Effects Of Straw With Different Treatment On Soil Heavy Metal Immobilization And Organic Carbon Mineralization

Soil heavy metal pollution remediation,farmland soil carbon sequestration and emission reduction and resource utilization of crop straw are hot research topics in agroecological environmental protection.At present,there are more literature focusing on soil heavy metal immobilization and soil organic carbon mineralization alone,but little attention has been paid to the synergistic fixation of soil heavy metals and organic carbon,and there are very few reports on the application of agricultural organic wastes for the synergistic fixation of soil heavy metals and organic carbon.In this study,rice straw treated in different ways（raw straw,decomposed straw,pyrolyzed straw,and burned straw）and biochar made from rice straw at different pyrolysis temperatures（300°C,450°C,600°C,and 750°C）was selected to characterize the physicochemical properties of each straw material,and the effects of different treated straws on the physicochemical properties,heavy metal effectiveness and morphology,and organic carbon mineralization of Cu and Cd contaminated soil were investigated by indoor aerobic incubation tests.The effects of different treatments of straw on the physicochemical properties,the effectiveness and morphology of heavy metals,the mineralization of organic carbon,the content and structure of water-soluble organic carbon,and the microbial community of Cu and Cd contaminated soils were investigated through indoor aerobic incubation tests.The main results are as follows:（1）Different treatments of straw significantly reduced the Cu content of DTPA extracted state in the soil,and the passivation effect showed that burning straw>pyrolysis straw>decomposing straw>virgin straw.The passivation effect of pyrolysis straw and burning straw on Cu was better,reducing the effectiveness of Cu in soil by21.5%and 21.7%,respectively.Pyrolysis straw reduced the content of DTPA-extracted state Cd in the soil by 20.0%,which was the most effective in passivating heavy metals in Cu and Cd mixed contaminated soil.Raw straw and decomposed straw elevated soil DTPA-extracted state Cd content,and there was a risk of aggravating soil heavy metal contamination.The effect of straw burning on soil Cd effectiveness was not significant.Soil p H,effective phosphorus and organic carbon content significantly influenced soil heavy metal morphology and thus altered heavy metal effectiveness.All straws reduced the percentage of Cu and Cd in the weakly acid-extracted state of the soil,converting the weakly acid-extracted Cu to the oxidizable and reducible states and the weakly acid-extracted Cd to the oxidizable and residual states.（2）Raw straw,decomposed straw and burned straw significantly increased the cumulative soil organic carbon mineralization by 13.5%-442.8%,while pyrolysis of straw reduced the cumulative soil organic carbon mineralization and had an inhibitory effect on soil organic carbon mineralization.Soilβ-glucosidase activity and microbial carbon content were significantly and positively correlated with soil organic carbon mineralization.（3）Biochar prepared by pyrolysis at 300°C-750°C significantly reduced the effectiveness of Cu and Cd in soil,resulting in a 19.1%-22.9%reduction in soil DTPA-extracted Cu content and a 5.3%-34.2%reduction in DTPA-extracted Cd content.The correlation analysis revealed that the total soil organic carbon content was significantly and positively correlated with the percentage of oxidizable state of heavy metals,and all biochars enhanced the total soil organic carbon content and promoted the conversion of weakly acid-extracted state of heavy metals to oxidizable state.With the increase of biochar pyrolysis temperature,the p H value and effective phosphorus content of the soil increased,which significantly promoted the conversion of heavy metals in the weakly acid-extracted state to the reducible and residual states.the biochar prepared at750℃had the largest percentage of ash,the highest p H value and effective phosphorus content of the treated soil,and the best comprehensive passivation effect on Cu and Cd in the soil.（4）The accumulated carbon mineralization of biochar-treated soils showed a trend of promoting and then inhibiting the mineralization of organic carbon as the pyrolysis temperature of biochar increased.After the pyrolysis temperature of biochar reached600℃,the accumulated carbon mineralization of biochar-treated soil gradually stabilized.The biochar prepared at high temperature was more carbon stable,and the treated soil had lower water-soluble organic carbon content and more complex structure,which significantly affected the amount of soil organic carbon mineralization.（5）Different treatments of straw affected the enzymatic activity and microbial activity of the soil by altering the biological effectiveness of heavy metals,thus contributing to the mineralization of soil organic carbon.biochar made by pyrolysis of straw at 500°C-600°C had a synergistic effect of fixing soil heavy metals and organic carbon.The biochar prepared at higher temperatures has high ash content,carbon content and stable carbon structure,which can protect soil organic carbon while increasing soil p H,effective phosphorus content and total organic carbon content,promote the conversion of heavy metals in the weak acid extraction state to other forms,reduce the effectiveness of soil heavy metals,thus affectingβ-glucosidase activity and microbial activity in soil,further reducing the decomposition rate of organic carbon in soil This will further reduce the decomposition rate of organic carbon in soil and lower CO₂emission.