Factors Affecting Arsenic Availability in Flooded Rice Soils.

Arsenic (As) concentrations in rice grain are high, compared to other grains.  The reason for this is As is a redox element and rice is grown under flooded conditions.  Arsenic availability in flooded soils is also influenced by iron (Fe) and manganese (Mn), both of which are redox elements.  After soils are flooded, microorganisms are forced to use alternate electron acceptors for respiration, such as Fe and Mn, after the oxygen is depleted.  This causes the redox potential (Eh) in the soil to decrease following flooding.  This reducing environment not only causes an increase in dissolved Fe and Mn in solution, it results in an increase in soluble phosphorus (P), because P that was adsorbed on Fe and Mn oxides and hydroxides is released.  Arsenic has four oxidation states: arsenate (5+), arsenite (3+), arsenic (0), and arsine (3-), although arsenic and arsine are not stable in soils.  When the soil redox potential is high (oxidized conditions), As is present as arsenate (As⁵⁺), while redox potential is low (reducing conditions), As⁵⁺ is reduced to As³⁺ (arsenite).  Under very reducing conditions As can be methylated by bacteria to form monomethylarsonic acid or dimethylarsinic acid.  As a result, soil solution As concentrations initially increase when a soil is flooded due to release from Fe and Mn oxides as they are reduced.  When the redox potential gets lower, As solubility increases further when As⁵⁺ is reduced to As³⁺.  Arsenic levels in soil solution and in rice grain have been shown to be lower in fields that are intermittently flooded, compared to continuously flooded, which is likely due to higher Eh.   Alternatively, soil solution As and As uptake by rice are higher when fields are fertilized with a readily available organic carbon source, such as poultry litter, due to more reducing conditions.