Influence Of Water Management On As And Pb Migration And Transformation In Paddy Soil And Rice(Oryza Sativa L.) System

In recent years,the contamination of heavy metal in agricultural soil has raised people’s great concern on the safety of our country’s food products.Controlling the contamination of heavy metal and improving the quality of food products has become research hotspots and also a great challenge in environmental management.Arsenic(As)and lead(Pb),which are typical toxic heavy metal elements,are the main heavy metal pollutants in our country’s vast paddy soil.The complex redox conditions caused by changes of the soil water conditions will affect the morphological transformation and bioavailability of As and Pb in the paddy soil.Exploring the influence of water management on the transporting and speciation transformation of heavy metal in paddy soil-rice system can be of great help for agricultural activities and assessing the environmental risk of heavy metal.In this study,rice and paddy soil was selected as our analytical object.By conducting pot experiment in greenhouse and applying traditional chemical analysis methods combined with scanning electron microscopy and X-ray energy spectroscopy(SEM-EDS),the effects of water conditions on the accumulation of As and Pb in rice at different growth stages(tillering stage,flowering stage,and mature stage)has been studied,then implement 7 different water management modes to explore the best water management mode that can reduce As and Pb accumulation in rice at the same time.Primary conclusions were summarized as follows:(1)The effects of water management on soil pH and Eh and the speciation of As and Pb were clarified.The soil Eh value decreases and the pH value increases under flooding conditions,and the opposite occurs under drought conditions.The proportion of non-specifically adsorbed As in rice soil is extremely low(0.06%～1.23%),and most of As exists in the form of amorphous iron oxide,crystalline iron oxide,and residue.The specifically sorbed As under flooding conditions increased significantly during the flowering stage.Changes in soil water conditions and growth stages did not affect the residual Pb.Soil carbonates-bound is easily converted to an exchangeable state under drought conditions.(2)Impact of water management and the growth stage of rice on iron plaque formation on rice root surface were investigated.Iron plaque was formed densely during the tillering stage,and decreased with the prolongation of the growth stage.Compared with tillering stage,the amount of iron plaque in the mature stage is only9.3%～28.2%.The amount of iron plaque on the root surface under flooding conditions was significantly higher than that under drought conditions.Compared with flooding conditions,the amount of iron plaque under drought conditions is only 8.9%～32.9%.The accumulation of As in rice plants was significantly positively correlated with the Fe and As contents in the iron plaque on the root surface(R~2>0.79,p<0.05),and there was no significant correlation between the lead content in the iron surface of rice roots and the amount of iron film formation.(3)The influence of water management on the accumulation and distribution in different tissues of rice plants were revealed,The accumulation of As and Pb in various parts of rice is as follows:root>leaf>stem>husk>grain.Rice plants reduced the accumulation of As,while significantly increasing Pb accumulation under drought or alternate flooding and drought conditions.The prolonged flooding can significantly reduce the accumulation of Pb in rice grains,and reduce the ratio of As(III)in rice grains while increasing the absorption and accumulation of total As.(4)The main growth stage of As and Pb absorbed by rice were identified.The key stage for As accumulation in rice plants is the flowering stage.Compared with the flowering stage,the As accumulation in the roots and stems in mature stage decreased by 67.0%～95.3%.Under drought conditions,the key stage for Pb accumulation in rice was before the flowering stage,but the key stage for Pb accumulation in rice was after the flowering stage under flooding conditions.(5)An optimal water management model for reducing both As and Pb in rice grains was proposed.The accumulation of arsenic and lead in rice grains can be well predicted by soil Eh and pH.The water management mode that flooding before the flowering period and followed by drying can reduce the accumulation of total As and total Pb in rice grains at the same time.The water management mode of flooded planting during the whole growth stage can reduce As(III)and total Pb accumulation in rice grains at the same time.