Greenhouse Gas Emissions and Arsenic Mobilization in Rice Paddy Fields: Coupling Mechanisms, Influencing Factors, and Simultaneous Mitigation Measures

Abstract

As an important agricultural ecosystem, greenhouse gas (GHG) emissions and arsenic (As) mobilization in rice paddy fields have gained significant attention on climate change and food safety. There is a certain correlation between the GHG and As migration in rice paddy fields. The oxidation of methane in paddy fields can provide electrons for the reduction and release of arsenate. Nitrate in rice paddy soil can promote the fixation of As by oxidizing Fe (II) to form iron oxide–As complexes or directly oxidize As (III) to As (V) to reduce the toxicity of As. However, incomplete denitrification of nitrate can lead to the emission of N₂O. This review systematically expounds the research advances, influencing factors and simultaneous mitigation measures of GHG emissions and As mobilization in rice paddy fields. It focuses on discussing the influence mechanisms of soil physical and chemical properties, water management measures, fertilization methods, and the addition of soil conditioner on As migration and GHG emission, and it looks forward to future research directions. It aims to provide a theoretical basis and practical guidance for reducing the risk of As contamination in rice fields, reducing GHG emission, and achieving sustainable development of rice production.

1. Introduction

As an important food crop, rice (Oryza sativa L.) provides the staple food for almost half of the world’s population, with Asia accounting for about 90% of the world’s rice production and consumption [1]. However, during the flooded production process of rice, paddy fields face greenhouse gas (GHG) emissions and arsenic (As) contamination [2,3,4]. Under anaerobic conditions, soil microorganisms are involved in the processes of methane generation and As migration, which are regulated by the coupled C/N/As biogeochemical cycles [5].

Rice paddy fields are one of the main agricultural sources of GHG, and the global warming potential (GWP) of rice is 2.5–5.5 times that of other major cereal crops [6]. Under the anaerobic conditions, organic matters in paddy field soils are metabolized for methane production through microbial fermentation. Methane is the second-largest greenhouse gas in the world after CO₂. According to the Emissions Gap Report 2024 released by the United Nations Environment Programme, global CH₄ emissions in 2023 reached 9.3 billion tonnes of CO₂ equivalent, accounting for 16.3% of total GHG emissions [7]. According to the China Natural Resources Statistics Bulletin 2024 and China Statistical Yearbook 2024, paddy fields account for approximately 22.6% of the total cultivated land area in China. However, due to the high methane emission characteristics, GHG emissions from rice paddy fields accounts for 48% of the total GHG emissions from cultivated land [8,9]. In addition, the application of nitrogen fertilizer during rice production provides nitrogen substrates for nitrification–denitrification processes. Agronomic measures such as alternate wetting and drying (AWD) irrigation or flooding sunning of rice paddy fields will stimulate nitrification–denitrification in soils and increase N₂O emissions [10,11].

As is a common toxic metalloid that is widespread in rice paddy soils, particularly in South and Southeast Asia, due to factors such as irrigation with As-contaminated water, the use of As-containing pesticides, and industrial and mining activities [12]. A national soil survey in China indicated that 2.7% of the sampling sites had As concentrations exceeding national standards [13]. Contamination in rice paddy fields has attracted widespread global attention due to its impact on food security [14,15]. The total As concentration in polished rice from various producing regions in China ranges from 65.3 to 274.2 ng/g, with an average of 114.4 ng/g. Moreover, the As intake by Chinese people through rice is higher than that through drinking water [16]. Long-term exposure to As can lead to health problems such as skin damage, cancer, and cardiovascular diseases [17]. In addition to affecting human health through the food chain, the accumulation of As in rice plants can have negative effects on plant growth, inhibiting grain filling and reducing rice productivity [18].

There is a coupling relationship between GHG emissions and As immobilization in rice paddy fields [19,20,21,22]. A conceptual diagram of As contamination and GHG emissions in rice paddy fields is shown in Figure 1. The coupling of As migration and GHG emissions in paddy fields is synchronously regulated by changes in redox potential. The As migration was mainly affected by the morphometry of As (As (V)/As (III)) and the adsorption–desorption equilibrium. While the emission of GHG, especially methane, is mainly related to the microbial community structure (such as methanogens, nitrifying/denitrifying bacteria) and metabolic activity, the morphological changes in As, the equilibrium of adsorption and desorption, and the activities of microorganisms related to soil methane metabolism are closely related to Eh, and there is a synergistic relationship. An increase in Eh is conducive to methane oxidation and reduces the toxicity of As, while a decrease in Eh leads to an increase in methane production and an enhancement in the migration of As. Simultaneously inhibiting GHG emissions and As migration in paddy fields is of great significance for ensuring food security and protecting the lowland ecological environment. In recent years, research on GHG emissions and As migration in rice paddy fields has made a series of progress. This paper reviews the research advances related to the coupling mechanism, influencing factors, and simultaneous mitigation measures of GHG emissions and As migration in rice paddy fields. Although existing reviews have, respectively, summarized the research progress on GHG emissions (such as the generation and mitigation mechanisms of methane and nitrous oxide) or As migration (such as the release driven by iron oxide reduction) in rice paddy fields, there are two limitations in the existing reviews: (i) There is a lack of systematic elaboration on the coupling mechanism between GHG emissions and As contamination, and existing studies mostly mention the correlation of the C/N/As cycle but fail to integrate the synergistic or antagonistic effects of key coupling pathways such as methane oxidation and As(V) reduction, nitrate reduction, and Fe(II)/As(III) oxidation. (ii) Existing reviews mostly discuss mitigation measures of GHG emissions or As migration alone, while the synergistic effects of water management, fertilization, and other measures on the two issues were ignored (such as the dual effect of alternative wetting and drying on methane emission reduction and As immobilization). This review takes "electron transfer" as the core, the coupling mechanism of GHG emissions and As migration is systematically analyzed, and the interaction pattern of the C/N/As cycle driven by microorganisms is revealed. Multi-dimensional collaborative strategies are proposed to provide theoretical and practical guidance for the sustainable development of rice fields.

2. GHG Emissions and as Migration in Rice Paddy Fields

2.1. GHG Generation and Emissions in Rice Paddy Fields

Rice paddy fields are not only the main agricultural source of methane; nitrogen fertilizer application in paddy fields also leads to N₂O emission. Due to their different response mechanisms to soil moisture, methane and N₂O emissions from paddy fields show a trade-off relationship [23]. During the flooded period, the aerobic decomposition of organic matter in paddy soils is inhibited. Additionally, the microbial anaerobic respiration is inhibited due to the limited availability of electron acceptors such as NO₃⁻ and SO₄²⁻, which were characterized by a relatively small radius, high electronegativity, weak polarizability, and high HOMO energy. Methane was produced from the decomposition of active organic matter including the plant and animal residues, root exudates, organic fertilizer, and other substances in the soil. In the anoxic environments during the flooding stage, the denitrifying microorganisms can also utilize high concentrations of Fe(II) produced by microbial Fe(III) reduction as electron donors for nitrate-reducing and Fe(II)-oxidizing microorganisms (autotrophic denitrification) [24]. Coupled with the abiotic oxidation of Fe(II) by reactive nitrogen, both processes contribute to N₂O emissions [25]. During the mid-term drainage (field drying) period, methane production is inhibited, aerobic oxidation is enhanced, and large amounts of N₂O produced from denitrification during the flooding period are emitted.

2.2. Migration and Transformation of as in Rice Paddy Fields

As mainly exists in soil in the form of inorganic arsenate and arsenite, and it is immobilized through adsorption or co-precipitation on the surface of iron oxides/hydroxides [26]. The toxicity and mobility of As vary with its species: inorganic As exhibits higher toxicity than organic As, and As(III) (arsenite) is more toxic and mobile than As(V) (arsenate) [27]. The migration and transformation of As in rice paddy fields are complex processes, influenced by multiple factors. Among these, the redox cycling and sequestration of Fe(III) oxide/hydroxide minerals have a significant impact on the mobility and bioavailability of As [28]. Currently, microbially mediated reductive dissolution of As-containing iron oxides/hydroxides is recognized as the key mechanism driving As contamination in soil [29]. For example, iron-reducing bacteria (IRB) can reduce Fe(III) to Fe(II), which promotes the release of As bound to iron oxides, thereby increasing As mobility [30]. Under the flooded conditions, As(V) is released during the reductive dissolution of Fe(III) minerals and further reduced to As(III), which is more toxic and mobile [25].

2.2.1. Influence of Soil Properties on as Dynamics

Soil physicochemical properties, especially texture, mineral composition, and redox potential, play a pivotal role in governing As migration and transformation. Clay-rich soils typically contain high concentrations of Fe, Mn, and Al oxides, which enhance As retention through strong adsorption [31]. In contrast, sandy soils with low clay content (<20%) and limited Fe/Mn oxides show higher As mobility, as observed in upland paddy areas of Southeast Asia. Mn oxides, due to their high redox potential, further modulate As behavior by delaying the reductive dissolution of Fe(III) oxides under anaerobic conditions. The content of Mn and Fe determines the retention of arsenic in the soil, while its fluidity is affected by organic matter and total phosphorus/available phosphorus [32]. In addition, in the flooding paddy soil, the oxidation–reduction reactions of metal elements in the soil will affect the soil pH. The flooded incubation resulted in a decrease in Eh, a rise in pH, and an increase in the content of As(III) in the soil solid phase. Soil incubation experiment results suggested that Eh decreases from +100 mV to −68 mV during the flooded incubation, and pH increases from approximately 6.5 to 7.0 accordingly. As(III) in the soil solid phase accounts for 80% of the total soil As [33].

2.2.2. Geographic Variations in as Behavior

Regional differences in As contamination sources and soil properties lead to distinct As dynamics across paddy ecosystems. In South and Southeast Asia (e.g., Bangladesh, Vietnam), As contamination is primarily linked to anthropogenic activities such as irrigation with As-rich groundwater and historical use of As-containing pesticides [34,35]. These regions’ paddy soils, often dominated by alluvial clays with high Fe content, exhibit intense microbial Fe(III) reduction under flooded conditions, driving As release from Fe oxides—up to 60% of total soil As can be mobilized into pore water during the rice growing season. In China, As contamination in paddy soils is widespread in mining-intensive regions (e.g., Hunan, Guangxi) [36,37]. For instance, the rice paddies in Hunan affected by mining and the high clay content combined with low Eh under continuous flooding lead to As(III) release. In contrast, paddy soils in the Yangtze River Delta, with neutral pH and moderate Fe oxide content, show lower As mobility due to stronger adsorption by Fe-Mn nodules.

2.2.3. Natural vs. Anthropogenic Factors Regulating as Immobilization

Natural factors inherently shape As retention in paddy soils. Iron oxide is abundant on Earth and is reported to have a high affinity for As [38]. Soil Eh, a key natural driver, controls Fe redox cycling: under oxic conditions, Fe(III) oxides are stable and retain As, while anaerobic conditions trigger Fe(III) reduction, releasing bound As. Anthropogenic activities, however, can disrupt or enhance these natural processes. For example, long-term application of ammonium-based fertilizers exacerbates soil reduction, promoting Fe(III) reduction and As release. Conversely, nitrate application stimulates nitrate reduction coupled with Fe(II) oxidation (NRFO), re-oxidizing Fe(II) to Fe(III) oxides and re-immobilizing the mobilized As [39]. Soil amendments, like biochar or Mn oxides, enhance As adsorption by increasing soil surface area and redox potential, reducing As mobility [2].

3. The Coupling Relationship Between GHG Emissions and as Migration in Rice Paddy Fields

Considering the complex interactions among C/N/As in rice paddy fields [5,40,41], microorganisms often induce opposing environmental outcomes for GHG (methane and N₂O) emissions versus As(III) mobilization [2]. The coupling relationship between GHG production, consumption, and As migration and transformation in paddy fields is illustrated in Figure 2. On the one hand, the coupled process of methane oxidation and As(V) reduction can reduce methane emissions. Under anaerobic conditions, methane can be anaerobically oxidized by anaerobic methanotrophic archaea (ANME) and bacteria or by specific ANME-bacteria consortia [42,43], with concurrent reduction in As(V) to As(III). ANME, either alone or in synergy with arsenate-reducing bacteria (AsRB) such as Geobacter, mediate a coupled pathway of anaerobic methane oxidation (AOM) and arsenate reduction through reverse methanogenesis and respiratory arsenate reduction, contributing 26–49% of total As release in wetland soils [20]. Under aerobic conditions, aerobic methane oxidation (AeOM) has also been shown to drive arsenate reduction in soils, contributing up to 76.2% of As in pore water of surface soils [21]. Thus, the coupled process of methane oxidation and As reduction reduces methane production but promotes As migration. Additionally, the toxic effect of released arsenate can inhibit methane production and promote methane oxidation by reducing the abundance and diversity of soil methanogens. Results from soil incubation experiments showed that methane emissions were reduced by 68.5% with the addition of 30 mg/kg of arsenate [44].

On the other hand, nitrate reduction coupled with iron oxidation (NRFO) and nitrate reduction coupled with As oxidation (NRAO) in rice paddy fields promote As immobilization. In the NRAO process, nitrate mitigates the toxicity of As(III) through direct oxidation with nitrate acting as an electron acceptor [22,45,46]. In the NRFO process, nitrate oxidizes Fe(II) to Fe(III) oxides/hydroxides, which reduce the mobility and bioavailability of As through adsorption, co-precipitation, and other mechanisms [47]. The nitrate-coupled iron/As oxidation metabolic pathways can inhibit the reduction and release of As, whereas As(III) is microbially oxidized to As(V) with lower mobility. However, incomplete denitrification of nitrate leads to increased N₂O production [22]. In addition to nitrate, manganese (Mn)-containing minerals, due to their high redox potential, can also inhibit the release of As caused by Fe(III) dissolution by oxidizing Fe(II) and As(III) [48,49,50]. Mn-containing minerals in paddy soils can also act as electron acceptors and photocatalysts, thereby reducing methane emissions from rice paddy fields [51].

The relevant chemical reactions can be expressed as the following equation:

CH₄ can be oxidized under anaerobic and aerobic conditions with high-valence cations or as O₂ electron acceptors, respectively, producing H⁺ and electrons that can be used for arsenate reduction, and coupling of methane oxidation and arsenate reduction occurs. The relevant chemical reaction formulas for methane oxidation, arsenate reduction, and their coupling are as follows:

Methane oxidation:

Anaerobic oxidation of methane:

CH₄ + SO₄²⁻ → HCO₃⁻ + HS⁻ + H₂O

CH₄ + NO₃⁻ → HCO₃⁻ + N₂ + H₂O

CH₄ + NO₂⁻ → HCO₃⁻ + N₂ + H₂O

CH₄ + Fe(OH)₃ → HCO₃⁻ + Fe²⁺ + H₂O

CH₄ + MnO₂ → HCO₃⁻ + Mn²⁺ + H₂O

Aerobic oxidation of methane:

CH₄ + O₂ → HCO₃⁻ + H₂O + H⁺

Arsenate reduction coupled with methane oxidation:

H₃AsO₄ + e⁻ + H⁺ (from methane oxidation) → H₃AsO₃ + H₂O

The arsenite can also be oxidized to arsenate by the high-valent metal oxide mineral such as MnO₂ and anions such as NO₃⁻ in rice paddy soil by the following reaction equations:

Soil minerals (MnO₂) on As forms:

MnO₂ + H₃AsO₃ + H⁺ → Mn²⁺ + H₃AsO₄ + H₂O

H₂O + MnO₂ + H₃AsO₃ → MnOOH* + H₃AsO₄

Fe²⁺ + H⁺ + MnO₂ → Mn²⁺ + Fe³⁺ + H₂O (As immobilization)

Nitrate reduction coupled with As(III) oxidation:

NO₃⁻ + H₃AsO₃ → H₃AsO₄ + N₂O/N₂

Microbial-mediated reduction and dissolution of arsenic-containing iron oxides/hydroxides is a generally accepted mechanism for As contamination in rice paddy soil, while ferrous oxidation is one of the main pathways for As fixation in rice paddy soil. The following is the chemical reaction formula for the fixation of As by the oxidation of Fe(II) with nitrate:

Nitrate-reduction coupled with Fe(II) oxidation:

NO₃⁻ + Fe²⁺ → Fe(OH)₃ + N₂ (As immobilization)

Under the flooded condition, organic matter in paddy soil was fermented for methane production, while N₂O generated from denitrification in the soil is emitted in large quantities during the alternating wetting and drying process of rice paddy fields. Meanwhile, arsenate in the soil can be reduced to arsenite, which has higher mobility and toxicity. These processes have significant impacts on global climate change and food security. The development of ecologically efficient and economically feasible remediation strategies to reduce GHG emissions from paddy fields, mitigate As contamination risks, and promote the green and sustainable development of the rice industry is of great significance.

4. Factors Affecting GHG Emissions and as Migration in Rice Paddy Fields

4.1. Physicochemical Properties of Soil

Soil properties such as texture, pH, redox potential (Eh), and mineral contents including iron and Mn have significant impacts on As migration and methane emissions [52,53]. Clay soils contain a high proportion of iron, Mn, and aluminum (Al) oxides, which exhibit strong adsorption capacity for As and can reduce As migration [54]. Moreover, Mn oxides in clay were often at high redox potential, which can delay the reductive dissolution of Fe(III) and As(V) by increasing environmental Eh [50,55]. However, the methane emissions in clay are higher than those in loamy and sandy soils due to the low oxygen content in clay soils [56]. Soil Eh in rice paddy fields comprehensively reflects the contents of oxygen and non-oxygen electron acceptors in the soil. When Eh drops below −200 mV, oxygen (as an electron acceptor) is depleted, and the reductive dissolution of iron under anoxic conditions leads to As release, accompanied by anaerobic metabolism of organic matter for methane production [14,57]. Soil pH significantly affects the speciation and mobility of As. Previous research results suggested that acidic (pH4) and alkaline (pH10) conditions can result in a change in anionic charge, which causes release of electrostatically bound As [58]. Soil pH can influence methane production by affecting organic matter decomposition and methanogen activity, with methanogens generally exhibiting the highest activity at pH 6–7 [59]. Under different pH conditions, arsenate reduction coupled with methane oxidation in paddy soils is dominated by different methanotrophs, thereby affecting the reductive release of As [60]. Furthermore, minerals such as iron and Mn in the soil also play important roles in As migration and methane production. Iron minerals can immobilize As through adsorption and co-precipitation, affecting As migration; Mn oxides can alter the soil redox environment, and amorphous Mn oxides can enhance As adsorption, directly or indirectly influencing As migration and methane production [14].

4.2. Water Management in Rice Paddy Fields

Water management is one of the important factors affecting As migration and methane emissions in rice paddy fields. Agronomic measures such as flooding and drainage can affect the redox potential of the soil, which in turn influences methane emissions and As migration and transformation. The traditional continuous flooding measures keep the soil in an anaerobic state for a long time, which is conducive to the reductive release of As and methane production [61]. Under the continuous flooding conditions, iron oxides in the soil are reduced, and a large amount of As adsorbed by them is released. At the same time, methanogens are active, leading to an increase in methane emissions. In contrast, intermittent irrigation (such as AWD) can improve soil aeration and regulate soil Eh, thereby reducing As migration and methane emission. The adoption of moderate AWD can reduce nitrogen loss, improve nitrogen fertilizer use efficiency, inhibit methane production, and decrease As accumulation in grains. Yang et al. used AWD measures controlled by soil water potential, leaf water potential, and groundwater level, which reduced the global warming potential (GWP) of rice paddy fields by 48.3–78.9% and the As content in grains by 50.3–66.5% [62]. However, the effect of intermittent irrigation is affected by factors such as the degree of drought, irrigation frequency, and timing. In general, allowing the soil to dry out more between flood events increases the environmental benefits, but excessive drought may inhibit rice growth and affect rice yields [63]. High irrigation frequency cannot give full play to its inhibitory effect on As migration and methane emission. Perry et al. found that a single mid-season drainage can reduce methane emissions from paddy fields by 20–77% and the average As content in grains by 20% [61]. In addition, other water management measures such as controlling water level depth and optimizing irrigation time can also inhibit As migration and methane emissions. Appropriately reducing the water level depth can increase soil aeration and reduce methane emissions [62]. Reasonable irrigation during the critical period of rice growth stages can not only meet the growth needs of rice but also reduce the environmental risks of As migration and GHG emissions [64].

4.3. Type of Fertilizers

The impact of fertilization on As migration and GHG emissions in paddy fields is relatively complex. Nitrogen fertilizer is commonly used in rice production, and different forms of nitrogen fertilizer have varying effects on methanogenesis and the migration and transformation of As. Ammonium nitrogen can enhance the reductive environment in the soil, potentially promoting As release and methane production. Since ammonium nitrogen consumes oxygen during nitrification, this is beneficial to the activities of AsRB and methanogens. In contrast, nitrate nitrogen can promote soil oxidation processes and inhibit the reductive release of As and methane generation. As an electron acceptor, nitrate nitrogen can participate in redox reactions in the soil, increase the soil redox potential, and is unfavorable to the activities of As-reducing and methanogenic microorganisms [39,65]. On the other hand, As can inhibit mineralization of organic nitrogen and nitrogen fixation, increase the microbial demand for nitrogen in detoxification, and strengthen the As-nitrogen coupling process, leading to increased nitrogen losses such as soil N₂O emissions [66]. Straw returning and organic fertilizer application also affect As migration and methane emissions. Studies have shown that rice straw returning can promote the reduction in soil Eh, facilitate the growth of AsRB to enhance As release [67,68], and improve As bioavailability by promoting As methylation [69]. In addition, excessive application of organic fertilizers increases the soil organic carbon content in the soil, providing more substrates for methanogens and thus leading to increased methane emissions. Meanwhile, the decomposition of easily degradable organic matter in organic fertilizers can promote the formation of a reductive environment in the soil, thereby facilitating As release [70]. However, the appropriate application of stable organic fertilizers may reduce As migration by improving soil structure and enhancing the adsorption capacity for As [71]. Furthermore, the microbial community in organic fertilizers may also affect the metabolic processes of As and methane, and the specific mechanism requires further research.

4.4. Other Factors

In addition to the aforementioned factors, rice varieties, temperature, light, precipitation, and other factors also affect As migration and methane emissions in rice paddy fields. Different rice varieties vary in their ability to absorb and accumulate As, which is related to the physiological characteristics of rice roots such as the iron plaques in rice root surface and the expression of As transporters [72,73]. Screening rice varieties with low As accumulation can reduce the risk of As accumulation in the food chain. Root exudates influence methane production and As release by affecting the substrate supply for methanogens, the coupling of arsenate reduction with methane oxidation in the soil, and the formation of secondary minerals in the soil. Small molecular organic acid root exudates reduce the concentration of As(III) in soil pore water by 35.1–65.7% [74,75]. The impact of temperature on As migration and methane emissions is mainly achieved by influencing microbial activity and chemical reaction rates. Generally, elevated temperatures promote the growth and metabolism of microorganisms, increasing methane emission; at the same time, temperature also affects the chemical equilibrium and migration rate of As in the soil [76]. Increased methane production can further promote As migration and enhance As bioavailability [2]. When the temperature rises from 28 °C to 33 °C, the coupling of arsenate reduction and methane oxidation is strengthened, and the production rate of As(III) increases by 36.1% [77]. Light affects plant photosynthesis and soil microbial activity, influencing As migration and methane emissions indirectly. Under sufficient light conditions, rice photosynthesis is enhanced, leading to increased root exudates, which may alter the structure and activity of soil microbial communities, thereby affecting As reduction and methane production. The Fenton-like reaction between micromolar levels of hydrogen peroxide in rainwater and soil Fe(II) can generate hydroxyl radicals, inhibiting arsenate reduction in the soil, promoting arsenite oxidation, and reducing As uptake by rice roots and As accumulation in grains [78]. Qin et al. found that the rainwater reduced As content in rice grains by approximately 33% and decreased N₂O and methane emissions by over 97% and 98%, respectively [78].

5. Simultaneous Mitigation Measures for GHG Emissions and as Mobilization in Rice Paddy Fields

5.1. Water Management Optimization

Agronomic measures for water management to increase oxygen content in paddy field soils can achieve multiple effects such as GHG emission reduction, water conservation, reduction in soil nutrient leaching, and reduction in As reduction and release. Optimizing water management is one of the effective ways to synchronously inhibit As migration and methane emissions. AWD changes the Eh of the soil, increases soil aeration during the drying phase, inhibits the activity of methanogens, and reduces methane production; at the same time, the oxidizing environment is conducive to the oxidation and fixation of As, reducing the mobility and bioavailability of As. Multiple studies have shown that AWD can significantly reduce methane emissions from paddy fields while reducing As uptake by rice [62,64]. However, the effectiveness of AWD implementation is constrained by multiple factors. For example, the degree and duration of drought will affect the growth and yield of rice. If the drying phase is too dry, rice may suffer from water stress and reduce yield. Linquist et al. found that, compared with continuous flooding, the GWP of rice paddy fields under AWD treatment decreased by 45–90%, and the As concentration in rice grains decreased by 64%, but the yield decreased by 1–13% [63]. Therefore, it is necessary to optimize the AWD irrigation scheme according to the climate, soil conditions, and rice varieties in different regions. During the critical periods of rice growth, such as the booting and filling stage, appropriately shorten the drought time or reduce the drought degree to minimize As migration and methane emissions under the premise of ensuring yield. In addition, combined with soil moisture monitoring technology, precise irrigation based on the actual soil moisture content can control the soil redox state more accurately and improve the synchronous mitigation effect of As migration and methane emissions. New oxygen-increasing irrigation measures inhibit methanogen activity and reduce methane production by increasing the Eh of paddy field soils, while inhibiting the reductive dissolution of iron, manganese, As, etc. Minamikawa et al. found that nano-bubble water irrigation can reduce methane emissions from paddy fields by 21% without affecting rice growth and reduce the content of dissolved As [79]. However, the large-scale application of nano-bubble water irrigation technology still needs to evaluate the field application cost and the whole process of greenhouse gas emissions.

5.2. Rational Fertilization

Rational fertilization is an important measure for simultaneous inhibition of As migration and methane emissions in rice paddy fields. In terms of nitrogen fertilizer management, selecting appropriate types and application rates of nitrogen fertilizers is crucial. Studies have shown that nitrate nitrogen fertilizers are more beneficial than ammonium nitrogen fertilizers in inhibiting As migration and methane emissions. This is because nitrate nitrogen can act as an electron acceptor in the soil, increasing soil Eh, promoting the oxidative fixation of As, and facilitating the oxidative removal of methane [46,80]. For example, in some paddy field experiments, treatments with nitrate nitrogen fertilizer application showed lower soil As bioavailability and significantly reduced methane emissions compared to those with ammonium nitrogen fertilizer application. Nitrate reduction can couple with the oxidation of As(III) and Fe(II) [47], inhibit Fe(III) reduction [81], and reduce As mobility. Therefore, increasing the application of nitrate-containing nitrogen fertilizers can reduce the transfer of As from soil to the food chain, alleviating the threat of As to human health. However, due to the poor retention of nitrate in the soil, excessive application of nitrate nitrogen fertilizers may lead to nitrate leaching in rice paddy fields, causing agricultural non-point source pollution and indirect N₂O emissions. Therefore, it is necessary to select suitable fertilizer types such as slow-release fertilizers and adopt fertilization methods like side-deep fertilization and precision fertilization based on soil nutrients and rice growth stages. This can ensure rice yield while reducing the environmental risks of As migration and GHG emissions. In addition, during the oxidation of As(III) through the microbial NRAO pathway, incomplete denitrification of nitrate can lead to increased N₂O production [22]. Anaerobic ammonium oxidation (anammox) induced by amino fertilizer application may provide a feasible approach to effectively inhibit N₂O generation in rice paddy fields, as nitrite produced from incomplete denitrification and ammonia is neutralized into N₂ [22]. Minerals such as iron oxides and manganese oxides during rice paddy field fertilization can oxidize NH₄⁺ into N₂ through processes like Feammox and Mnammox, which are similar to anammox, thereby reducing N₂O production [2,82]. Meanwhile, the continuous cyclic regeneration of iron and manganese oxides through iron oxidation, Mnammox, and NRFO/NRMO processes enables the oxidation and fixation of As through oxidation, adsorption, and co-precipitation. Silicon fertilizers can also influence methane production and arsenic (As) methylation processes in paddy fields. Specifically, silicate application induces a synergistic effect between methanogens and As-methylating microorganisms, which helps significantly reduce methane emissions and methylated As concentrations in As-rich paddy soils [83]. Additionally, silicon fertilizers can regulate soil microbial functional genes involved in GHG production and consumption, alter microbial community structure and activity, and thereby reduce GHG emissions. For instance, in treatments with silicate fertilizer application, the cumulative methane emissions from indica and japonica rice were significantly reduced by 31% and 28%, respectively, while their cumulative N₂O emissions were also significantly reduced by 21% and 17%, respectively [84]. Although silicon fertilizers are a novel counter-measure strategy, they still face challenges in large-scale application. The cost-effectiveness of producing and applying silicon fertilizers on a wide range of agricultural lands needs to be further evaluated. Additionally, the long-term impact of continuous silicon fertilizer application on soil structure and other non-target soil properties is still unclear.

The application of organic materials should not be ignored. Straw returning, organic fertilizer, and manure application are important measures to maintain soil fertility in rice paddy fields. However, organic materials can increase methane emissions by providing substrates for methanogens. Additionally, humic acid substances in organic fertilizers can act as electron shuttles to accelerate the transfer of electrons to acceptors such as iron oxides, enhancing microbial arsenate respiration and driving the reductive release of As [85,86]. Combining appropriate water management measures can improve soil organic matter in rice paddy fields while inhibiting methane emissions and As migration. Islam et al. found that, under organic management, a 71–85% reduction in methane emissions and a 17–65% decrease in grain As content were achieved by adopting AWD in the early growth stage of rice. Under the management of combined organic and inorganic fertilization, early AWD achieved a 51–76% reduction in methane emissions and a 28–39% decrease in grain As content. Although N₂O emissions increased by 23–305%, its contribution to GWP was less than 20%. Under early AWD treatment, the GWP at both area and yield scales decreased by 67–83% [87].

5.3. Soil Amendment Additions

Adding soil amendments is an effective method to inhibit As migration and methane emissions simultaneously in rice paddy fields. The soil amendments used for As contamination remediation and GHG emission reduction in rice paddy fields are shown in

Table 1. The effects and mechanisms of different As contamination passivators on the mitigation effects of As mobility and GHG emissions from rice paddy fields.

Types	Dosages	Mitigation Effects on GHG Emissions and As Migration	Mechanisms	References
Calcium sulfate: Iron oxide at a mass ratio of 9:1 (IBP)	0.3% of soil mass	Arsenite concentration reduced by 78%, AOM increased by 55%	Promote the coupled reduction in AOM and Fe(III), reduce electron transfer from AOM to As(V) reduction	[93]
Activated carbon with BET-specific surface area of 871 m2/g	2% of soil mass	CO2 emissions reduced by 47.8%, methane emissions reduced by 97.6%, As release almost completely inhibited	Inhibiting Fe(III) reduction, reducing As release and carbon emissions	[94]
Synthetic birnessite with average manganese oxidation state of 3.73	Birnessite at 0.5% (w/w), with soil manganese concentration of 3000 mg/kg	Methane emissions reduced by 47–54%, As release reduced by 38–85%	Birnessite catalyzes the polymerization of DOM to form refractory DOM, inhibits methanogenesis, increases methane oxidation, inhibits As release by reducing iron reduction.	[90]
Pyrolytic carbon (at 550 °C) @birnessite at a mass ratio of 1:1	5% of soil mass	98.11% of total As is immobilized, CO2, methane, and N2O emissions are reduced by 14–20%, 6–42% and 21–60% respectively	Immobilize As through NRAO and iron oxidation/adsorption; promote AOM to reduce methane emissions; N2O emission reduction through reactions such as Mnammox and Feammox	[2]
Combined treatment of nitrate and birnessite	Sodium nitrate is 0.25% of soil mass, and birnessite is 0.5% or 0.83% of soil mass	The combined treatment shows almost no As release, and N2O emissions are reduced by at least 87%	Birnessite inhibits the activity of denitrifying enzymes, reduces denitrification electron consumption; NRMO and MnO2 regeneration promote As(III) immobilization and N2O emission reduction	[91]

Notes: AOM, anaerobic oxidation of methane; NRAO, nitrate reduction coupled with As(III) oxidation; and NRMO, nitrate reduction coupled with Mn(II) oxidation.

. Iron-based amendments mitigate As mobility primarily through oxidation, absorption, and co-precipitation: (i) Oxidation: They promote nitrate-reducing Fe(II) oxidation (NRFO), where Fe(II) is oxidized to Fe(III) under nitrate-reducing conditions; the generated Fe(III) further oxidizes toxic As(III) to less mobile As(V) [2,88]. (ii) Adsorption: Fe(III) (hydr)oxides have abundant surface functional groups that form inner-sphere complexes with As(V) via ligand exchange [89]. (iii) Co-precipitation: As(V) is incorporated into the crystal structure of secondary Fe(III) minerals during their formation, permanently sequestering As [2]. Additionally, iron-based amendments increase soil redox potential (Eh), which inhibits the activity of As-reducing bacteria that mediate As(V) reduction, thereby reducing reductive dissolution of As-bearing minerals and As release [50]. Manganese oxides inhibit As mobility through their strong oxidizing capacity. As(III) can be oxidized to As(V) by MnO₂ due to the high standard redox potential of MnO₂ [90]. The generated Mn²⁺ can form Mn-As coprecipitates, while the residual MnO₂ surfaces adsorb As(V) via electrostatic attraction and surface complexation. Moreover, their role in polymerizing soil organic matter reduces the availability of dissolved organic matter (DOM), which otherwise complexes with As to enhance its mobility [90]. Their ability to stimulate anammox-related pathways (promoting nitrate reduction to N₂) also indirectly reduces N₂O emission, but this process is synergistic rather than direct for As immobilization [91]. The combined treatment of nitrate and birnessite further enhances As(III) oxidation (via nitrate as an electron acceptor) and As(V) adsorption by Mn oxides, thus effectively inhibiting As migration [92]. Biochar supplementation can reduce As mobility through physical adsorption, soil property regulation, and microbial mediation. (i) Physical adsorption: Its well-developed porous structure and abundant surface functional groups (e.g., -COOH, -OH) provide adsorption sites for As(III)/As(V); for example, oxygen-containing groups can form hydrogen bonds with As(III), while cationic sites electrostatically attract anionic As(V) [93]. (ii) Soil property regulation: Biochar improves soil aeration (increasing Eh), which inhibits reductive dissolution of Fe/Mn (hydr)oxides and reduces As release from soil solids to solution [93]. (iii) Microbial mediation: As a microbial carrier, biochar enriches functional microorganisms such as Geobacter (facilitates NRFO) and Streptomyces (mediates As(III) oxidation), enhancing biotic As(III) oxidation conversion and subsequent immobilization by Fe(III) minerals [2]. These processes collectively reduce As mobility in the soil–plant system, as evidenced by decreased As accumulation in rice grains [92]. The effectiveness of different amendments varies depending on factors such as soil type (e.g., pH, organic matter content), amendment dosage, and application method, so selection and optimization need to be carried out according to specific conditions.

5.4. Microbial Agent Applications

Measures for microbial regulation in paddy soil provide new ideas for the simultaneous mitigation of As migration and GHG emissions. Iron-oxidizing microorganisms and As-oxidizing microorganisms can oxidize Fe(II) to Fe(III) and As(III) to As(V), respectively, thereby reducing the mobility, bioavailability, and toxicity of As. For example, the strain Pseudogulbenkiania sp. 2002 can generate lepidocrocite minerals through the NRFO pathway to immobilize As [95]. Another example is the strain Paracoccus sp. SY isolated from paddy soil, which can achieve autotrophic growth by oxidizing As(III) under both aerobic and anaerobic conditions [96]. However, the above reports on As immobilization did not provide results related to GHG emissions. The loss of microbial diversity in paddy soil can inhibit methane oxidation, enhance microbial methanogenesis, leading to increased methane emission, and at the same time promote the reductive dissolution of As-containing iron minerals, increasing As release [97]. Regulating the soil microbial community structure and enhancing the microbial diversity in paddy soil may also achieve the simultaneous mitigation of As migration and methane emissions in rice paddy fields. For instance, increasing the quantity and diversity of methane-oxidizing bacteria in the soil can promote methane oxidation and reduce methane emissions. Meanwhile, using genetic engineering technology to construct microbial strains with specific functions is expected to more effectively achieve the goal of synchronous inhibition of As migration and methane emissions. However, microbial regulation technology still faces some challenges in practical applications, such as the survival and colonization ability of microorganisms and competition with indigenous microorganisms, which need further research and solutions. Recent studies have also reported the impact of microbial viruses on methanogenesis, methane oxidation, and As reduction in paddy fields. Free viruses and induced viruses affect the abundance of ANME-2d and Geobacter in rice paddy soil through different infection modes, thereby influencing methanogenesis, methane oxidation, and As reduction processes. This provides a new reference for the synchronous inhibition of methane emission reduction and As migration in rice paddy fields [98]. However, microbial interventions, such as introducing specific strains to promote or inhibit certain processes, may also lead to unintended microbial shifts. For instance, the introduction of a strain to enhance anammox activity might disrupt the existing ecological balance in the soil microbiome. Some native microorganisms that are beneficial for other soil functions, like nitrogen fixation or organic matter decomposition, could be outcompeted by the introduced strain. This could potentially lead to a decline in soil fertility in the long run.

5.5. Summary of the Potential Measures for Simultaneous Mitigation of GHG Emissions and as Migration

Simultaneous mitigation of GHG emissions and As migration in paddy fields can be addressed through multiple strategies, as summarized in Figure 3. In rice breeding, efforts should focus on developing new varieties that, while maintaining high yields, exhibit low GHG emissions and reduced As uptake by roots. For water management, the AWD regime should be optimized by tailoring irrigation schemes to regional climate, soil characteristics, and rice varieties. During critical growth stages (e.g., booting and grain filling), drying periods should be shortened, or intensity reduced, and precision irrigation integrated with soil moisture monitoring should be implemented. Additionally, optimizing floodwater levels during inundation can enhance soil oxidizing conditions. In fertilization practices, inorganic nitrogen sources should be carefully selected: nitrate-based fertilizers are preferable for inhibiting As migration and methane emission, but their application rates must be controlled. Slow-release formulations and side-deep fertilization techniques can minimize indirect GHG emissions caused by nitrogen leaching. Applying amino-based fertilizers and incorporating iron/manganese oxides can reduce N₂O emissions. Organic fertilizer application should be combined with water management to mitigate methane emissions and As release. Soil amendments, including inorganic materials (e.g., iron oxides, manganese oxides) and organic amendments (e.g., biochar), can immobilize As, modulate soil properties, and suppress both methane emissions and As migration. Their application requires optimization based on site-specific conditions. Microbial regulation strategies involve utilizing iron- and As-oxidizing microorganisms and adjusting microbial community structures to reduce As mobility and methane emissions. Screening high-efficiency functional strains such as iron-oxidizing, As-oxidizing, and methane-oxidizing bacteria to enhance soil microbial diversity can achieve simultaneous inhibition of As migration and GHG emissions. Furthermore, regulating rhizosphere microorganisms can reduce root uptake of As.

6. Summary and Future Perspectives

6.1. Summary

The coupling between GHG emissions and As immobilization in rice paddies is governed by interconnected biogeochemical cycles of C, N, and As, with electron transfer and Eh as core regulators. Key findings reveal the following:

(1)

Synergistic mechanisms

Oxidizing soil conditions (e.g., AWD) simultaneously suppress methanogen activity (reducing CH₄ emissions) and inhibit reductive dissolution of As-bound iron oxides (lowering As migration). Conversely, reducing environments enhance both CH₄ production and As(III) release, driven by microbial Fe(III) reduction and AsRB.

(2)

Coupled pathways

AOM coupled with arsenate reduction reduces CH₄ but increases As(III) mobility, contributing 26–49% of As release in wetlands. In contrast, NRFO/NRAO immobilizes As but may elevate N₂O via incomplete denitrification.

(3)

Mitigation strategies

Multi-dimensional measures achieve co-benefits. Water management (e.g., optimized AWD) reduces CH4 emissions by 45–90% and grain As by 64% while balancing yield. Nitrate-based fertilizers and slow-release formulations enhance As immobilization via NRFO and reduce GHG, though N₂O risks require controlled application. Amendments (iron/manganese oxides, biochar) immobilize As through adsorption/oxidation and lower GHG by regulating soil aeration and microbial activity. Microbial regulators (e.g., iron-oxidizing bacteria) promote As(V) formation and methane oxidation, with potential for targeted strain engineering. These strategies integrate soil properties, microbial functions, and agronomic practices to reconcile food safety, yield, and low-carbon sustainability.

6.2. Future Perspectives

To advance simultaneous GHG reduction and As mitigation in rice systems, future research should focus on the following below.

6.2.1. Short-Term Actionable Priorities

(1)

Optimization of existing agronomic practices

Refine region-specific water and fertilizer management protocols based on soil type and climate. For example, in As-contaminated areas of Southeast Asia, standardize AWD irrigation schedules and promote nitrate-based slow-release fertilizers with 20–30% reduced application rates to balance As immobilization and N₂O control. Develop simple soil Eh monitoring tools for farmers to adjust irrigation timing, ensuring yield stability while reducing GHG emissions by 30–40%.

(2)

Localized application of soil amendments

Prioritize low-cost, regionally available amendments and establish application guidelines through small-scale field trials. Collaborate with agricultural extension services to train farmers in amendment integration with existing practices (e.g., combining biochar application with straw return under AWD).

(3)

Policy incentives and knowledge dissemination

Design subsidy programs for farmers adopting verified mitigation measures (e.g., AWD + nitrate fertilizer) and develop region-specific “green rice” certification standards that include GHG and As thresholds. Create user-friendly manuals and mobile apps to disseminate best practices, linking farmers to carbon credit markets where applicable.

6.2.2. Long-Term Research Directions

(1)

In-depth analysis of multi-factor interaction mechanisms

Employ integrated multi-omics to dissect microbial community responses to combined stresses. Quantify how factors like root exudates, viral–microbial interactions, and mineral transformations synergistically regulate C/N/As cycles. For example, identify keystone taxa that simultaneously enhance methane oxidation and As(III) immobilization, and model their responses to climate variability.

(2)

Precision agriculture

Deploy IoT-based real-time monitoring systems (soil moisture, Eh, As speciation) coupled with AI-driven irrigation/fertilizer recommendation algorithms. Pilot “smart paddy” systems in 5–10 major rice-growing regions to optimize resource use, aiming for 50% reduction in monitoring labor and 15–20% improvement in mitigation efficiency.

(3)

Advanced materials

Develop new types of amendments that release oxidants (Mn(IV)) under reducing conditions to suppress As mobilization and methane production. Test its long-term stability and eco-toxicity in soil–plant systems.

(4)

Development of regionally adaptive and sustainable strategies

Construct integrated assessment models that couple biogeochemical processes with socioeconomic factors (e.g., farmer income, policy enforcement costs). For example, in China’s Yangtze River Delta, model trade-offs between AWD implementation costs, yield losses, and GHG/As benefits to inform regional policy.