Effects Of Mercapto-palygorskite Immobilization On Cadmium Speciation In Alkaline Soil And Wheat Accumulation Mechanism

China is one of the countries with the largest wheat planting area and yield in the world,and due to suitable soil types and climatic conditions,northern China is the main wheat producing area.However,due to long-term sewage irrigation,industrial and mining waste gas,waste residue emissions,and other human activities,the Cd content in soil and wheat grains in northern China exceeds the standard.Cd is a toxic heavy metal,posing a significant threat to the soil environment and food security.As a new immobilization material,mercapto-modified palygorskite has shown good passivation performance in the remediation of Cd pollution in acid paddy fields in southern China in recent years,and can effectively reduce the availability of Cd in soil and the content of Cd in grains.However,currently,there are relatively few experimental studies on alkaline soil contaminated with Cd in northern China,especially the application effects of MP in the field,specific passivation mechanisms,the mechanism of Cd reduction in wheat,the impact on Cd transport in the soil wheat system,and the inhibition and control effects of key organs on Cd during wheat filling process are still unclear.Therefore,this article conducted relevant researches on the above issues using a combination of field and pot experiments,with the aim of providing theoretical and technical support for the prevention and control of Cd contaminated alkaline wheat fields in north China.The main research results are as follows:（1）In soil with a depth of 0-20 cm,MP significantly decreased the DTPA-Cd content in the bulk soil by about 60.7%,and increased the GWD and R_0.25 values.Similarly,the content of DTPA-Cd in soil aggregates also significantly decreased by 40.2-63.6%.MP had no significant effects on soil aggregate mass distribution,p H value,CEC value,and enzyme activity.The content of OM,DOC,available Fe,Mn,and S in soil aggregates significantly increased by 15.0-19.1%,19.2-41.7%,24.7-41.2%,and 12.5-35.1%,respectively.The Cd content of aggregates,especially the exchangeable Cd（EXC-Cd）was significantly decreased by 5.4-28.1%and Fe-Mn oxide bound Cd（OX-Cd）was increased by 22.3%-50.4%,respectively.In addition,MP has different effects on the GSF value of soil aggregates,but the AF_Xvalue has a downward trend at a soil depth of 0-20 cm.In soil depths of 20-40 cm and 40-60 cm,MP had almost no significant impact on the above indicators,but had a slight impact on the Cd content,GSF,and AF_X values in individual aggregates.Smaller particle size aggregates（<1 mm）and larger particle size aggregates（>1 mm）contribute 59.1%and 22.00%to the reduction of effective Cd in the bulk soil.The partial least squares structural equation model（PL-SEM）shows that S promotes the production of effective Fe,Mn,OM,and DOC,while the content of DOC inhibits the formation of EXC-Cd,while effective Fe and Mn promote the production of OX-Cd.（2）In non-rhizosphere soil,that was under the sole action of MP,the available Cd content in soil with different particle size aggregates was significantly reduced by 58.15-61.77%and changed the Cd fractions.The main performance is that the content of EXC-Cd was significantly reduced and the content of OX-Cd was significantly increased.In the rhizosphere soil,that was under the combination effect of rhizosphere effect and MP immobilization effect,the available Cd content was further reduced by 68.92-75.74%,and the Cd fractions was further changed,but the total Cd content in soil did not significantly change.No matter in rhizosphere soil or non-rhizosphere soil,MP did not interfere with the stability of microbial communities.However,under the combined effect,the number of microbial markers in soil increased,and the number of microbial markers in bacteria was greater than that of fungi,and the number of microbial markers was inversely proportional to the size of aggregates.The collinear network of different forms of Cd and microorganisms under the synergistic effect of rhizosphere effect and MP immobilization effect was more complex,but its complexity was not significantly related to the particle size of aggregates.This combined effect promotes more bacterial and fungal groups related to redox,heavy metal chelation,Fe element transformation,and humus generation to participate in the immobilization process of Cd.However,no matter in which group,the effect of fungi on the Cd immobilization process was greater than that of bacteria,and this effect was further enhanced under the synergistic effect of rhizosphere effect and MP immobilization effect.（3）The addition of MP significantly reduced the Cd content of wheat root and shoot from 68.91%to 74.32%and from 70.68%to 77.2%,respectively,while significantly increasing the content of sulfhydryl compounds GSH and PCs by 15.40-21.26%and 33.43-48.83%,respectively.At the same time,the distribution ratio of Cd in cell walls and organelles of wheat decreased,while the distribution ratio of soluble component Cd increased,but the content of Cd in all components was significantly reduced.MP thickens the cell wall,causing vesicles to appear on the cell membrane,with almost no plasmolysis.The organelle structure within the cell is more complete,and the thylakoids on the chloroplast are arranged more neatly and clearly.Transcriptome analysis further confirmed the above results.MP enhanced the transcription process of cell wall components（lignin and cellulose）,promoted cell membrane production and regulated protein expression,increased intracellular plant hormone signals,significantly expressed genes for antioxidant synthesis,and further enhanced wheat photosynthesis,alleviating the toxicity of Cd to wheat.（4）MP had a strong immobilization effect on Cd in alkaline farmland,which could play a role throughout the wheat growing season,but would not affect the soil p H value.The contents of DTPA-Cd and EXC-Cd in soil decreased by 34.88-49.71%and 49.36-84.81%respectively during the entire stage,while OX-Cd increased by 34.61-43.60%.At maturity,the Cd content in grains decreased from 0.118mg/kg to 0.069 mg/kg,which was lower than the national standard limit（0.1 mg/kg）.The roots and nodes of wheat are the key organs that MP inhibits and controls the accumulation of Cd in grains,and the effect of MP on them is relatively weaker at flowering and filling stages than at other stages.MP can reduce the transport and accumulation of Cd to grains by regulating the antagonistic or synergistic effects of coexisting elements（Ca,Mg,Fe,Mn,Cu,Zn）on Cd.（5）Ater adding MP,the DTPA-Cd and EXC-Cd in soil at various stages significantly decreased by 36.50-55.81%and 63.41-83.87%,respectively,while OX-Cd significantly increased by 67.84-106.63%,but there was no significant periodic change,and there was no significant seasonal fluctuation in microbial community.At maturity,the Cd content in wheat grains decreased from 0.120 mg/kg to0.073-0.095 mg/kg.Path model analysis showed that root is the most critical organ for MP to affect Cd transfer to grains,and the reduction rates of Cd at different stages were 27.51-38.39%（pre-filling stage,FS1）,0-7.78%（peak-filling stage,FS2）,and 27.76-42.07%（mature stage,MS）,respectively.Combining the changes of soil Cd content indicators and soil microbial indicators at different stages,it can be seen that MP has a direct and periodic effect on the Cd content of wheat roots.The addition of MP changed the elemental composition of wheat root system during the filling process,and increased the content of sulfhydryl compounds in the early filling and mature stages.The effect of MP on wheat roots is weakest at the peak filling stage.During the pre-filling and mature stages,MP reduces the accumulation of Cd by regulating root cell membrane metabolism,amino acid metabolism,secondary metabolism,nucleotide metabolism,and vitamin metabolism.