soil Cd activity and inhibition of wheat Cd accumulation.Under the optimal conditions,the soil p H,organic matter content and available potassium content could be increased by 0.37 units,18.9%and 143.4%;and soil dehydrogenase,manganese peroxidase,alkaline phosphatase andβ-glucosidase activity could be enhanced by 25.6%,24.3%,24.3%and 25.5%,respectively.The content of soil available Cd was decreased by 30.8%,and that of Cd in wheat grain,straw and root was reduced by 64.2%,30.8%and 49.7%,respectively.Overall,this work illustrates the mechanism for Mn oxide-based materials to remediate Cd-contaminated weakly alkaline upland arable soil and inhibit wheat Cd accumulation,providing a new way for the remediation of Cd-contaminated weakly alkaline upland soil in northern China,as well as improving the application of Mn oxide in soil remediation. |
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