The Mitigation Effect Of Composite Modified Biochar On Cd/Pb Migrationin In Soil-Chinese Cabbage System And The Response Of Soil Dom

Biochar is a promising environmentally friendly soil amendment and has been widely used in the remediation of heavy metal pollution in soil.However,the mitigation effect of silicon and phosphorus omposite modified biochar on Cd/Pb migration in soil-vegetable system and soil fertility regulation effect,together with their mechanism are not yet fully understood.At the same time,understanding the response of soil dissolved organic matter（DOM）components and environmental effects under silicon and phosphorus composite modified biochar,is great significant for evaluating environmental effects and application risks of composite modified biochar.In this study,we used pot experiments,studied the preparation of composite modified biochar B5PSi₆₀₀（corn straw:diatomite:heavy superphosphate（m/m/m）=5:1:1,600°C）and urea further modified B₅PN₆₀₀(B₅PSi₆₀₀:urea（m/m）=4:1)and pure biochar(B₆₀₀,600°C)with corn straw,diatomite and heavy superphosphate as raw materials,under different application amounts（0.1%,1%,and3%）,on the migration behavior of heavy metal Cd/Pb in soil-Chinese cabbage system,soil fertility regulation effect and the growth characteristics of Chinese cabbage.Meanwhile,we used spectroscopic analysis techniques to reveal the effects of different biochar applications on soil DOM characteristics and try to explore the relationship between soil DOM with the growth and heavy metal migration of Chinese cabbage under biochar application.1)The application of composite modified biochar effectively passivated the Cd/Pb contaminated soil and improved the soil ecological environment.The application of composite modified biochar effectively reduced the availability of Cd/Pb in soil and promoted the transformation of acid-soluble and reducible speciation of soil Cd/Pb into residual and oxidizable speciation.The reduction effect of different biochar on Cd/Pb in soil was B₅PSi₆₀₀>B₅PN₆₀₀>B₆₀₀>CK（control）,and the reduction effect was more obvious with the prolongation of culture time.Compared with CK,the immobilization rates of Cd in soil by B₅PSi₆₀₀（3%）and B₅PN₆₀₀（3%）were 21.59%and 15.04%,respectively,and the immobilization rates of Pb in soil were19.62%and 16.16%,respectively.The application of B₅PSi₆₀₀（3%）showed the best immobilization effect on soil heavy metals.The composite modified biochar B₅PSi₆₀₀ and B₅PN₆₀₀ significantly promoted the increase of soil cation exchange capacity（16.12%,12.91%）and soil organic matter（13.59%,23.00%）.At the same time,it also significantly increased the content of available nitrogen（8.15%,8.92 times）,available phosphorus（13.00 times,11.47 times）,available potassium（1.33 times,1.85 times）and available silicon in soil,and their content increased with the increase of application amount.Overall,B₅PN₆₀₀ application significantly increased soil nitrogen content;B₅PSi₆₀₀ application significantly increased soil phosphorus and silicon content.Furthermore,the application of composite modified biochar promoted the activities of soil catalase,alkaline phosphatase,and urease,while reducing the activity of sucrase.2)The application of composite modified biochar significantly reduced the migration and accumulation of Cd/Pb in the soil-Chinese cabbage system,and promoted the growth of Chinese cabbage.The application of composite modified biochar reduced the accumulation of Cd/Pb in Chinese cabbage,and reduced the enrichment coefficient and migration coefficient of Cd/Pb in the Soil-Chinese cabbage system.Among them,B₅PSi₆₀₀（3%）application had the most obvious mitigation effect on the migration and accumulation of Cd/Pb in Chinese cabbage,the enrichment coefficients of Cd/Pb decreased by 38.74%and 53.48%,respectively,and the transfer coefficients of Cd/Pb decreased by 34.89%and 20.28%,respectively.Meanwhile,compared with CK,the composite modified biochar significantly increased the height,root length,and biomass of Chinese cabbage.At the same time,the composite modified biochar significantly promoted the increase of nutrient elements（N,P,K,Si）in Chinese cabbage.Compared with CK,B₅PN₆₀₀（0.1%）increased the N content in Chinese cabbage by 190.21%;B₅PSi₆₀₀（3%）increased the contents of P,K,and Si in Chinese cabbage increased by 351.93%,20.22%and 50.60%respectively.In addition,the composite modified biochar weakened the oxidative stress of heavy metals on Chinese cabbage,increased the chlorophyll content,and reduced the antioxidant enzyme,superoxide dismutase activity,and malondialdehyde content of Chinese cabbage.Among them,B₅PSi₆₀₀ significantly weakened the oxidative stress of Chinese cabbage.3)The application of composite modified biochar increased the content of soil DOM and its bioavailability,and reduced the potential risk of pure carbon to enhance the effectiveness of heavy metals.Pure biochar and composite modified biochar had high DOM content,and bioavailability index（BIX）of modified biochar DOM increased after composite modification.Parallel factor analysis showed that B₆₀₀DOM had higher percentage of fulvic acid components,while B₅PN₆₀₀ and B₅PSi₆₀₀ were lower percentage of fulvic acid components and higher percentage of protein-like components.Compared with CK,the content of soil DOM increased after the application of three biochars.Parallel factor analysis showed that the percentage of fulvic acid-like components in soil DOM decreased after the application of B₅PN₆₀₀ and B₅PSi₆₀₀ compared with B₆₀₀.Redundancy analysis（RDA）showed that DTPA-Cd was positively correlated with fulvic acid-like components（S1）and negatively correlated with protein-like components and DOM content.DTPA-Pb was positively correlated with DOM content and protein-like components,and negatively correlated with fulvic acid-like components（S1+S2）.Meanwhile,the fulvic acid-like components（S1+S2）were positively correlated with plant biomass and Cd accumulation,indicating that the DOM fulvic acid-like components promoted the growth of Chinese cabbage and had the potential risk of improving Cd mobility.Compared with pure biochar,with the application of composite modified biochar,the fulvic acid component of soil DOM decreased,which could reduce the potential risk of the application of pure biochar on the increase of heavy metal migration.