Governing Controls of Sulfur On Arsenic Uptake By Rice in Paddy Soil.

Sulfur (S) in the form of sulfide and sulfhydryl groups has a high affinity for arsenic (As), leading to a strong association of As with sulfide minerals and organic matter.  Further, the formation of As sulfide complexes or precipitates may limit the uptake of this toxin within rice (or other aquatic plants). In fact, addition of sulfate to paddy soils decreases the total uptake, grain concentration, and toxicity of As in rice. Several mechanisms may be responsible for these observations, including:

• As-S mineral precipitation under sulfate reducing conditions
• As adsorption on precipitated sulfides
• Increased iron-plaque formation
• Changes in aqueous speciation of As (e.g. increased methylation or thiolation)
• Increased ability for detoxification and stabilization of As in planta. 
• Increased complexation by sulfhydryl groups.

While all of the mechanisms have been observed in controlled experiments, their relative importance in paddy soils is unknown. Furthermore, the influence of S from an organic source has not previously been investigated. In a pot trial, with two naturally As contaminated rice paddy soils from Cambodia, we investigate the influence of sulfur on As mobilization and rice uptake by adding no amendment, inorganic sulfate (gypsum) or one of the following organic amendments: dried rice straw, charred rice straw or cattle manure. In order to elucidate the significance of different mechanisms, we monitor pore water chemistry (e.g. Eh, pH, As and S speciation) throughout the experiment and investigate the initial soil and amendment chemical composition, as well as changes in solid phase As and S speciation by X-ray absorption spectroscopy (XAS). To isolate the effect of S, we also monitor a range of other elements affecting the As uptake in rice (e.g. iron, phosphate, silica). In parallel, we perform batch experiments to determine the influence of inorganic and organic sulfur on the bulk release and precipitation mechanisms.