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 (CaSO4·H2O) 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 SO42−-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.
Highlights
Rice is the staple food for people in South and South-East Asia and may constitute over 70% of the total caloric intake in some areas [1]
In the soil with higher inherent sulfate content (91 mg SO4 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 didnot not observe a substantial effect of addition gypsum on addition ondissolved either the dissolved or rice uptake of to the extent an impact was detected, it trended in opposite direction concentrations or rice uptake of As
Summary
Rice is the staple food for people in South and South-East Asia and may constitute over 70% of the total caloric intake in some areas [1]. Human exposure to As from drinking contaminated groundwater in these areas has been the subject of extensive research since the 1990s [2]. The attention has increasingly been drawn to the additional exposure pathway through rice consumption [3,4,5]. Arsenic in its more reduced form, arsenite (as H3 AsO3 at most soil pH values), acts as a silicic acid analogue and is taken up through the aquaporins in rice roots [7]. The more oxidized form of As, arsenate (As(V)), behaves to phosphate and, is taken up by plants through the phosphate transport channels [8,9,10,11]
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