Dynamics And Influence Factors Of Arsenic Species In The System Of As-contaminated Paddy Soil-rice Plants

In recent years, arsenic contamination occured in many countries of the world, including China.Rice, as one of the staple crops in Asia, can accumulate higher level of arenic than other upland cropsfor the special anaerobic characters of paddy soil and special hollow medullary cavity. But theconcentration of total arsenic is not the only index to evaluate its health risk to huamn body, Arsenictoxicity is more closely related to the As species. Inorganic arsenic and organic arsenic are main speciesin soil-plant system. Inorganic arsenic mainly includes arsenite and arseniate, organic arsenic mainlyincludes MMAVand DMAV. The toxicity of the order is AsIII> AsV> MMAV> DMAVand the toxicityof inorganic arsenic is higher than that of organic arsenic. Physi-chemical and biological soil propertiescould impact the variation of arsenic species in soil, such as organic matter, soil redox status and soilmicroorganisms. Therefore, we chose the As-contaminated paddy soil to study the dynamics andconcentrations of arsenic species in soil solution incubated in different culture temperature, soilsterilization or not, addition of exogenous silica and organic matter, and the effect of exogenous silicaand organic matter on arsenic speciation in soil solution and arsenic accumulation in different tissues ofrice under flooding and alternation of flooding and drying. And try to analyze dynamics and Influencefactors of arsenic species in the system of As-contaminated paddy soil-rice plants and screen the mainpathways to reduce bioavailability of arsenic in soil and by rice plants uptake of arsenic. The mainresults showed as follows:(1) Dynamics of Arsenic species in As-contaminated paddy soil solution incubated in differenttemperatures during flooded periodDynamics of Arsenic species in As-contaminated paddy soil solution were conducted using highperformance liquid chromatography inductively coupled plasma-mass spectrometry (HPLC-ICP-MS)when autoclaved or non-autoclaved soils were incubated in different temperatures (5℃,27℃and50℃)during flooded period. The results showed that AsIII(arsenite), AsV(arsenate) and DMAV(dimethylarsinic acid) were detected in soil solution (porewater) during flooded period, but no MMAV(monomethylarsinic acid) was found in the porewater. DMAVconcentration was the highest and up to23.7ng/mL in soil solution when autoclaved soil was incubated in50℃chamber after flooded for23days, which indicated that some kinds of micro-organisms in autoclaved soil could survive in hightemperature condition and might be involved in arsenic methylation in soil solution. In summary, totalarsenic concentrations in soil solution was501ng/mL when soil was incubated in27℃and flooded for23days, which is lower than other flooded periods on the base of rice growing conditions. Therefore, it suggested that the local farmers should irrigate paddy fields according to the cycle of flooded andnon-flooded, which can save water resources and reduce the arsenic level in soil solution.(2) Effects of Exogenous Silicon and Organic Matter on Arsenic Species in As-contaminated paddy soilsolution during flooded periodDynamics of Arsenic Species in As-contaminated paddy soil solution were conducted using highperformance liquid chromatography inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) inorder to explore effects of exogenous silicon and organic matter on arsenic species in soil solution whenautoclaved and non-autoclaved soil submerged. The results showed that AsIII(arsenite), AsV(arsenate)and DMAV(dimethylarsinic acid) were detected in soil solution (porewater) during flooded period, butno MMAV(monomethylarsinic acid) found in the porewater. when soil flooded more than20d, theaddition of silicon increased AsIIIand AsVconcentrations in autoclaved soil solution; the addition oforganic matter significantly reduced AsIII, AsV, DMAVand total arsenic concentrations innon-autoclaved soil solution (P<0.05). Finally, total arsenic concentrations in soil solution was99.2ng/mL when soil flooded for20days, which is lower than other flooded periods on the base of ricegrowing conditions. Therefore, it suggested that the local farmers should irrigate paddy fields accordingto the cycle of flooded and non-flooded and appropriate amount of organic fertilization, which canreduce arsenic toxicity and control its bioavailability in soil solution.(3) The effect of exogenous silica and organic matter on As species in soil solution and arsenicaccumulation in different tissues of rice under flooding and alternation of flooding and dryingRice panicles formed only in the control and the treatment of silica application under floodedcondition. Under alternation of flooding and drying condition, rice paniclesformed only in the controland the treatment of organic matter application. straws biomass of each treatment were lower than thatof the control. Therefore, adding organic matter did not promote the growth of rice.AsIII(arsenite), AsV(arsenate) and DMAV(dimethylarsinic acid) were detected in paddy soilsolution, but no MMAV(monomethylarsinic acid) found. under flooded condition, Arsenite ispredominant species of As in porewater; Under alternation of flooding and drying condition, Arsenate ispredominant species of As. The two water regimes had significant effect on total arsenic concentrationsin soil solution for rice growing for130and170days (P<0.05). Under flooded condition, total arsenicconcentration in soil solutions of each treatment were significantly higher than that under alternation offlooding and drying conditions. total arsenic concentrations in soil solution of each treatment decreasedsharply When the rice plants grown for210days, and total arsenic concentrations in soil solution of thecontrol and the treatment of organic matter were lower than that of the other two treatments.Whether flooding or alternate of flooding and drying, different treatments had a significant effect onthe total arsenic concentrations in rice straws (P<0.05). the Si application only and+Si+OMsignificantly decreased the total arsenic concentrations in the straws. Total arsenic concentrations instraws was1632μg/kg when under flooded condition with silicon application, which was lowest thanother treatments. Therefore, appropriate amount of silicon addition can reduce the accumulation ofarsenic in rice straws. Total arsenic concentrations in husks was375μg/kg when under flooded condition with silicon,which was lowest than other treatments. Silicon significantly reduced the husk total arsenicconcentrations (P<0.05). Under alternation of flooding and drying condition, organic mattersignificantly reduced the husk total arsenic concentrations (P<0.05). The two kinds of water regimeshad significant effect on husks total arsenic concentration of control (P<0.05). under alternation offlooding and drying condition, total arsenic concentrations in husks of control was significantly lowerthan that under flooded condition.AsIII(arsenite) and AsV(arsenate) were detected in rice grains, but no DMAV(dimethylarsinic acid)and MMAV(monomethylarsinic acid) found in grains. Total arsenic concentrations in grain was687μg/kg when under alternation of flooding and drying condition of control, which was highest than othertreatments. Total arsenic concentrations in grain was308μg/kg when under flooded condition withsilicon addition, which was lowest than other treatments. Therefore, appropriate amount of silicon canreduce the accumulation of arsenic in rice grain grown in arsenic contaminated soil from As pollutionfarmland.