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. 2025 Aug 13;9(8):e2025GH001410. doi: 10.1029/2025GH001410

Concentrations and Health Implications of As, Hg, and Cd and Micronutrients in Rice and Emissions of CH4 From Variably Flooded Paddies

Angelia L Seyfferth 1,2,✉, Matt A Limmer 1, Brian P Jackson 3, Benjamin R K Runkle 4
PMCID: PMC12344449  PMID: 40810033

Abstract

The flooded soil conditions under which rice is typically grown are beneficial for boosting yield and decreasing herbicide inputs but may pose a food safety and environmental health risk. Flooded soils lead to reducing conditions and anaerobic metabolisms of soil microorganisms, which mobilizes arsenic from soil into soil solution, where it can be absorbed by rice roots and transported to grain. These conditions also promote the production and emission of methane (CH4)—a potent greenhouse gas. To evaluate how water management affects metal(loid) grain concentrations and CH4 emissions, we conducted a 2‐year field study in which rice paddy water was managed under a range of soil redox conditions that spanned from flooded to non‐flooded. We observed that growing rice under less flooded conditions decreased CH4 emissions and concentrations of grain total As, grain inorganic As, grain total Hg, and grain inorganic Hg relative to flooded conditions, with more reductions observed as conditions were drier; grain organic As and Hg (MeHg) species also decreased with drier conditions particularly in Year 1. However, the driest conditions tested led to a 50%–97% increase in grain Cd concentrations that exceeded the CODEX limit and grain yield reductions as high as 25% and 40% in Year 1 and 2, respectively. While concentrations of toxic metal(loid)s could be manipulated by water management, micronutrient concentrations were similar or decreased with drier conditions, potentially increasing grain Cd bioaccessibility to humans. Because practices for rice water management are gaining momentum, more research should monitor grain Cd levels along with micronutrients.

Keywords: row rice, closer to zero, cadmium, arsenic, mercury, methane, redox

Key Points

  • Rice is uniquely prone to high grain As concentrations because it is grown in flooded conditions, which also lead to CH4 emissions

  • Growing rice in less flooded conditions decreased CH4 emissions and concentrations of rice grain As and Hg species, but increased Cd

  • Because rice farmers are rapidly adopting water savings practices, efforts should ensure that grain Cd does not exceed food safety limits

1. Introduction

Rice is a widely consumed staple around the world, and is often used as a first food for infants (Meharg et al., 2008); thus, understanding the factors that influence concentrations of toxic metals and metalloids in rice grain and bran is critical for protecting human health. The international Codex Alimentarius has set limits for toxic metals and metalloids in traded commodities; the CODEX limits for inorganic arsenic (As) and cadmium (Cd) polished rice are 0.2 and 0.4 mg kg−1, respectively (Codex Alimentarius, 2023). Recently, the U.S. Food and Drug Administration (USFDA) developed a “Closer‐to‐Zero” plan that focuses on minimizing concentrations of arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb) in foods fed to babies and young children (“US FDA. Closer to Zero: Action Plan for Baby Foods,” 2021). In California, rule AB 899 was passed that mandates that foods intended for infants and young children be tested for Pb, As, Cd, and Hg once per month, and that the data be made publicly available (Stats, 2023). Of these, the first three (As, Cd, and Hg) are of potential concern in rice. While adults in the U.S. who consume rice a few times per week are likely not at risk of negative health outcomes from As, Hg, and Cd in rice, infants and subpopulations who consume high quantities of rice are at increased risk (EFSA Panel on Contaminants in the Food Chain et al., 2024). Infants have a higher ingestion rate relative to their body mass, which increases the exposure of a given concentration of a toxic metal(loid) in rice to their bodies (Meharg et al., 2008). In addition, infants and young children have limited diets, and it has been observed that diets low in micronutrients like Zn, Ca, and Fe can increase the body burden or bioaccessibility of toxic metal(loids) in humans (Casarett, 2008; Mehri, 2019; Peng et al., 2023; Vance & Chun, 2015). The combination of concentration and speciation affects metal(loid) toxicity, particularly for As and Hg. Inorganic Hg (iHg) is far less toxic to humans than methylmercury (MeHg). In contrast, inorganic As (iAs) species are more toxic than the organic As (oAs) species dimethylarsinic acid (DMA(V)) and monomethylarsonic acid (MMA(V)) (Iarc, 2012), whereas the organic dimethylmonothioarsenate (DMMTA) species is similarly as toxic or more toxic than iAs(III) (Moe et al., 2016; Naranmandura et al., 2009, 2011). Speciation also affects rice uptake and grain accumulation of these compounds, as MeHg and oAs species are more readily transported to rice grain than their inorganic forms (Rothenberg & Feng, 2012; Zhou et al., 2015). It is critical to understand the factors that affect the concentrations and speciation of metal(loid)s in rice and, therefore, potential human exposure.

Of the aforementioned toxic metal(loid)s, rice is particularly prone to elevated concentrations of As because (a) it is typically grown in flooded paddies where As is mobilized, and (b) once mobilized, it is taken up through highly efficient Si transporters (Ma et al., 2008; Takahashi et al., 2004). In most upland or oxic soils, arsenic is strongly bound to soil minerals via adsorption and co‐precipitation (Fendorf & Kocar, 2009), and therefore most crops are protected from elevated As in soils. However, rice is typically grown in flooded paddy soils where reduction‐oxidation (redox, EH) potential is low, which allows Fe(III)‐ and As(V)‐reducing bacteria to thrive (Chan et al., 2023; Edwards et al., 2015); these microbes reductively dissolve As‐sorbing Fe(III) oxides and the iAs(V) on them, releasing the more mobile iAs(III) to soil solution (i.e., porewater) (Cummings et al., 1999, 2000; Pedersen et al., 2006; Stuckey et al., 2016; Takahashi et al., 2004; Yamaguchi et al., 2011; Zobrist et al., 2000). Thus, reducing conditions lead to higher concentrations of Fe, Mn, and As in soil porewater and lower EH values. Once mobilized into soil solution, arsenic is taken up into rice via the highly efficient silicon (Si) transport pathway because the dominant As form under these conditions, inorganic arsenite [iAs(III)], is chemically indistinguishable from Si to root Si transporters (Ma et al., 2008). Notably, some soil microorganisms methylate iAs(III) to various organic As (oAs) forms including MMA(V), DMA(V), and DMMTA; of these, DMA has been the most widely detected in rice grain and can also be transported via the Si pathway (Li, Ago, et al., 2009; Limmer, Wise, et al., 2018).

The low soil redox conditions that promote As mobilization and methylation in flooded paddy soil also promote methane (CH4) production and efflux as well as Hg mobilization and methylation (Jia et al., 2013; Rothenberg et al., 2014; Zheng et al., 2024). It is well known that the low Eh conditions in flooded paddy soils lead to high emissions of CH4, a potent greenhouse gas for which rice is responsible for 10% of anthropogenic emissions (Saunois et al., 2019). Compared to As and CH4, factors that promote Hg mobilization and methylation in rice is far less studied. It is thought that elemental Hg, inorganic Hg (iHg), and MeHg are all plant‐available, but the exact mechanisms of uptake are unknown. Microbially‐mediated iron reduction, sulfate reduction, and methanogenesis can all promote methylation of inorganic Hg, but the activity of sulfate‐reducing bacteria are thought to be the dominant driver of MeHg production in paddy soil (Rothenberg et al., 2016, 2017). For this reason, dietary exposure to Hg and As tend to co‐occur via consumption of rice foods (Rothenberg et al., 2017). While the USFDA finalized guidance on an industry action level for iAs in infant rice cereal of 100 μg kg−1 to protect against neurodevelopmental effects, no such action levels exist for organic forms such as DMMTA or for MeHg in rice. MeHg is especially harmful to the developing fetus (Clarkson & Magos, 2006) and rice may be a substantial source of MeHg in human diets in places where rice is consumed as a staple (Rothenberg et al., 2018, 2021). In addition to toxicity, the speciation of As and Hg affects their localization in edible rice parts. Methylated Hg and oAs species readily accumulate in polished grain, whereas iAs and iHg species concentrate in rice bran (Carey et al., 2010; Limmer & Seyfferth, 2022; Rothenberg et al., 2011). Thus, while promoting methylation may lower the risks of rice consumption from As, it enhances the risk from Hg consumption. The different toxicities and localizations of chemical forms of Hg and As complicate understanding the health risks associated with rice consumption; however, CH4 emissions as well as As and Hg grain concentrations and thus risk can be lowered by growing rice under less flooded conditions.

Growing rice under drier (more oxic) soil conditions decreases CH4 emissions and the phytoavailability of As and Hg, but it tends to increase the phytoavailability of Cd, particularly in acidic soils. Rather than the traditional maintenance of continuously flooded conditions from plant emergence throughout the growing season, the water savings management practice of “alternate wetting and drying” or AWD has been the most widely researched (Lampayan et al., 2015; Linquist et al., 2015). In AWD, the water table is allowed to drain below the soil surface and then reflooded at least once during the growing season. While AWD has been adopted in some areas, the rate of adoption of AWD in the United States is far lower than the rate of adoption of furrow‐irrigated rice, or “row rice” (Reba & Massey, 2020). In Arkansas, for example, AWD has remained less than 5% of acreage while furrow‐irrigated rice has increased from 0% to 20% since 2017 (Hardke et al., 2024). In ‘row rice’, rice is often planted directly into the beds used for soybeans without constructing levees, and some of the rice is therefore grown under more oxic conditions similar to other row crops (Della Lunga, Brye, Henry, & Slayden, 2021). Growing rice under less flooded conditions decreases As mobilization by limiting the activity of Fe(III)‐ and As(V)‐respiring microorganisms and limits As and Hg methylation by limiting the activity of sulfate‐reducers and methanogens. The result is lower methane emissions, lower phytoavailability of As and Hg, and thus lower grain As and Hg. Conversely, these more aerobic water managements render Cd more phytoavailable particularly in acidic soils (Arao et al., 2009; Honma, Ohba, Kaneko, et al., 2016; Hu et al., 2013; Limmer & Seyfferth, 2024; Linam et al., 2022; Reddy & Patrick, 1977; Wan et al., 2019; Yao et al., 2022; Zhao & Wang, 2020) by promoting sulfide oxidation and thus the release of Cd from sparingly soluble CdS (de Livera et al., 2011). Like As and Hg, Cd also poses risk to human health (Faroon et al., 2013). The CODEX value of Cd in rice is 0.4 mg kg−1 and it ranks number 7 on the Agency for Toxic Substances and Disease Registry's Substance Priority List, while As and Hg rank number 1 and 3, respectively. Therefore, lowering As and Hg while increasing Cd in rice may not be protective of human health.

While many studies focus on As in rice, fewer studies have focused on how water management affects Hg and Cd, and far fewer examine all three toxins in the same study along with CH4 emissions. Moreover, most studies that report rice concentrations of As, Hg, and Cd rely on market‐basket approaches with no paired soil or biogeochemical information. With the adoption of oxic rice production gaining momentum, the assumed low risk of Cd in rice using historical data where rice is traditionally flooded (e.g., Pokharel & Wu, 2023) may not be valid for rice of the future where water savings practices (e.g., row rice) are in place. Moreover, given supply chain and government interest in implementing climate‐smart rice irrigation practices like AWD and furrow irrigation, which can reduce methane emissions by 40%–80% (Das et al., 2024; M. Leavitt et al., 2023; Linquist et al., 2018; Runkle et al., 2019), attention to consequences for grain metal(loid) concentrations and speciation is particularly urgent. The purpose of this study is to examine how growing rice under a range of soil redox conditions affects the concentrations of metal(loid)s in rice including As, Hg, and Cd, speciation of As and Hg, and micronutrient metals, with implications for the risk to human health upon consumption of the grain produced. For a systems approach, we also explore how CH4 emissions are impacted by water management. We hypothesized that growing rice under flooded conditions would lead to higher grain As and Hg, particularly in methylated forms, and CH4 emissions compared to less flooded conditions, which would promote higher grain Cd concentrations.

2. Methods

2.1. Experimental Design

A 2‐year field study was conducted to evaluate the impacts of variable redox conditions on the concentrations of As, Hg, and Cd in rice. Rice was grown at the University of Delaware's Rice Investigation, Communication, and Education (RICE) facility in 2016 and 2017. This outdoor facility consists of an array of 2 × 2 m rice paddies instrumented and used for rice research (Limmer, Mann, et al., 2018). Unlike other work at the UD RICE Facility, for this study six rice paddies were unlined. The unlined paddies were on a slight slope that allowed for variable water saturation of the soil and thus established a gradient in soil redox conditions with Paddy 1 to Paddy 6 ranging from generally anoxic to oxic. The soil was a Typic Hapludult of the Elsinboro series and is characterized as a sandy clay loam (30% sand, 44% silt, 26% clay). Acid‐digestible soil concentrations of total As, Cd, and Hg were measured in soil samples collected at harvest in both years; five subsamples to 15 cm depth were collected per paddy, composited, and sieved to 2 mm and these samples contained 5.5 (±0.5) mg kg−1 As, 0.093 (±0.013) mg kg−1 Cd, and 0.024 (±0.003) mg kg−1 Hg. The soil was instrumented with soil redox sensors at 10 and 15 cm depth that continuously recorded soil redox potentials relative to the standard hydrogen electrode as described previously (Limmer et al., 2023).

Rice (Oryza sativa L. “Jefferson”) was started from seed and grown to the 4‐leaf stage in a greenhouse before being transferred outdoors for 2 days to acclimate. Rice seedlings were then hand transplanted into the rice paddies in 7 rows of 7 (49 plants total) in late May. During rice growth, porewater was collected weekly using rhizon samplers and analyzed for pH, Eh, Fe(II), total Fe, As, S, Cd, Mn, iAs, oAs, and dissolved organic carbon (DOC) with methods described previously (Limmer et al., 2023; Limmer, Mann, et al., 2018; Linam et al., 2023; Seyfferth et al., 2016; Seyfferth & Fendorf, 2012; Teasley et al., 2017). Data for porewater Cd is not shown because many samples did not have measurable Cd in porewater (∼25% of detections in Year 1 and <5% of detections in Year 2), and porewater Hg was not measured due to limited funding. Rice was grown to grain maturity and hand‐harvested in early September when rough rice yield and straw biomass were recorded. Methane fluxes were measured weekly using the closed chamber technique with a chamber capable of covering the entire paddy. Air was sampled every 2 s for 5 min using a portable greenhouse gas analyzer (Los Gatos Research, San Jose, California). The methane fluxes were calculated using the slope of the methane concentration over time and adjusted for temperature using the ideal gas law. Only fits with R 2 ≥ 0.9 are reported.

2.2. Plant Analyses

Roots were cleaned of soil and used to evaluate three element pools: metal(loid) content in root plaque, Fe mineral composition of root plaque, and plaque‐free elemental analysis per established methods (Amaral et al., 2017; Limmer, Mann, et al., 2018). Briefly, cleaned root systems were separated into two halves where one half was subjected to cold dithionite‐citrate‐bicarbonate (DCB) extraction to remove plaque from roots (Taylor & Crowder, 1983). The DCB extracts were analyzed for total As, Fe, Mn, and P using ICP‐OES. The plaque‐free roots were rinsed with water and used for acid digestions (described below). The second half was sonicated in water to dislodge intact Fe plaque, which was collected through vacuum filtration onto nitrocellulose filters (Amaral et al., 2017). These filters were then subject to Fe extended x‐ray absorption fine structure (EXAFS) analysis at the Stanford Synchrotron Radiation Lightsource on beamline 11‐2. The samples were measured in fluorescence mode and duplicate spectra were averaged, background subtracted and normalized in Athena. Linear combination fitting (LCF) was performed using 2‐line ferrihydrite, lepidocrocite, goethite, and siderite of the k 3 spectra from k = 2.5–12 Å−1.

Plant elemental analysis included measuring concentrations of total As, Hg, and Cd as well as speciation of As and Hg in rice grain and bran, and total As and Cd in other plant parts. We also measured nutrient concentrations including Fe, Zn, Cu, Ca, and Mn in rice grain. Grain was dehusked using a benchtop dehusker and polished using a benchtop polisher where polished grain and bran were collected separately. Polished grain, bran, flag leaf, straw, and plaque‐free roots were each digested in trace metal grade (TMG) HNO3 in a microwave digester, diluted with 18 MΩ cm water, and analyzed using ICP‐MS for total As and Cd. Arsenic speciation in polished grain was assessed by extracting iAs and oAs species in diluted TMG HNO3 (Maher et al., 2013) and analyzed using HPLC‐ICP‐MS using the separation method of Jackson (2015). For total Hg, samples were analyzed at the Trace Metal Analysis Core at Dartmouth College where samples were acid digested in 3:1 HNO3/HCl at high temperature and pressure (MARS 6, CEM Corporation, Matthews, NC), diluted with DI water and analyzed by triple quadrupole ICP‐MS (Agilent 8900, Wilmington, DE). Speciation of Hg into iHg and MeHg was performed by species‐specific isotope dilution GC‐purge and trap ICP‐MS with a Brooks Rand MERX purge and trap system interfaced to a quadrupole ICP‐MS (Agilent 7900, Wilmington, DE) (Chen et al., 2009; Taylor et al., 2008). Briefly, plant tissue samples were extracted in 3 ml tetramethyl ammonium hydroxide, spiked with an in‐house synthesized 201MeHg and inorganic 199Hg. An aliquot of the extract was buffered to pH > 6 in DI water with citrate buffer and 50 μL of ethylation reagent (tetraethylborate) was added to the vial. Mercury species were purged from the vial using nitrogen, trapped on a tenax trap in an argon gas flow, thermally desorbed, separated by a GC column, and the resulting element‐specific chromatogram was recorded by ICP‐MS.

2.3. QA/QC

Accuracy and precision of concentration measurements were ensured through various procedures. Microwave digestions of plant materials were performed with standard reference materials during each run, and blanks, duplicates, and standard checks were used in the ICP‐MS analyses. Duplicates and checks agreed within 10%. For total As and Hg analysis in soil, standard reference material NIST 2709 San Joaquin soil was used with recoveries ranging from 91% to 100% for As and 102% to 112% for Hg. NIST 1568a rice flour was used for total As, Cd, Cu, Mn, Ca, and Zn with recoveries of 113%, 133%, 110%, 90%, and 113%, respectively. NIST 1568b rice flour was used for As species and total Hg in rice grain with recoveries of 102% and 95% for iAs and DMA, respectively, and 83% for total Hg, while NIST 2976, mussel tissue was used for Hg speciation with recoveries of 90%–91%. The detection limits for total As and Cd in plant tissue were 0.004 mg/kg, and most samples were above these limits. The detection limit for total Hg and MeHg were 1 μg/kg and 0.12 μg/kg, respectively, and all measurements were above these limits. The detection limit for iAs and DMA was 0.004 mg/kg and some samples were below these limits, depending on the water management. If a sample was below the detection limit, it was assigned a value at half of the detection limit.

2.4. Statistics

Regression analyses were used to understand the role of soil redox potential (EH) on plant metal(loid) concentrations, porewater concentrations, root plaque mineral composition, and CH4 emissions. Parameters that were not linear were transformed to achieve linearity. A partial correlation analysis was then used to determine biogeochemical drivers of plant metal(loid) concentrations while controlling for soil redox potential. All statistical analyses were performed in SPSS v. 29.0.2.0.

3. Results

3.1. Impacts of Water Management on Redox Conditions and Grain Yield

This experiment allowed us to examine how plant levels of As, Cd and Hg, yield, and methane fluxes are affected by a gradient in soil redox potential (Figure 1) induced in rice paddies with the same soil over two growing seasons. As anticipated, Paddies 5 and 6 had consistently higher redox potential (EH) relative to the other paddies and higher seasonal averages, whereas Paddy 1 (year 1) and Paddies 1–3 (year 2) had relatively lower redox conditions (Figure 1, Table 1). Except for Paddy 4 in Year 1, the most flooded paddies had about 25%–40% higher grain yield than the least flooded paddies (Figure S1 in Supporting Information S1).

Figure 1.

Figure 1

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Soil redox potential (EH, relative to standard hydrogen electrode (SHE)) over the (a) year 1 and (b) year 2 growing seasons in the UD RICE Facility showing the gradient in soil redox potentials achieved due to differences in water management. Data are daily average EH values at 10 cm depth calculated from 15‐min resolution data.

Table 1.

Average Soil EH Values (mV) From 10 cm Probes Buried in Paddies in Each Year

Paddy 1 Paddy 2 Paddy 3 Paddy 4 Paddy 5 Paddy 6
Year 1 −177 −160 149 203 640 623
Year 2 100 −241 79 110 508 564
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Porewater biogeochemical data show that the gradient in redox conditions across the six rice paddies differed (Figure 2 and Figure S2 in Supporting Information S1). The pH of the most flooded paddies stayed fairly constant around pH 7, whereas the least flooded paddies had slightly acidic pH, particularly evident in Year 2, and the EH also responded predicably with slightly higher values for the least flooded paddies (Figures S2a–S2d in Supporting Information S1). The paddy redox gradient resulted in variable porewater total Fe, total As, iAs, and oAs concentrations with higher levels reached in more flooded rice paddies for both years (Figures 2a–2d). In the most flooded paddies, concentrations tended to reach a maximum around 60–80 days past transplantation, with a later‐season increase in oAs particularly evident (Figures 2e–2h). Porewater Mn was highest and S was lowest in the most flooded paddies (Figures S2e–S2h in Supporting Information S1).

Figure 2.

Figure 2

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Porewater (a, b) total Fe, (c, d) total As, (e, f) inorganic As, and (g, h) organic As concentrations measured from six paddies at the UD RICE Facility that were subject to 2 years of a flooding gradient where Paddy 1 is the most flooded and Paddy 6 is the least flooded.

3.2. Impacts of Water Management on Plant Metal(loid) Concentrations

In both years, paddies that were more flooded had higher levels of total As and Hg and lower levels of Cd in rice grain than paddies that were drier (Figure 3). More flooded paddies generally had higher concentrations and proportions of oAs and MeHg than paddies that were drier. Grain concentrations of total As and Hg were higher in year 1 than in year 2 in the most flooded paddy, and grain concentrations of Cd were higher in year 1 than in year 2 in the least flooded paddy (Figure 3). The grain Cd concentration exceeded the CODEX limit of 0.4 mg kg−1 in Paddy 6 in each year, whereas grain iAs remained below its CODEX limit value; there is no CODEX limit for Hg in rice. The trends observed for decreasing As and increasing Cd with drier conditions were also consistent in all measured plant fractions including grain, bran, husk, flag leaf, straw and root (Figure 4). Plant Hg concentrations were only measured in grain and bran.

Figure 3.

Figure 3

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Concentrations of (a) As, (b) Hg, and (c) Cd in polished rice grain harvested from six paddies at the UD RICE Facility that were subject to 2 years of a flooding gradient where Paddy 1 is the most flooded and Paddy 6 is the least flooded. Darker shades represent the inorganic fraction of each element, and the lighter shades in (a, b) represent the methylated component of As and Hg, respectively. Black horizontal lines in (a, c) represent the CODEX limit for the inorganic fraction of those elements in polished rice; there is no CODEX limit for Hg in rice. The red horizontal line in (a) represents the USFDA action limit for inorganic As in infant rice cereal.

Figure 4.

Figure 4

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Concentrations of (a, b) total As and (c, d) Cd measured in rice grain, bran, husk, flag leaf, straw, and root from six paddies at the UD RICE Facility that were subject to 2 years of a flooding gradient where Paddy 1 is the most flooded and Paddy 6 is the least flooded.

Levels of some measured nutrients in polished grain and the amount and mineral composition of Fe plaque were also affected by year and water management. Concentrations of Cu, Mn, and Ca in polished grain were higher in year 1 than in year 2 for all water managements, whereas grain Zn was similar in year 1 and 2 (Figure 5). Grain Fe was below detection and is thus not reported. There was a slight increasing trend for grain Cu with drier conditions, whereas decreasing trends were observed for grain Mn in both years and grain Ca in year 1. Grain Zn concentrations were not affected by water management. Paddies with high levels of porewater Fe(II) also tended to have more Fe plaque on the roots (Figure S3 in Supporting Information S1). This plaque Fe primarily consisted of ferrihydrite (∼50%), with lepidocrocite and goethite comprising most of the rest of Fe minerals (Figure S3 in Supporting Information S1).

Figure 5.

Figure 5

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Concentrations of the micronutrients (a) Cu, (b) Mn, (c) Ca and (d) Zn in polished rice grain harvested from six paddies at the UD RICE Facility that were subject to 2 years of a flooding gradient where Paddy 1 is the most flooded and Paddy 6 is the least flooded.

3.3. Disentangling Drivers of Grain Metal(loid) Levels

Because plants were obtained at grain maturity only, we used the average soil redox potential (EH) to predict plant metal(loid) concentrations; soil EH acted as a master variable that drove mobility and plant‐availability of As, Cd, and Hg in soil (Table S1 in Supporting Information S1). Grain concentrations of total As and Hg decreased with increasing EH while the opposite was true for Cd (Figure 6a; Table S1 in Supporting Information S1). In addition, methylated As and Hg species also decreased with increasing EH (Figure 6b; Table S1 in Supporting Information S1). However, it is important to disentangle the multiple drivers for each metal(loid), particularly those for which multicollinearity exists that could lead to spurious correlations. For example, increased soil EH increased levels of plant Cd but also decreased porewater Mn and increased porewater S (Figures S2 and S4 in Supporting Information S1). Therefore, the increased levels of plant Cd at higher soil EH could be due to both increased mobility of Cd and a lack of Mn at high EH. Similarly, grain iAs increased linearly as porewater As increased (r 2 = 0.82, p < 0.001, Figure S5 in Supporting Information S1), but both increased as soil EH decreased. Disentangling such relationships will allow us to better understand how variables that are influenced by soil redox potential affect plant levels of metal(loid)s.

Figure 6.

Figure 6

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Concentrations of (a) total As, Cd, and Hg and of (b) methylated As (oAs) or methylated Hg (MeHg) in polished rice grain as a function of soil EH for averaged over the growing season for both years where EH was measured from the EH sensor buried at 10 cm depth in each rice paddy. Exponential fits are shown for each data set, but linear fits were also significant (p < 0.05) with similar r 2 values.

We performed a partial correlation analysis to disentangle redox potential from the other measured variables (Tables S2–S7 in Supporting Information S1). When controlling for soil EH, total grain As was positively correlated with all plant As fractions, grain Hg and MeHg, porewater total As, porewater Fe(II), and the proportion of plaque as lepidocrocite but negatively correlated with grain Cd (Table S2 in Supporting Information S1). Grain iAs was similarly positively correlated with all measured As plant fractions, grain Hg and MeHg, porewater total As and Fe(II), and the proportion of plaque as lepidocrocite, but also positively correlated porewater inorganic As and negatively correlated with root Cd and the proportion of plaque as goethite (Table S3 in Supporting Information S1). Grain oAs was positively correlated with all plant As fractions except for root As, all measured plant Hg fractions, porewater As, and the proportion of plaque as lepidocrocite while negatively correlated with root Cd (Table S4 in Supporting Information S1). Grain Hg was positively correlated with grain As, grain and bran MeHg, grain and bran iHg, Grain Mn, porewater As, and the proportion of plaque as lepidocrocite (Table S5 in Supporting Information S1). Grain MeHg was positively correlated with grain and bran As, grain and bran Hg, grain and bran iHg, bran MeHg, grain Mn, and the proportion of plaque as lepidocrocite (Table S6 in Supporting Information S1). Grain Cd was positively correlated with plant fractions of Cd and negatively correlated with bran, husk, straw, root As, porewater As, and the concentrations of As, Mn and Fe in root plaque (Table S7 in Supporting Information S1).

3.4. Impacts of Water Management on Carbon Cycling and Methane Fluxes and Emissions

In both years, DOC concentrations and methane emissions were highest for the most flooded paddies (Figure 7). In year 1, when the paddies were first established, the DOC concentrations were low and never exceeded 4 mmol L−1, but were higher in year 2 (Figures 7a and 7b). The most flooded paddies were sources of methane, and the least flooded paddies were sometimes methane sinks, but the temporal patterns differed by year (Figures 7c and 7d; Table 2). In year 1, the most flooded paddies became sources of methane around 40 days after transplanting, whereas in year 2 the most flooded paddy was an immediate source of methane. Emissions were generally higher in Year 2 than in Year 1, particularly in the most flooded paddies, with Paddy 1 and 2 having cumulative emissions of 48 and 1.7 kg CH4 ha−1, respectively, in Year 1 and 400 and 154 kg CH4 ha−1, respectively, in Year 2 (Table 2). In contrast, the least flooded paddies were not sources and sometimes sinks of methane with Paddy 5 and 6 having cumulative fluxes emissions of −0.4 and −2.7 kg CH4 ha−1, respectively, in Year 1 and 0 and −4 kg CH4 ha−1, respectively, in Year 2 (Table 2).

Figure 7.

Figure 7

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Time course of (a, b) porewater DOC concentrations and (c, d) methane fluxes measured from six paddies at the UD RICE Facility that were subject to 2 years of a flooding gradient where Paddy 1 is the most flooded and Paddy 6 is the least flooded. Note change in y‐axis scale in both pairs from Year 1 to Year 2.

Table 2.

Cumulative Methane Fluxes From Paddies in Both Years (kg CH4 ha−1)

Paddy 1 Paddy 2 Paddy 3 Paddy 4 Paddy 5 Paddy 6
Year 1 48 1.7 0.3 −1.3 −0.4 −2.7
Year 2 400 154 0.9 3.7 0.0 −4.0
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4. Discussion

This study illustrated how the toxic elements As, Cd, and Hg, species of As and Hg, and the potent greenhouse gas CH4 are affected by water management in rice. Growing rice under flooded conditions resulted in the highest grain As and Hg concentrations (total and species), the highest CH4 emissions, but the lowest Cd. These findings are consistent with prior work where impacts of water management were assessed on total grain As or/and Hg or/and Cd or/and CH4 emissions (Arao et al., 2009; Conrad, 2002; Della Lunga, Brye, Slayden, et al., 2021; Honma, Ohba, Kaneko, et al., 2016; Honma, Ohba, Kaneko‐Kadokura et al., 2016; Li et al., 2005; Linam et al., 2022; Linquist et al., 2015; Rothenberg et al., 2016; Runkle et al., 2021; Seyfferth, Limmer, & Wu, 2019; Wan et al., 2019). Growing rice under non‐flooded conditions strongly decreased the concentrations of grain As and Hg as well as CH4 emissions, but strongly increased grain Cd concentrations to levels that exceed the CODEX limit (Codex Alimentarius, 2023) for Cd in rice despite very low soil Cd (0.093 mg kg−1). While Cd is not redox active, under low soil redox potentials with sufficient sulfide generation and particularly in acid soils, Cd exists as sparingly soluble CdS and is mobilized along with S under high (oxic) redox potentials as S2− is oxidized to SO4 2− (de Livera et al., 2011). The higher porewater S in the least flooded paddies (Figures S2g and S2h in Supporting Information S1) supports that this process occurred in our study, which coincided with higher grain Cd (Figure S4b in Supporting Information S1). These non‐flooded conditions biogeochemically represent furrow‐irrigated rice, a practice that is gaining momentum in many areas, including the Mid‐South U.S. (Hardke et al., 2017). Despite the well‐researched practice of AWD, the “row rice” is less well‐studied yet is being adopted faster than AWD (Reba & Massey, 2020). Our data show that while this practice decreases grain As and Hg and decreases CH4 emissions, it may lead to unsafe grain Cd levels in some soils. Risk assessment studies that rely on older literature or only on flooded rice (e.g., Pokharel & Wu, 2023) may miss the Cd risk that non‐flooded rice may pose to human health, and more work is clearly needed on the impacts of row rice on rice Cd in a variety of soils.

Many rice water management studies to date with multiple metal(loid)s or methane and metal(loid)s have focused on total concentrations of the metal(loid) (Carrijo et al., 2018; Honma, Ohba, Kaneko, et al., 2016; Honma, Ohba, Kaneko‐Kadokura et al., 2016; Hu et al., 2013; Linam et al., 2022; Linquist et al., 2015; Wan et al., 2019; Windham‐Myers et al., 2014; Yao et al., 2022) and fewer on either As speciation (Arao et al., 2009; Hu et al., 2023; Li et al., 2019; Li, Stroud, et al., 2009; Ma et al., 2014; Seyfferth, Amaral, et al., 2019; Somenahally et al., 2011) or Hg speciation (Rothenberg et al., 2016). In our study, we were able to measure inorganic and organic species of As and Hg. Because the toxicity of organic and inorganic species of As and Hg differ, these data provide important insight into how water management has the potential to impact human health. The strongly reducing conditions in the most flooded paddies resulted in relatively high levels of methylated As in both years and methylated Hg in year 1. By year 2, there was mainly methylated Hg but in lower concentration across the variably managed rice paddies. While we did not measure volatile species here, other work (Chen et al., 2017; Huang et al., 2012; Mestrot et al., 2011; Shang et al., 2017; Y. Wang et al., 2022) suggests that volatilization of some methylated As and Hg species occurred from year 1 to year 2 and that, combined with plant uptake, resulted in slightly lower levels in year 2 compared to year 1. Despite these year‐to‐year differences, our data support previous findings that a large proportion of the increased grain Hg and As under strongly reducing conditions is due to methylated species, and it is these species that are most affected as conditions become less and less reducing (Li et al., 2019; Tanner et al., 2018). Some of the microbial processes that mobilize and methylate As are similar to those for Hg (Benoit et al., 2001; Chen et al., 2019; Compeau & Bartha, 1985; Reid et al., 2017; Slowey & Brown, 2007).

Soil EH strongly affected the concentration and proportion of iron minerals that comprised root plaque, which likely influenced plant metal(loid) concentrations. Ferrihydrite is a poorly‐crystalline iron oxyhydroxide mineral with high reactivity and surface area for As retention (Dixit & Hering, 2003) that decreased in concentration with drier water management (Figure S3 in Supporting Information S1). Lepidocrocite has received less attention than ferrihydrite but also plays a role in As retention (Park et al., 2018; Pedersen et al., 2006; Wang & Giammar, 2015) and can form from either the transformation of ferrihydrite or rapid changes in redox conditions (Cornell & Schwertmann, 2003; Grigg et al., 2022; Hansel et al., 2005; Schulz et al., 2023; ThomasArrigo et al., 2019). In contrast, goethite is a well‐crystalline mineral that has relatively less reactive surface area for As retention (Bowell, 1994). Some have also suggested that Cd and Hg can interact with Fe plaque (Liu et al., 2008; Y. Wang et al., 2022; H. Zhou et al., 2018). Here, we observed that drier conditions (higher soil EH) led to lower proportions of ferrihydrite and lepidocrocite and higher proportions of goethite in root plaque (Table S1 in Supporting Information S1). While controlling for the effect of soil EH, the proportion of lepidocrocite was still positively correlated with grain As and grain Hg species (Tables S2–S6 in Supporting Information S1), suggesting that lepidocrocite may play a role in the retention of these metal(loid)s. In contrast, the proportion of goethite was negatively correlated with grain iAs (Table S3 in Supporting Information S1), which suggests that as transformation to goethite occurs, there is more retention and less uptake of iAs that can be stored in the grain.

One of the major rice production challenges is how to limit the concentrations of toxic elements while providing nutritious food, particularly for those on limited diets. While grain polishing can remove inorganic As and Hg, it often removes micronutrients as well. The water management used here was able to dramatically affect concentrations of toxic elements As, Hg, and Cd in polished grain without affecting grain Zn concentrations. Many higher plants, including rice, can allocate micronutrients to the seed to improve fitness regardless of the growing conditions (Assunção et al., 2022). Here, the rice plants were able to allocate similar concentrations of Zn to seed despite the changing biogeochemical conditions that impacted As, Hg, Cd, and CH4. However, grain concentrations of Mn, Cu, and Cd were affected by either growing year or by water management with either decreases or no change as paddies were drier (Figure 5). It is well established that the higher status of micronutrients can limit the body burden of toxic metal(loid)s in mammals (Casarett, 2008; Peng et al., 2023). Thus, as Cd levels increased, the sum of micronutrients in rice grain decreased (Figure S6 in Supporting Information S1), potentially making Cd more bioaccessible as paddies are kept drier. This potential increase in bioaccessible Cd under drier conditions combined with the increasing usage of non‐flooded rice warrants further investigation, particularly at the production scale.

Growing rice under less flooded conditions has the potential to cause yield declines, which must be considered. Here, we saw a 25%–40% grain yield reduction between flooded and non‐flooded conditions (Figure S1 in Supporting Information S1). These findings are slightly higher than prior work that showed row rice or severe (dry) AWD leading to 10%–22% yield reductions relative to flooded conditions (Carrijo et al., 2017; Chlapecka et al., 2021) and likely represent a worst‐case scenario. Because some farmers are adopting row rice in the Mid‐South, this suggests that they are willing to risk a slight yield decline for the benefit of saving time by not creating levees (Leavitt et al., 2025; Nalley et al., 2022).

5. Conclusions

We conducted a 2‐year field study to evaluate the impact of water management on metal(loid) concentrations and CH4 emissions in rice paddies. We observed that growing rice with drier conditions greatly decreased CH4 emissions and concentrations of grain As and grain Hg species in rice grain. However, under the driest conditions, we observed grain Cd concentrations that exceeded the CODEX limit for rice. Considering the impacts on other grain micronutrients, our findings suggest that the human bioaccessibility of elevated Cd under the driest conditions could be more than what is suggested by Cd concentrations alone. While this study focused on 6 rice paddies over 2 years in one soil, more data with a variety of soils is warranted to determine potential health benefits and risks to rice consumers with water management. Nevertheless, as rice producers are adopting water savings strategies, our data strongly suggest that levels of Cd and micronutrient levels in grain should be monitored to ensure a safe food supply, particularly food products intended for infants and young children.

Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

Supporting information

Supporting Information S1

GH2-9-e2025GH001410-s001.docx (524.4KB, docx)

Acknowledgments

This work was partially supported by the Institute for the Advancement of Food and Nutrition Sciences (IAFNS; Grant UARKANSAS‐20220412), USDA NIFA (Grant 2023‐67017‐39185), and the NSF (Grants 1350580 and 1930806). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE‐AC02‐76SF00515. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (P30GM133894). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS or NIH. We thank Sasil for inspiration on the figures. ALS and MAL designed the study and performed the experiment. ALS, MAL, and BJ analyzed the data. The first draft of the manuscript was written by ALS and all authors commented on previous versions of the manuscript. ALS, MAL, and BRKR secured funding. All authors read and approved the final manuscript.

Seyfferth, A. L. , Limmer, M. A. , Jackson, B. P. , & Runkle, B. R. K. (2025). Concentrations and health implications of As, Hg, and Cd and micronutrients in rice and emissions of CH4 from variably flooded paddies. GeoHealth, 9, e2025GH001410. 10.1029/2025GH001410

Data Availability Statement

All numerical data is available in Figshare (Seyfferth, 2025).

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Citations

  1. Seyfferth, A. L. (2025). Dataset for 2016 and 2017 UD RICE facility investigating metals and methane in variably water‐managed rice paddies [Dataset]. figshare. 10.6084/m9.figshare.28544180 [DOI]

Supplementary Materials

Supporting Information S1

GH2-9-e2025GH001410-s001.docx (524.4KB, docx)

Data Availability Statement

All numerical data is available in Figshare (Seyfferth, 2025).

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