Relevance of Reactive Fe:S Ratios for Sulfur Impacts on Arsenic Uptake by Rice

Abstract

Human arsenic exposure from rice consumption is a global concern. Due to the vast areas of naturally contaminated soils in rice-producing regions, the only possibility for reducing hazardous exposure is to prevent As uptake and translocation to rice grain. Sulfur inhibits As mobility both in soil and plant, indicating that soil S content may be a primary factor controlling As uptake; indeed, gypsum (CaSO 4 ·H 2 O) has been proposed as a potential amendment. Here, we investigated S controls on rice As uptake within two naturally contaminated soils (15.4 and 11.0 mg As per kg soil, respectively) from Cambodia, by adding gypsum at two levels (20 and 60 mg per kg soil). We found that although gypsum initially decreased As release to soil solution, the concentrations then increased compared to the control treatment. Further, As concentrations in rice biomass were generally insignificantly affected by the gypsum treatments and trended in opposite directions between the two soils. Single and multivariate statistical tests indicated that Fe exerted stronger control on As uptake in rice than S and that the initial ratio of reactive Fe to sulfate-S had an overriding impact on As uptake in rice. However, in the soil with higher inherent sulfate content (91 mg SO 4 2− -S per kg soil) the additional S provided by gypsum appeared to increase the ability of the rice plant to prevent As translocation to grain. We conclude that S may contribute to regulating grain As concentrations, but that the effect is highly dependent on S:Fe(As) ratios. Thus, at modest amendment rates, gypsum has limited potential for minimizing As concentration in rice when applied to naturally contaminated soil, particularly if the reactive Fe(III) content is high.

Arsenic Uptake and Accumulation Mechanisms in Rice Species

“…AsV also acts as a phosphate analogue with respect to transport across the root plasma membrane [86]. Pigna et al [87] reported that As toxicity in crops may be prevalent in situations where As pollution coexists with low available P. Sulphur (S) is an element that interacts toughly with As, particularly under reducing conditions; the reduced forms of S can make a binding with As(III) [88]. Srivastava et al [89] stated that S is important for plant growth as it regulates As tolerance over complexation of As by S-containing ligands (glutathione [GSH; γ-Glu-Cys-Gly] and PCs [GSH oligomers]).…”

Methylated Thioarsenates and Monothioarsenate Differ in Uptake, Transformation, and Contribution to Total Arsenic Translocation in Rice Plants

“…One driver for the occurrence of thiolation is sulfur (S) addition, even though naturally occurring S concentrations are already sufficient for As thiolation in paddy soils . Sulfur fertilization has recently been suggested to lower grain As concentration; − however, no conclusive data have been obtained so far.…”

Monothioarsenate Uptake, Transformation, and Translocation in Rice Plants

“…Thioarsenates are structural analogues to arsenate and form under sulfur-reducing conditions from arsenite by OH – /SH – -ligand exchange and oxidative addition of zerovalent sulfur. − Sulfur-reducing conditions can occur in flooded rice paddy fields, , especially when sulfur (S) is applied as fertilizer. This has recently been suggested to lower grain As concentrations ,− with the beneficial side effect that emissions of methane are significantly decreased . No conclusive data about the effects of S fertilization have been obtained so far.…”