Passivation Effect And Regulation Of Calcium And Magnesium Based Remediation Materials On Combined Pollution Of Cadmium And Arsenic In Paddy Fields

Cadmium and Arsenic（Cd-As）compound pollution in cultivated soil is serious and complex to control.Effective remediation of Cd-As compound pollution paddy field is the difficulty to realize the safe utilization of farmland at present.Calcium（Ca）and magnesium（Mg）are important ions regulated by plant signal network.The migration,absorption and transport of Cd-As in the"soil crop"system are closely related to the regulation of Ca²⁺and Mg²⁺.The study on the passivation effect of Ca and Mg on paddy soil and the regulation characteristics of Cd/As enrichment in rice is of great significance for repairing the paddy soil polluted by Cd/As and reducing the content of Cd/As in rice.In this study,the field experiment of Cd/As compound pollution of paddy field in one season and two consecutive seasons was carried out by applying five kinds of Ca and Mg passivators（phosphate rock powder conditioner,terbesite calcium soil conditioner,calcium magnesium phosphate fertilizer,attapulgite nutrient soil and quicklime）,the pot experiment of Cd/As compound pollution of paddy field soil by applying single Ca and Mg（calcium oxide,calcium carbonate and magnesium carbonate）,and the water culture experiment of Cd/As compound stress by adding exogenous Ca and Mg（Ca Cl₂and Mg Cl₂）,At the same time,the regulation of Cd/As transport by Ca/Mg in the"soil rice"system was explored in the whole research process.The main research results show that:（1）The passivation effect of five Ca and Mg passivators on Cd-As compound polluted paddy soil.The application of Ca and Mg passivators did not significantly improve the soil pH and soil organic matter in calcareous paddy fields.The five passivators（except quicklime）all reduced the proportion of weak acid extractable Cd in soil.Among them,the proportion of weak acid extractable Cd in attapulgite nutrient soil decreased by 15.33%compared with CK in the first quarter;Except for attapulgite nutrient soil,other passivators reduced the proportion of weak acid extractable state of soil as.Among them,the proportion of weak acid extractable state of terbesite calcium soil conditioner and calcium magnesium phosphate fertilizer decreased by 7.25%and 7.05%respectively in the first quarter compared with CK.Only calcium magnesium phosphate fertilizer reduced the available Cd content in the first and second seasons,which was 0.10 mg·kg^-1and 0.3 mg·kg^-1lower than CK respectively;Calcium magnesium phosphate fertilizer and attapulgite nutrient soil significantly reduced the content of available as in the first quarter,which was 0.37mg·kg^-1and 0.42 mg·kg^-1lower than CK respectively.（2）Effects of five Ca and Mg passivators on Cd/As content in brown rice in Cd as composite polluted paddy field.In the first quarter,the Cd content of brown rice treated only with terbesite calcium was lower than that of CK,and the Cd content of brown rice treated with other passivators was higher than that of CK;In the second season,except for attapulgite nutrient soil,the Cd content of brown rice treated with other passivators was significantly lower than that of CK.In the first quarter,the content of as in brown rice treated with five passivators was lower than that of CK,among which terbesite calcium and quicklime were the most significantly lower than that of CK,decreased by 0.50 mg·kg^-1and 0.52 mg·kg^-1respectively;In the second quarter,the as content of brown rice treated with calcium,quicklime and calcium magnesium phosphate fertilizer was lower than that of CK,and the as content of brown rice treated with quicklime was the most significantly lower than that of CK,which was 2.03 mg·kg^-1.（3）The passivation effect of single Ca and Mg on Cd-As compound polluted paddy soil.Calcium oxide and calcium carbonate significantly increased soil pH at tillering stage,while calcium oxide,calcium carbonate and magnesium carbonate significantly decreased soil organic matter at jointing stage,filling stage and mature stage.Calcium oxide,calcium carbonate and magnesium carbonate all increased the percentage of Cd weak acid extractable in soil at each growth stage.After adding calcium oxide,the percentage of Cd weak acid extractable in soil at each growth stage was higher than that of other treatments.The percentage of Cd weak acid extractable in soil at each growth stage was 41.2%,39.50%,34.25%and 32.02%respectively.Calcium oxide and calcium carbonate treatment increased the proportion of as weak acid extractable in soil at tillering stage and jointing stage,magnesium carbonate decreased the proportion of as weak acid extractable in soil at tillering stage,filling stage and mature stage,and magnesium carbonate treatment significantly increased the proportion of as weak acid extractable in soil at jointing stage.The effective Cd content of calcium oxide,calcium carbonate and magnesium carbonate treatment at jointing stage,filling stage and mature stage decreased,but the effective Cd content of magnesium carbonate treatment at tillering stage did not decrease.Calcium oxide,calcium carbonate and magnesium carbonate increased the content of available as in each growth period,and the increase of available as in calcium oxide treatment was the largest.（4）Effect of single Ca and Mg passivators on Cd/As content of brown rice in Cd-As composite polluted paddy field.The content of Cd/As in root of calcium oxide treatment at jointing stage,filling stage and mature stage was higher than that of CK,and the content of Cd/As in root of other treatments varied in different stages.Calcium oxide and calcium carbonate treatment increased Cd/As content in rice leaves at tillering stage,filling stage and mature stage,while magnesium carbonate treatment increased as content in rice leaves at jointing stage,filling stage and mature stage.The content of Cd/As in rice stems at different growth stages was significantly reduced by magnesium carbonate treatment,and the content of Cd/As in rice stems at grain filling stage was increased by calcium oxide and calcium carbonate.The content of Cd/As in mature brown rice treated with calcium oxide,calcium carbonate and magnesium carbonate decreased significantly,but could not significantly reduce the content of Cd/As in grain filling stage.（5）Effects of exogenous Ca and Mg Addition on Cd/As content in various organs of hydroponic rice under Cd-As compound stress.Under low concentration of Cd/As combined stress,7 treatments at tillering stage and a single low addition of Ca at maturity stage significantly inhibited Cd/As uptake by rice roots,while Ca and Mg treatments at other growth stages did not significantly inhibit Cd/As uptake by rice roots;Except at tillering stage,the seven treatments significantly inhibited as uptake by rice stems,and a single high addition of Ca significantly promoted Cd uptake by rice stems;Single and combined mg and Ca,Mg treatments significantly inhibited as uptake by rice leaves at each growth stage,and the seven treatments significantly promoted Cd uptake by rice leaves at each growth stage.Under high concentration Cd/As combined stress,Ca and Mg did not significantly inhibit Cd/As uptake by rice roots at all growth stages;The compound addition of Ca and Mg promoted the uptake of as by rice stems at the filling stage,the single Ca treatment significantly promoted the uptake of Cd by rice stems at various growth stages,and Mg inhibited the uptake of Cd by rice stems;Single Ca and Mg treatments inhibited as uptake by rice leaves at jointing stage and filling stage,and seven treatments promoted Cd uptake by rice leaves.Under Cd/As combined stress,the single addition of Mg inhibited the absorption of as by Rice Panicles and brown rice;The single addition of Ca promoted the uptake of Cd by Rice Panicles and brown rice.（6）Effects of exogenous Ca and Mg Addition on Cd/As transport in hydroponic rice under Cd-As combined stress.Under low concentration Cd/As combined stress,Ca and Mg treatment inhibited the transport of as from root to leaf,from root to stem,from stem to leaf,from stem to rice hull and from rice hull to brown rice at tillering stage,jointing stage,filling stage,jointing stage,and mature stage;It can promote the transportation of as from root to stem and stem to leaf at maturity and from stem to ear at grain filling stage.Under the combined stress of high concentration Cd/As,a single high addition of Ca could promote the transport of as from root to stem and from stem to leaf at jointing stage and mature stage,and inhibit the transport of as from stem to leaf at grain filling stage;Single addition of Mg promoted as transport from root to leaf at tillering stage.Under the combined stress of Cd/As concentration,Ca and Mg treatments promoted the transport of Cd from root to leaf at tillering stage and from stem to ear at filling stage.Under the combined stress of high concentration Cd/As,the transport of Cd from stem to leaf at jointing stage and filling stage treated with Ca and Mg,and the single addition of a large amount of Mg could promote the transport of Cd from stem to leaf at mature stage;Ca and Mg treatment inhibited the transport of Cd from stem to rice hull and from rice hull to brown rice.