Effect of sulfur amendments on grain arsenic in flooded rice paddy soils
Date
2025
Authors
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Publisher
University of Delaware
Abstract
Arsenic (As) accumulation in rice grain poses substantial human health risks, so finding ways to lower its concentration is critically important. Rice is particularly prone to As accumulation because it is typically grown under flooded management where anaerobic paddy conditions mobilize As from the soil and facilitate plant uptake. Different species of As exist in rice paddy systems with inorganic As (iAs), a Group I carcinogen, being predominant; thus, attention has largely focused on iAs in rice grain. Organic forms of As (oAs) also exist in rice paddies and in rice grain. While some species such as dimethylarsinic acid (DMA) are less acutely toxic to humans than inorganic As, other oAs forms such as dimethyl monothioarsenate (DMMTA) are more cytotoxic than iAs, and emerging evidence suggests that DMMTA may pose a yet unknown risk to humans. Previous research has shown that sulfur (S) amendments can decrease As in rice, but new evidence suggests they may also simultaneously increase DMMTA concentrations. Much of the previous work on S amendments used soils that were highly contaminated with As or/and had unrealistically high levels of S added. To date, the effect of S amendments on grain As in non-contaminated United States (U.S.) soils, is unknown. To evaluate the impact of S-amendments on As in rice, we conducted a pot study with three geochemically distinct paddy soils (e.g. soil texture, pH, total As, and S) obtained from production-scale fields in central Arkansas. The experiment included four treatments in triplicate in each soil: elemental S (300 mg S kg-1), ammonium sulfate (50 mg S kg-1) applied at either preflood or at midseason, and non-amended control. These S levels were chosen because they are in the range that farmers would apply to field-grown rice. Rice was sewn into the soil and amendments were applied at recommended times: preflood elemental S (5 mos. prior to seeding), preflood ammonium sulfate (2 mos. prior to seeding), and midseason ammonium sulfate (at booting d 77-109 after seeding) and grown to grain maturity (d 140-150 after seeding). Each week, porewater and Indicator of Reduction in Soil (IRIS) films were sampled to monitor chemistry over time. Plant tissues were analyzed post-harvest to compare As concentrations among the soils and treatments. In contrast to previous research, none of the S amendments decreased grain As concentrations relative to untreated controls under the conditions used here (i.e., non-contaminated soils with realistic S application rates). The elemental S increased grain As concentrations by 47% in the acidic silt loam soil, while S amendments had no impact on the other soils. Soil had a larger effect on grain As concentrations with coarser-textured soils having higher grain As concentrations than finer-textured soils despite lower total soil As concentrations. The results suggest that S amendments, when applied at recommended rates to soils with background As concentration, are not an effective strategy for limiting total grain As in these three Arkansas rice paddy soils.
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Keywords
Arsenic, Pot study, Rice, Soil, Sulfur