A new insight on the effects of iron oxides and dissimilated metal-reducing bacteria on CH4 emissions in constructed wetland matrix systems

Pollutant removal and greenhouse gas emissions in horizontal subsurface flow constructed wetlands with iron ore treating ammonium-rich wastewater

Horizontal subsurface flow constructed wetlands (HFCWs) are capable of eliminating organic matter and nitrogen while emitting less methane (CH4) and nitrous oxide (N2O) than free water surface flow wetlands. However, the simultaneous removal of pollutants and reduction of greenhouse gases (GHG) emissions from high-strength wastewater containing high levels of organic matter and ammonium nitrogen (NH4+-N) has not get been investigated. The influent COD concentration affected the efficiency of nitrogen removal, GHG emissions and the presence of iron from iron ore, but the COD and TP removal efficiencies remained unaffected. CO2 and CH4 fluxes were significantly influenced by influent COD concentrations, whereas less N2O emissions were obtained during 7d. The highest CO2 and CH4 fluxes, along with the GHG emissions, were observed in HFCWs with COD concentrations of 375.6 mg/L and NH4+-N concentrations of 159.0 mg/L at a COD/N ratio of 2.4. Conversely, the lowest CH4 (-1.72 mg/m2/h) and N2O fluxes (0.13 mg/m2/h) were recorded in HFCWs with COD concentrations of 375.6 mg/L and NH4+-N concentrations of 162.4 mg/L at a COD/N of 4.5, although nitrogen removal was weak in these HFCWs. HFCWs at a COD/N ratio of 3.6 exhibited greater removal of nitrogen and other pollutants, along with a lower global warming potential (GWP). In conclusion, the concentrations of organic matter and NH4+-N in wastewater affected both pollutant removal and GHG emissions. The simultaneous enhancement of pollutant removal and the reduction of GHG emissions can be achieved in HFCWs with a COD/N ratio of 3.6.

Microbial community assemblage altered by coprecipitation of artificial humic substances and ferrihydrite: Implications for carbon fixation pathway transformation

The suppression of soil carbon mineralization has been demonstrated to be effectively facilitated by carbon‑iron interactions, yet the specific mechanisms by which artificial humic substances (A-HS) coupled with ferrihydrite influence this process remain insufficiently explored. This study is to investigate how the A-HS, specifically artificial fulvic acid (A-FA) and artificial humic acid (A-HA), coupled with ferrihydrite, affect carbon mineralization under anaerobic system that simulates paddy flooding conditions. The object is to investigate trends in carbon emissions and to delineate microbial community structure and functional pathways. The findings indicate that A-HA and A-FA substantially reduce CO2 and CH4 emissions, with A-FA having a particularly pronounced effect on carbon fixation, halving CO2 concentrations. The low concentration of Fe(II) observed suggest that A-FA and A-HA impede the dissimilatory iron reduction (DIR) process. Detailed 16S rDNA sequencing and gene prediction analyses reveal changes in microbial community structures and functions, highlighting Methanobacterium as the dominant hydrogenotrophic methanogens. The reductive citric acid cycle, predominantly utilized by Clostridium carboxidivorans, was identified as the principal carbon fixation pathway. This work provides a novel insight into the microbial mechanisms of carbon sequestration and highlights the potential of A-HS in improving soil fertility and contributing to climate change mitigation through enhancing soil carbon storage.

Combined metabolomic and microbial community analyses reveal that biochar and organic manure alter soil C-N metabolism and greenhouse gas emissions

The use of biochar to reduce the gas emissions from paddy soils is a promising approach. However, the manner in which biochar and soil microbial communities interact to affect CO2, CH4, and N2O emissions is not clearly understood, particularly when compared with other amendments. In this study, high-throughput sequencing, soil metabolomics, and quantitative real-time PCR were utilized to compare the effects of biochar (BC) and organic manure (OM) on soil microbial community structure, metabolomic profiles and functional genes, and ultimately CO2, CH4, and N2O emissions. Results indicated that BC and OM had opposite effects on soil CO2 and N2O emissions, with BC resulting in lower emissions and OM resulting in higher emissions, whereas BC, OM, and their combined amendments increased cumulative CH4 emissions by 19.5 %, 31.6 %, and 49.1 %, respectively. BC amendment increased the abundance of methanogens (Methanobacterium and Methanocella) and denitrifying bacteria (Anaerolinea and Gemmatimonas), resulting in an increase in the abundance of mcrA, amoA, amoB, and nosZ genes and the secretion of a flavonoid (chrysosplenetin), which caused the generation of CH4 and the reduction of N2O to N2, thereby accelerating CH4 emissions while reducing N2O emissions. Simultaneously, OM amendment increased the abundance of the methanogen Caldicoprobacter and denitrifying Acinetobacter, resulting in increased abundance of mcrA, amoA, amoB, nirK, and nirS genes and the catabolism of carbohydrates [maltotriose, D-(+)-melezitose, D-(+)-cellobiose, and maltotetraose], thereby enhancing CH4 and N2O emissions. Moreover, puerarin produced by Bacillus metabolism may contribute to the reduction in CO2 emissions by BC amendment, but increase in CO2 emissions by OM amendment. These findings reveal how BC and OM affect greenhouse gas emissions by modulating soil microbial communities, functional genes, and metabolomic profiles.

Effective remediation of agricultural drainage at three influent strengths by bioaugmented constructed wetlands filled with mixture of iron‑carbon and organic solid substrates: Performance and mechanisms

Agricultural drainage containing a large quantity of nutrients can cause quality deterioration and algal blooming of receiving water bodies, thus needs to be effectively remediated. In this study, iron‑carbon (FeC) composite-filled constructed wetlands (Fe-C-CWs) were employed to treat farmland drainage at three pollution levels, and organic solid substrates (walnut shells) and phosphate-accumulating denitrifying bacteria (Pseudomonas sp. DWP1) were supplemented to enhance the treatment performance. The results showed that the Fe-C-CWs exhibited notably superior removal efficiency for total nitrogen (TN, 52.0-58.2 %), total phosphorus (TP, 67.8-70.2 %) and chemical oxygen demand (COD, 56.7-70.4 %) than the control systems filled solely with gravel (28.5-32.5 % for TN, 33.2-40.5 % for TP and 30.2-55.0 % for COD) at all influent strengths, through driving autotrophic denitrification, Fe-based dephosphorization, and organic degradation processes. The addition of organic substrates and functional bacteria markedly enhanced pollutant removal in the Fe-C-CWs. Furthermore, use of FeC and organic substrates and denitrifier inoculation decreased CO2 and CH4 emissions from the CWs, and reduced global warming potential of the CWs at low influent strength. Pollutant removal efficiencies in the CWs were only marginally impacted by the increasing influent loads except for NO3--N, and pollutant removal mass was largely increased with the increase of influent strengths. The microbial community in the FeC composite-filled CWs exhibited distinct distribution patterns compared to the gravel-filled CWs regardless of the influent strengths, with obviously higher proportions of dominant genera Trichococcus, Geobacter and Ferritrophicum. Keystone taxa associated with pollutant removal in the Fe-C-filled CWs were identified to be Pseudomonas, Geobacter, Ferritrophicum, Denitratisoma and Sediminibacterium. The developed augmented Fe-C-filled CWs show great promises for remediating agricultural drainage with varied pollutant loads.

Plant-microbe involvement: How manganese achieves harmonious nitrogen-removal and carbon-reduction in constructed wetlands

Carbon deficits in inflow frequently lead to inefficient nitrogen removal in constructed wetlands (CWs) treating tailwater. Solid carbon sources, commonly employed to enhance denitrification in CWs, increase carbon emissions. In this study, MnO2 was incorporated into polycaprolactone substrates within CWs, significantly enhancing NH4+-N and NO3--N removal efficiencies by 48.26-59.78 % and 96.84-137.23 %, respectively. These improvements were attributed to enriched nitrogen-removal-related enzymes and increased plant absorption. Under high nitrogen loads (9.55 ± 0.34 g/m3/d), emissions of greenhouse gases (CO2, CH4, and N2O) decreased by 147.23-202.51 %, 14.53-86.76 %, and 63.36-87.36 %, respectively. N2O emissions were reduced through bolstered microbial nitrogen removal pathways by polycaprolactone and MnO2. CH4 accumulation was mitigated by the increased methanotrophs and dampened methanogenesis, modulated by manganese. Additionally, manganese-induced increases in photosynthetic pigment contents (21.28-64.65 %) fostered CO2 sequestration through plant photosynthesis. This research provides innovative perspectives on enhancing nitrogen removal and reducing greenhouse gas emissions in constructed wetlands with polymeric substrates.

CH4 control and nitrogen removal from constructed wetlands by plant combination

Global warming trend is accelerating. This study proposes a green and economical methane (CH4) control strategy by plant combination in constructed wetlands (CWs). In this study, a single planting of Acorus calamus L. hybrid constructed wetland (HCW-A) and a mixed planting of Acorus calamus L. and Eichhornia crassipes (Mart.) Solms hybrid constructed wetland (HCW-EA) were constructed. The differences in nitrogen removal performance and CH4 emissions between HCW-A and HCW-EA were compared and analyzed. The findings indicated that HCW-EA demonstrated significant improvements over HCW-A, with NH4+-N and TN removal rates increasing by 21.61% and 16.38% respectively, and CH4 emissions decreased by 43.36%. The microbiological analysis results showed that plant combination promoted the enrichment of Proteobacteria, Alphaproteobacteria and Bacillus. More nitrifying bacteria carrying nxrA genes and denitrifying bacteria carrying nirK genes accelerated the nitrogen transformation process. In addition, the absolute abundance ratio of pmoA/mcrA increased, reducing the release of CH4.

Agricultural runoff treatment by constructed wetlands filled with iron-carbon composites in winter: Performance augmentation by organic solids and denitrifying bacteria addition

Iron-carbon composite-filled constructed wetlands (Fe-C CWs) were employed to treat agricultural runoff in the winter season in this study, and organic substrates and phosphate-accumulating denitrifying bacteria were supplemented to improve the treatment performance. Fe-C CWs performed significantly better in pollutant removal than the control system filled with only gravel by effectively driving autotrophic denitrification, Fe-based dephosphorization and organic degradation. Organic substrate and functional bacteria addition further augmented the performance, and immobilized bacterial cells were more effective than free cells. Fe-C and organic substrates decreased the greenhouse gas emission fluxes of the CWs, and denitrifier inoculation alleviated N2O emission. The microbial community in the Fe-C substrates showed a very distinct distribution pattern compared to that in the gravel, with notably higher proportions of Trichococcus, Thauera and Dechloromonas. Bioaugmented Fe-C-based CWs are highly promising for agricultural runoff treatment, especially at low temperatures.

Biochar and hematite amendments suppress emission of CH4 and NO2 in constructed wetlands

Constructed wetlands (CWs) are considered a widely used cost-effective technology for pollutant removal. However, greenhouse gas emissions are a non-negligible problem in CWs. In this study, four laboratory-scale CWs were established to evaluate the effects of gravel (CW_B), hematite (CW_Fe), biochar (CW_C), and hematite + biochar (CW_Fe-C) as substrates on pollutants removal, greenhouse gas emissions, and associated microbial characteristics. The results showed that the biochar-amended CWs (CW_C and CW_Fe-C) enhanced the removal efficiency of pollutants, with 92.53 % and 93.66 % of COD and 65.73 % and 64.41 % of TN removal, respectively. Both single and combined inputs of biochar and hematite significantly reduced CH₄ and N₂O fluxes, with the lowest average of CH₄ flux obtained in CW_C (5.99 ± 0.78 mg CH₄ m^-2 h^-1) and the least N₂O flux in CW_Fe-C (287.57 ± 44.84 μg N₂O m^-2 h^-1). The substantial reduction of global warming potentials (GWP) was obtained in the applications of CW_C (80.25 %) and CW_Fe-C (79.5 %) in biochar-amended CWs. The presence of biochar and hematite mitigated CH₄ and N₂O emissions by modifying microbial communities with higher ratios of pmoA/mcrA and nosZ genes abundances, as well as increasing the abundance of denitrifying bacteria (Dechloromona, Thauera and Azospira). This study demonstrated that biochar and the combined use of biochar and hematite could be the potential candidates as functional substrates for the efficient removal of pollutants and simultaneously reducing GWP emissions in the constructed wetlands.

Factors Influencing Gaseous Emissions in Constructed Wetlands: A Meta-Analysis and Systematic Review

Constructed wetlands (CWs) are an eco-technology for wastewater treatment and are applied worldwide. Due to the regular influx of pollutants, CWs can release considerable quantities of greenhouse gases (GHGs), ammonia (NH₃), and other atmospheric pollutants, such as volatile organic compounds (VOCs) and hydrogen sulfide (H₂S), etc., which will aggravate global warming, degrade air quality and even threaten human health. However, there is a lack of systematic understanding of factors affecting the emission of these gases in CWs. In this study, we applied meta-analysis to quantitatively review the main influencing factors of GHG emission from CWs; meanwhile, the emissions of NH₃, VOCs, and H₂S were qualitatively assessed. Meta-analysis indicates that horizontal subsurface flow (HSSF) CWs emit less CH₄ and N₂O than free water surface flow (FWS) CWs. The addition of biochar can mitigate N₂O emission compared to gravel-based CWs but has the risk of increasing CH₄ emission. Polyculture CWs stimulate CH₄ emission but pose no influence on N₂O emission compared to monoculture CWs. The influent wastewater characteristics (e.g., C/N ratio, salinity) and environmental conditions (e.g., temperature) can also impact GHG emission. The NH₃ volatilization from CWs is positively related to the influent nitrogen concentration and pH value. High plant species richness tends to reduce NH₃ volatilization and plant composition showed greater effects than species richness. Though VOCs and H₂S emissions from CWs do not always occur, it should be a concern when using CWs to treat wastewater containing hydrocarbon and acid. This study provides solid references for simultaneously achieving pollutant removal and reducing gaseous emission from CWs, which avoids the transformation of water pollution into air contamination.

Recent advances in constructed wetlands methane reduction: Mechanisms and methods

Constructed wetlands (CWs) are artificial systems that use natural processes to treat wastewater containing organic pollutants. This approach has been widely applied in both developing and developed countries worldwide, providing a cost-effective method for industrial wastewater treatment and the improvement of environmental water quality. However, due to the large organic carbon inputs, CWs is produced in varying amounts of CH4 and have the potential to become an important contributor to global climate change. Subsequently, research on the mitigation of CH4 emissions by CWs is key to achieving sustainable, low-carbon dependency wastewater treatment systems. This review evaluates the current research on CH4 emissions from CWs through bibliometric analysis, summarizing the reported mechanisms of CH4 generation, transfer and oxidation in CWs. Furthermore, the important environmental factors driving CH4 generation in CW systems are summarized, including: temperature, water table position, oxidation reduction potential, and the effects of CW characteristics such as wetland type, plant species composition, substrate type, CW-coupled microbial fuel cell, oxygen supply, available carbon source, and salinity. This review provides guidance and novel perspectives for sustainable and effective CW management, as well as for future studies on CH4 reduction in CWs.

Detection and Quantification of Candidatus Methanoperedens-Like Archaea in Freshwater Wetland Soils

Candidatus Methanoperedens-like archaea, which can use multiple electron acceptors (nitrate, iron, manganese, and sulfate) for anaerobic methane oxidation, could play an important role in reducing methane emissions from freshwater wetlands. Currently, very little is known about the distribution and community composition of Methanoperedens-like archaea in freshwater wetlands, particularly based on their alpha subunit of methyl-coenzyme M reductase (mcrA) genes. Here, the community composition, diversity, and abundance of Methanoperedens-like archaea were investigated in a freshwater wetland through high-throughput sequencing and quantitative PCR on their mcrA genes. A large number of Methanoperedens-like mcrA gene sequences (119,250) were recovered, and a total of 31 operational taxonomic units (OTUs) were generated based on 95% sequence similarity cut-off. The majority of Methanoperedens-like sequences can be grouped into three distinct clusters that were closely associated with the known Methanoperedens species which can couple anaerobic methane oxidation to nitrate or iron reduction. The community composition of Methanoperedens-like archaea differed significantly among different sampling sites, and their mcrA gene abundance was 1.49 × 10⁶ ~ 4.62 × 10⁶ copies g^-1 dry soil in the examined wetland. In addition, the community composition of Methanoperedens-like archaea was significantly affected by the soil water content, and the archaeal abundance was significantly positively correlated with the water content. Our results suggest that the mcrA gene is a good biomarker for detection and quantification of Methanoperedens-like archaea, and provide new insights into the distribution and environmental regulation of these archaea in freshwater wetlands.

Metagenomics reveals biogeochemical processes carried out by sediment microbial communities in a shallow eutrophic freshwater lake

Introduction

As the largest shallow freshwater lake in the North China Plain, Baiyangdian lake is essential for maintaining ecosystem functioning in this highly populated region. Sediments are considered to record the impacts of human activities.

Methods

The abundance, diversity and metabolic pathways of microbial communities in sediments were studied by metagenomic approach to reveal patterns and mechanism of C, N, P and S cycling under the threat of lake eutrophication.

Results

Many genera, with plural genes encoding key enzymes involved in genes, belonging to Proteobacteria and Actinobacteria which were the most main phylum in bacterial community of Baiyangdian sediment were involved in C, N, S, P cycling processes, such as Nocardioides (Actinobacteria), Thiobacillus, Nitrosomonas, Rhodoplanes and Sulfuricaulis (Proteobacteria).For instance, the abundance of Nocardioides were positively correlated to TN, EC, SOC and N/P ratio in pathways of phytase, regulation of phosphate starvation, dissimilatory sulfate reduction and oxidation, assimilatory sulfate reduction, assimilatory nitrate reduction and reductive tricarboxylic acid (rTCA) cycle. Many key genes in C, N, P, S cycling were closely related to the reductive citrate cycle. A complete while weaker sulfur cycle between SO₄ ^2- and HS^- might occur in Baiyangdian lake sediments compared to C fixation and N cycling. In addition, dissimilatory nitrate reduction to ammonia was determined to co-occur with denitrification. Methanogenesis was the main pathway of methane metabolism and the reductive citrate cycle was accounted for the highest proportion of C fixation processes. The abundance of pathways of assimilatory nitrate reduction, denitrification and dissimilatory nitrate reduction of nitrogen cycling in sediments with higher TN content was higher than those with lower TN content. Besides, Nocardioides with plural genes encoding key enzymes involved in nasAB and nirBD gene were involved in these pathways.

Discussion

Nocardioides involved in the processes of assimilatory nitrate reduction, denitrification and dissimilatory nitrate reduction of nitrogen cycling may have important effects on nitrogen transformation.

Microbial response mechanism of plants and zero valent iron in ecological floating bed: Synchronous nitrogen, phosphorus removal and greenhouse gas emission reduction

Iron-based ecological floating beds (EFBs) are often used to treat the secondary effluent from wastewater treatment plant to enhance the denitrification process. However, the impact and necessity of plants on iron-based EFBs have not been systematically studied. In this research, two iron-based EFBs with and without plants (EFB-P and EFB) were performed to investigate the response of plants on nutrient removal, GHG emissions, microbial communities and functional genes. Results showed the total nitrogen and total phosphorus removal in EFB-P was 45-79% and 48-72%, respectively, while that in EFB was 31-67% and 44-57%. Meanwhile, plants could decrease CH₄ emission flux (0-3.89 mg m^-2 d^-1) and improve CO₂ absorption (4704-22321 mg m^-2 d^-1). Plants could increase the abundance of Nitrosospira to 1.6% which was a kind of nitrifying bacteria dominant in plant rhizosphere. Among all denitrification related genera, Simplicispira (13.08%) and Novosphingobium (6.25%) accounted for the highest proportion of plant rhizosphere and iron scrap, respectively. Anammox bacteria such as Candidatus_Brocadia was more enriched on iron scraps with the highest proportion was 1.21% in EFB-P, and 2.20% in EFB. Principal co-ordinates analysis showed that plants were the critical factor determining microbial community composition. TN removal pathways were mixotrophic denitrification and anammox in EFB-P while TP removal pathways were plant uptake and phosphorus-iron coprecipitation. In general, plants play an important directly or indirectly role in iron-based EFBs systems, which could not only improve nutrients removal, but also minimize the global warming potential and alleviate the greenhouse effect to a certain extent.

Removal of organochlorine pesticides and metagenomic analysis by multi-stage constructed wetland treating landfill leachate

Constructed wetlands (CWs) can effectively treat landfill leachate (LL). However, there is limited research on the removal of organochlorine pesticides (OCPs) refractory organics during LL treatment in CWs. In this study, multi-stage subsurface flow CWs was used to treat LL, and the removal fate of hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT) in CWs was investigated. The structural differences between plant roots and substrate microbial communities were compared and the Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathway of organic matter was analyzed based on metagenomic analysis. The results showed that substrate adsorption (50.55%-72.74%) and microbial degradation (20.38%-27.89%) were the main ways to remove OCPs. The Proteobacteria occupied a dominant position in the CWs system, among which Betaproteobacteria (34.37%-35.90%) were contained in the substrate, and Alphaproteobacteria (21.19%-23.84%) was a more dominant microorganism in plant roots. Formaldehyde assimilation and serine pathway were the main pathways of methane metabolism. This study provides a reference for the removal mechanism of OCPs to promote the application of CWs technology in LL treatment.

Manganese-doped ferrihydrite/cellulose/polyvinyl alcohol composite membrane: Easily recyclable adsorbent for simultaneous removal of arsenic and cadmium from soil

Simultaneous removal of arsenic (As) and cadmium (Cd) from soil has been a matter of great concern. In this study, manganese-doped ferrihydrite/cellulose/polyvinyl alcohol composite membranes were prepared via loading manganese-doped ferrihydrite on cellulose mesh and then wrapping with polyvinyl alcohol. The obtained composite membranes exhibited excellent mechanical properties and could be easily separated from soil. The adsorption capacities of Cd(II) and As(III) on the composite membrane were 11.11 mg/g and 72.08 mg/g respectively. After mixing the composite membrane with polluted soil at dosage of 0.3% (w/w), the removal efficiency of the toxicity characteristic leaching procedure leachable As and Cd reached 65% and 69% respectively with one cycle. The stability and reusability of composite membrane were demonstrated by no decrease in tensile strength and the retention of 94% and 96% of As(III) and Cd(II) removal efficiency respectively after four adsorption cycles. The prepared composite membrane had application prospect in the remediation of As and Cd co-polluted soil.

Effects and mechanisms of constructed wetlands with different substrates on N2O emission in wastewater treatment

Nitrous oxide (N₂O) emissions from constructed wetlands (CWs) are accompanying problems and have attracted much attention in recent years. CWs filled with different substrates (gravel, biochar, zeolite, and pyrite) were constructed to investigate the nitrogen removal performance and N₂O emissions, which named C-CWs, B-CWs, Z-CWs, and P-CWs, respectively. C-CWs showed the poorest nitrogen removal performance in all CWs. Although B-CWs exhibited the highest fluxes of N₂O emissions, the percentage of N₂O emissions in nitrogen removal (0.15%) was smaller than that of C-CWs (0.18%). In addition, microbiological analysis showed that compared with C-CWs, CWs filled with biochar, zeolite, and pyrite had higher abundance of nitrifying and denitrifying microorganisms and lower abundance of N₂O producing bacteria. In conclusion, biochar, zeolite, and pyrite were more favorable kinds of substrate than the conventional substrates of gravel for the nitrogen removal and reduction of N₂O emissions from CWs.

The anaerobic oxidation of methane in paddy soil by ferric iron and nitrate, and the microbial communities involved

The anaerobic oxidation of methane (AOM) mediated by microorganisms is a key process in the reduction of methane emissions, and AOM-coupled electron acceptors have been shown to regulate methane emissions into the atmosphere in marine systems. Paddy fields are a significant source of methane and account for 20% of global methane emissions, but the effect of electron acceptors on the methane emission process in flooded paddy fields has been poorly characterized. This study aimed to determine whether the electron acceptors ferric iron and nitrate, and biochar, acting as an electron shuttle, can regulate the AOM process in paddy soil, with or without interaction between biochar and these two electron acceptors. We also aimed to characterize which microorganisms are actively involved. Here, we added ¹³C-labeled CH₄ (¹³CH₄) into anaerobic microcosms to evaluate the role of electron acceptors by measuring the methane oxidation rate and the enrichment of ¹³C-labeled CO₂ (¹³CO₂). We then combined DNA-stable isotope probing with amplicon sequencing to study the active microorganisms. We found for the first time that, in addition to nitrate, ferric iron can also effectively promote AOM in paddy soil. However, there was no significant effect of biochar. Ferric iron-dependent AOM was mainly carried out by iron-reducing bacteria (Geobacter, Ammoniphilus and Clostridium), and nitrate-dependent AOM was mainly by nitrate-reducing bacteria (Rhodanobacter, Paenibacillus and Planococcus). Our results demonstrate that the AOM process, regulated by the electron acceptors ferric iron and nitrate, can alleviate methane emission from paddy soil. The potentially active microorganisms related to electron acceptor reduction may be crucial for this methane sink and deserve further research.