The Impact Of Crop Straws On The Immobilization Of Arsenic And Cadmium And Their Bioavailability In Alkaline Soils

Northern China’s agricultural soils are facing a severe threat of combined arsenic（As）and cadmium（Cd）contamination,posing a challenge to the safe production of food crops.Due to the distinct chemical properties of As and Cd,their coexistence presents a significant remediation challenge.China generates a large amount of crop straw annually,with 43.2%of straw being used for field incorporation.Studies have shown that soluble organic carbon（dissolved organic carbon,DOC）in crop straw can combine with widely distributed iron oxides in soil,forming organo-ferrihydrite coprecipitates（OFCs）that immobilize heavy metals（HMs）in the soil.Therefore,investigating the fixation mechanisms of As and Cd by straw-derived OFCs holds important practical significance.Furthermore,considering the differences in soil alkalinity and DOC composition among various types of incorporated straw in northern regions,it is necessary to further explore the fixation mechanisms of As and Cd by different types of incorporated straw.The following are the main research findings of this study:Through As K-edge X-ray absorption near-edge structure（XANES）spectroscopy analysis,the adsorption of As（III）on organo-ferrihydrite coprecipitates（OFCs）synthesized using maize or rape straw-derived dissolved organic carbon（Ma-DOC or Ra-DOC）and its molecular mechanisms under different conditions were revealed.The research results demonstrated that ferrihydrite（Fh）exhibited higher adsorption capacity for As（III）compared to OFCs.The presence of dissolved organic carbon（DOC）enhanced the adsorption of As（III）particularly at p H 5-6.Furthermore,XANES spectroscopy revealed that approximately 30%of adsorbed As（III）on OFCs was oxidized to As（V）at p H 7-8,primarily due to the presence of functional groups（such as O-H,COOH,and C=O）in the DOC that facilitated the oxidation of As（III）to As（V）.Additionally,ion strength analysis indicated that inner complexation was the main adsorption mechanism of As（III）on OFCs.Two different types of alkaline soils（industrial and irrigated）were investigated to assess the impact of 1%and 2%addition of maize straw（MS）through 56 days of aging on the bioavailability of As and Cd.The research findings revealed that the addition of MS led to a decrease in p H by 0.76（industrial soil）and0.83（irrigated soil）,respectively.Additionally,the DOC concentration increased by 54.40 mg/kg（industrial soil）and 100.00 mg/kg（irrigated soil）.Furthermore,the addition of straw significantly influenced the bioavailability of As and Cd.The aging experiment demonstrated that after 56 days,the Na HCO3-extractable As and DTPA-extractable Cd content increased by 50%and 37.9%,respectively,in the industrial soil,while in the irrigated soil,the Na HCO3-extractable As and DTPA-extractable Cd content increased by 39.50%and 37.40%,respectively.Moreover,the sequential extraction results using the BCR method confirmed that the addition of MS increased the exchangeable fraction of As and Cd.Solid-state 13C nuclear magnetic resonance（NMR）indicated that alkyl C and alkyl O-C-O played a significant role in the migration of As and Cd in both soils.Furthermore,analysis of 16S r RNA revealed that the addition of MS promoted the activation of arsenic and cadmium in both soils by Acidobacteria,Chloroflexi,Actinobacteria,and Bacillus.A pot experiment was conducted to investigate the effects of 1%and 2%maize straw（MS）on the bioavailability and binding forms of As and Cd in smelting-contaminated soil（SS）and irrigation-contaminated soil（IS）.The study also examined the mechanisms of As and Cd uptake by winter wheat and the response of rhizosphere soil bacterial communities.The results showed that the addition of 2%MS significantly increased the bioavailability of As in the soil,by 37%（SS）and 39%（IS）（p<0.05）,while there was no significant change in the bioavailable Cd content.After the addition of 2%MS,the As concentration in wheat grains increased from 0.22 mg/kg to 0.51 mg/kg in the smelting-contaminated soil and from 0.59 mg/kg to 0.84 mg/kg in the irrigation-contaminated soil,exceeding the maximum permissible concentration set by national food safety standards（0.5 mg/kg）.The Cd content remained below 0.10 mg/kg in all treatments,complying with the maximum permissible intake value for food safety.The study indicated that the addition of straw enhanced the desorption of As mediated by dissolved organic carbon,resulting in increased Na HCO₃-extractable As in the soil and promoting its transfer to wheat grains.At the same time,the addition of straw reduced the DTPA-extractable Cd content in the soil,thereby decreasing Cd transport to wheat leaves and grains.The results of the BCR fractionation extraction indicated that the addition of 2%MS increased the fraction of labile As in the soil and transformed soluble Cd into a fixed Cd fraction.Further analysis of 16S r RNA revealed that the addition of straw promoted the relative abundance of Actinobacteria,Bacteroidetes,and Firmicutes in the soil.Therefore,MS demonstrated a significant effect in reducing Cd accumulation in wheat leaves and grains,potentially serving as an effective measure to ensure the safe production of wheat in Cd-contaminated agricultural fields.