Study On Transportation And Translation Of Arsenic In The Soil-rice System And Possible Mechanisms

Nowadays,Chinese government and experts has paid high attention to the issues of As contemination in soil and food.Besides,the translocation and transformation of arsenic in the system paddy soil-rice plants,as essential biogeochemical behaviors,are crucial to the quality and safety of agricultural products.A series experiments were conducted to provide theoretical basis for the development of management practices to reduce As concentrations in rice.1.The effects of exogenous phosphorus on arsenic fractions in rhizosphere and its bioavailability to rice and wheat seedlingsA compartmented rhizo-bag culture system was used to study the effects of exogenous phosphorus on arsenic fractions in rhizosphere and its bioavailability to rice (Oryza sativa L.)and wheat(Triticum aestivum L)seedlings growing in arsenic-contaminated soil from Chenzhou of Hunan province.The results showed that the proportions of arsenic fractions in rhizosphere and non-rhizosphere soils for rice and wheat growth followed the order:well-crystallized hydrous oxides of Fe and Al(45%～52%)＞amorphous and poorly-crystalline hydrous oxides of Fe and Al(26%～34%)＞specifically-sorbed(12%～14%)＞residual phases(4%～7%)＞non-specifically sorbed (0.09%～0.25%).Moreover,there were two main fractions of iron in soil:amorphous and poorly-crystalline hydrous oxides of Fe and Al(77%～82%)and well-crystallized hydrous oxides of Fe and Al(20%～24%).There was significantly positive correlation between the concentrations of Fe and As in shoots of rice and wheat seedlings.P100 increased the shoot biomass of rice and wheat significantly when compared with P0 treatments.SAU and SPU of wheat were lowest at P0 treatment and the translocation factors of As and P from roots to shoots in wheat were lower than those in P0 treatment.Although SAU and SPU of wheat increased in the P100 treatment significantly and the translocation factor of P was up to the highest level,the translocation factor of arsenic was the lowest compared to P0 and P50 treatments.Therefore,arsenic phytotoxicity might be reduced through the hyperaccumulation of P in wheat shoots.The results also showed that SPU and SAsU was lowest in rice plants.These results suggested that it would be the best way to plant wheat in low-medium arsenic contaminated soils and also increase the amount of P fertilizer application.During the period of rice growing,P50 treatment would be the best choice for decreasing arsenic risks in rice plants. 2.The behaviors of arsenic in the soil-rice system using two types of arsenic contaminated soilsA rhizo-bag cultivated system was used to investigate the translocation and transformation of arsenic in the soil-rice system of two types of arsenic contaminated soils with different iron concentrations during the whole growing periods.Meanwhile,arsenic and iron fractions were analyzed in soil and iron plaques using the sequential extraction procedure.The results illustrated that:pH values in the rhizosphere soils(R)were lower than those of non-rhizosphere soil(NR)and the R-Eh was higher than that of the NR-Eh. The activities of CAT(Catalase)enzyme soils,which can reflect the redox abilities of soils, had certain positive correlation with the soil pH.The concentrations of avaliable Fe in Chenzhou soil was not significantly increased after adding Fe(OH)₃ to the soil,but the contents of avaliable As were decreased during these three stages.Moreover,the more the contents of avaliable Fe in soils,the more the contents of avaliable As in soils will be.The main proportions of arsenic fractions in iron plaque were well-crystallized hydrous oxides of Fe and Al and amorphous and poorly-crystalline hydrous oxides of Fe and Al(＞90%).The quantities of iron plaques on rice roots were maily influenced by the supply of Fe as well as the oxidated environment,and low soil pH inhibited the formation of iron dioxides.Meantime,iron plaque was aging and its adsorption capabilities to As was decreased correspondingly.Iron plaque's adsorption capabilities to As was not completely influenced by its quantity,also it is influenced by fractions Fe in iron plaques.Comparing two kinds of soils with different Fe concentrations,we concluded that:It would be the best way to cultivate rice on arsenic contaminated soils with higher iron concentrations because iron plaque can decrease arsenic risks in rice plants as buffer. Although,it still needs to be careful to apply iron fertilizers in As contaminated soils with low iron concentrations,because the SAU of rice were dramatically increased though the contents of avaliable As were decreased after adding Fe(OH)₃ to the soil.3.The adsorption and desorption of arsenic in iron plaqueA batch experiment was conducted to study the adsorption and desorption of arsenic in iron plaques on root surface of the mature rice.The adsorption isotherm of As was better fitted to Langmuir equation(P＜0.01).The capability of arsenic absorbed in iron plaque was much higher than that in iron and manganese oxides in soil.However,the adsorptions of As in iron plaques had not reached its peak on the conditions of this study,.The desorption isotherm of As was better fitted to Freundlich equation(P＜0.05).The desorbed percentage of As from iron plaques were increased with the increase of the P concentrations applied and reached its peak of 7.7%.