| At present,the problem of excessive arsenic(As)content in rice has been widely concerned.The high enrichment capacity of rice for As under flooded conditions makes rice one of the main ways for humans to ingestion inorganic As.It is of great significance to clarify the geochemical behavior of As in paddy fields for soil quality and crop safety.Bioavailability of As in paddy fields is mainly regulated by changes in redox potential(Eh)caused by waterlogging and changes in geochemical factors(such as pH and Fe/Mn oxide solubility,etc.)caused by Eh changes.Under complex redox conditions,it is very important to study the dynamic changes and action mechanisms of different forms of Fe/Mn oxides and As.At present,there are few studies on the interaction between iron and manganese nodules and As in soil,and the studies on FeMn nodules are limited to themselves,ignoring their impact on the environment.Therefore,in this study,the farmland around Hongze Lake was taken as the research area.Firstly,the As content characteristics of soil-rice grains were systematically investigated,and the effects of Fe-Mn nodules on the enrichment of As in rice were analyzed.Then,the ecological effect of Fe-Mn nodules on rice and the mechanism of their influence on As in soil were investigated by pot experiment.The main research results are as follows:(1)The soil-rice survey in the study area found that the content of As in soil was significantly higher than the risk screening values(Soil environmental quality Risk control standard for soil contamination of agricultural land,GB 15618-2018)for soil contamination of agricultural land at sites where Fe-Mn nodules existed,but the content of As in rice was significantly negatively correlated with the content of As in soil.The effect of Fe-Mn nodules in the system should be considered when risk assessment is carried out on the farmland soil containing Fe-Mn nodules.(2)Pot experiment showed that adding Fe-Mn nodules could significantly reduce the content of inorganic As in rice leaves.Under long-term flooding conditions,Fe-Mn nodules could reduce the bioavailability of As in soil,and the main form of As changed from crystallinity Fe-Mn oxide bound arsenic to amorphous Fe-Mn oxide bound arsenic,and the proportion of residual arsenic increased.As in the solution was mainly due to the reduction and release of Mn(hydrogen)oxides.Fe-Mn nodules would release a large amount of Mn into the solution and delay the reduction of Fe(hydrogen)oxides at the same time,and then the formation of Mn and Fe secondary minerals would lead to the re-adsorption of As,reducing the concentration of As in the solution.The increase of amorphous Fe and Mn content in soil solid phase also reduced the concentration of As in solution.Therefore,Fe-Mn nodules positively regulate the migration and transformation of As in soil-solution,which will reduce the enrichment of As in rice.(3)Organic matter(rice straw)was added on the basis of the presence of Fe-Mn nodules,and it was found that it promoted the enrichment of As in rice and enhanced the absorption of inorganic As in rice.Therefore,straw returning to paddy fields containing Fe-Mn nodules was not suitable.Under long-term flooding conditions,organic matter could increase the bioavailability As in the soil containing Fe-Mn nodules,and the main form of As changed from stable state to unstable state,indicating that organic matter promoted the reduction dissolution and morphologic transformation of As crystallized in the soil containing Fe-Mn nodules.The addition of organic matter slightly reversed the positive regulation of Fe-Mn nodules on soil solution.Organic matter promoted the reduction dissolution of Fe(hydrogen)oxides and the release of As,As well As the competitive adsorption of dissolved organic carbon(DOC)and As on the surface of soil minerals and the formation of dissolved As-DOC complex,resulting in the increase of As concentration in solution. |
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