Soil Redox Potential Research Articles

A Classification Approach for Corrosion Rating of Soil to Buried Water Pipelines: A Case Study in Budhanilkantha-Maharajganj Roadway Areas of Nepal

The present study was focused to estimate six characteristics of soil specimens taken from 0.3 to 1.5 m sampling depth of six sampling sites of Budhanilkantha-Maharajganj roadsides using standard methods, and was assessed their corrosive nature to the buried-metallic pipelines using an empirical corrosion rating model. The estimated soil pH, moisture, resistivity, redox potential (ORP), chloride and sulfate ions were 6.4-7.9, 7-45%, 4.5 × 103-45.5 × 103 Ohm. cm, 317-514 mV (SHE), 12-86 ppm, and 40-294 ppm, respectively, in all the soil sample specimens. The experimental results indicated that the soils could be rated as mildly corrosive to less corrosive groups to the buried galvanized-steel and cast iron pipes in the study areas. A good positive or negative correlation coefficient between resistivity, moisture, chloride and sulfate contents implies that these soil parameters have an equal contribution to the rating of soil corrosivity. A polyethylene-sheet wrapping (i.e., encasement) around the galvanized-steel and cast iron water pipelines or the use of non-conducting materials of gravel/sand around the burying ground could be sufficient for the extension of their life up to 50 years or more. The empirical model is successfully applied for the corrosion rating of soil samples and could be progressive in the future for soil corrosion rating of soils to the underground waterworks. Present findings would be insightful and suggestive in making the corrosive land maps of the studied areas which would be helpful for the potable water pipeline works in other urban areas of Nepal.

Long-Term Effects of Sheep Grazing in Various Densities on Marsh Properties and Vegetation Dynamics in Two Different Salt-Marsh Zones

For conservation management of grassland ecosystems, an important question is under which conditions large grazers induce compositional and structural variation in plant communities, which is a prerequisite for high biodiversity. Here we used two long-term projects on the mainland salt marshes of the Wadden Sea to test the hypothesis that long-term grazing management with different stocking densities results in plant communities with distinctively different plant species composition and vegetation structure. The two projects took place on a low clayey and a high sandy salt marsh with different stocking densities of sheep: 0, 1.5, 3.5, 4.5 and the initially 10 sheep ha−1, where measurements were collected 11, 15, 19 and 23 years after the start of the project. Moreover, grazers affect abiotic conditions by reducing soil-redox potential and surface elevation, thereby driving composition and structure of salt-marsh vegetation. On the low salt marsh, a continued high stocking density (10 sheep ha−1) resulted in succession from the early-successional Puccinellia maritima community to the late-successional Atriplex portulacoides community. On the high salt marsh, the early-successional Festuca rubra community was maintained under all stocking densities. Cessation of grazing resulted in succession to the Elytrigia atherica community in both salt-marsh types. Intermediate stocking densities (1.5, 3 or 4.5 sheep ha−1) resulted in a mosaic of tall vegetation and patches of lawn, i.e. short-grazed vegetation, where Puccinellia maritima lawn occurred interspersed with patches of the Festuca rubra and tall Elytrigia atherica communities in both salt-marsh types. Effects of grazers were influenced by the presence of watering points near the sea wall. To conclude, our results show how joint interactions between grazers and abiotic conditions drive vegetation diversity and heterogeneity, with implications for ecosystem functions and services such as wildlife biodiversity and coastal protection.

Temporal Responses of Microbial Communities to Anaerobic Soil Disinfestation

Anaerobic soil disinfestation (ASD) is an organic amendment-based management tool for controlling soil-borne plant diseases and is increasingly used in a variety of crops. ASD results in a marked decrease in soil redox potential and other physicochemical changes, and a turnover in the composition of the soil microbiome. Mechanisms of ASD-mediated pathogen control are not fully understood, but appear to depend on the carbon source used to initiate the process and involve a combination of biological (i.e., release of volatile organic compounds) and abiotic (i.e., lowered pH, release of metal ions) factors. In this study, we examined how the soil microbiome changes over time in response to ASD initiated with rice bran, tomato pomace, or red grape pomace as amendments using growth chamber mesocosms that replicate ASD-induced field soil redox conditions. Within 2 days, the soil microbiome rapidly shifted from a diverse assemblage of taxa to being dominated by members of the Firmicutes for all ASD treatments, whereas control mesocosms maintained diverse and more evenly distributed communities. Rice bran and tomato pomace amendments resulted in microbial communities with similar compositions and trajectories that were different from red grape pomace communities. Quantitative PCR showed nitrogenase gene abundances were higher in ASD communities and tended to increase over time, suggesting the potential for altering soil nitrogen availability. These results highlight the need for temporal and functional studies to understand how pathogen suppressive microbial communities assemble and function in ASD-treated soils.

Response of tidal marsh vegetation to pulsed increases in flooding and nitrogen

Worldwide, human activities have modified hydrology and nutrient loading regimes in coastal wetlands. Understanding the interplay between these drivers and subsequent response of wetland plant communities is essential to informing wetland management and restoration efforts. Recent restoration strategies in Louisiana proposes to use sediment diversions from the Mississippi River to build land in adjacent wetlands and reduce the rate of land to open water conversion. In conjunction with sediment delivery, diversions can increase nutrient loads and water levels in the receiving basins. We conducted a greenhouse mesocosm experiment in which we exposed three common tidal freshwater and brackish marsh plants (Panicum hemitomon, Sagittaria lancifolia, and Spartina patens) to two nitrate loading rates [high (35 g N m2 year−1) and low (0.25 g N m2 year−1)], and two flooding treatments (with and without diversion pulsing). Experimental units were set at two different elevations within the treatment tanks to simulate both a healthy and degraded marsh. Plant growth metrics and soil physicochemical properties were measured monthly. Final total biomass was determined at the study’s conclusion. Growth responses differed between species but were not significantly influenced by the treatments. Soil redox potential decreased significantly following the increase in flooding associated with the diversion pulse, but recovered to pre-diversion levels after a 3-month recovery period. Our study suggests short flooding pulses with a recovery period may be key for maintaining healthy marshes, however there remains a need for longer-term empirical studies to understand marsh response to pressures associated with river sediment diversions over time.

Arsenic and cadmium accumulation in rice and mitigation strategies

Arsenic (As) and cadmium (Cd) are two toxic elements that have a relatively high risk of transfer from paddy soil to rice grain. Rice is a major dietary source of these two elements for populations consuming rice as a staple food. Reducing their accumulation in rice grain is important for food safety and human health. We review recent progress in understanding the biogeochemical processes controlling As and Cd bioavailability in paddy soil, the mechanisms of their uptake, translocation and detoxification in rice plants, and strategies to reduce their accumulation in rice grain. Similarities and differences between the two elements are emphasized. Some knowledge gaps are also identified. The concentrations of As and Cd in rice grain vary by three orders of magnitude, depending on the bioavailability of the two elements in soil, rice genotype and growing conditions. The redox potential in paddy soil has a profound but opposite effect on As and Cd bioavailability, whereas soil pH affects Cd bioavailability more than As bioavailability. A number of key genes involved in As and Cd uptake, translocation, sequestration, and detoxification in rice have been characterized. Allelic variations of several genes underlying the variations in Cd accumulation have been identified, but more remains to be elucidated, especially for As. Two types of strategies can be used to reduce As and Cd accumulation, reducing their bioavailability in soil or their uptake and translocation in rice. Reducing the accumulation of both As and Cd in rice simultaneously remains a great challenge.

Hematite enhances the immobilization of copper, cadmium and phosphorus in soil amended with hydroxyapatite under flooded conditions

Hydroxyapatite (HA) is often applied as chemical amendment in soils contaminated with trace metals such as copper (Cu) and cadmium (Cd). Large amounts of iron oxides in red soil may interacts with phosphate released from HA and influence trace metal immobilization of HA. Here we simulated a red paddy soil with 1–5% wt iron oxides by adding hematite and evaluated the Cu and Cd availability in soil amended with HA under flooded conditions. Changes in phosphorus and iron oxide fractions were also evaluated after a 42-day flooding incubation experiment. Results showed that the addition of HA-only and hematite-only decreased soil redox potential and increased pore water pH compared to the control. HA combined with hematite could effectively decrease phosphate, Cu and Cd in soil pore water compared to HA-only. Additionally, HA combined with hematite could also increase soil pH and decrease soil CaCl2-extractable Cu and Cd. In particular, HA combined with 5% hematite was most effective in reducing soil exchangeable fractions of Cu and Cd by 53.7% and 65.6% compared to the control, respectively. The addition of HA-only increased water-soluble phosphorus, NaHCO3-extractable inorganic phosphorus, NaOH-extractable inorganic phosphorus, and HCl-extractable phosphorus. Conversely, HA combined with hematite treatments decreased NaHCO3-extractable inorganic phosphorus by 11.3–43.0% compared to HA-only. Vivianite and metal–phosphate precipitates were not observed using the Visual MINTEQ model, X-ray diffraction, and chemical analysis. The addition of hematite with or without HA increased free and crystal iron oxide fractions, while it substantially enhanced amorphous iron oxides in the soil. Thus, this study indicates that soil with high hematite content could enhance Cu and Cd immobilization while decreasing phosphorus availability in the red paddy soil amended with HA under the flooded conditions.

Soil Organic Carbon and Carbonates are Binding Phases for Simultaneously Extractable Metals in Calcareous Saltmarsh Soils.

The ability of the simultaneously extracted metals/acid volatile sulfides (∑SEM/AVS) index to ascertain environmental risk from potentially toxic elements in calcareous saltmarsh soils was tested using structural equation modeling. This technique allows the detection of both direct and indirect relationships among AVS, SEM, and other soil variables, representing results in a graphical view. The dataset included 90 soil samples from 21 different sites belonging to 6 different saltmarshes and featured a wide range of soil chemicophysical properties. Variables included in the a priori model were hydroperiod, pH, soil redox potential, labile organic carbon, carbonates, total iron, and total amount of potentially toxic elements (PTEs). The best optimized model pointed out the main soil properties that affect AVS accumulation and SEM speciation in these soils. Effect plots of AVS and SEM calculated with the partial linear mixed-effects models included in the piecewise structural equation modeling showed a significant and positive influence of pH and carbonates on AVS and a highly significant effect of carbonates and labile organic carbon on SEM. Single SEM components were also considered separately, to define the potential contributions of labile organic carbon or carbonates as alternative binding phases. Simultaneously extracted Cu, Ni, and Zn were preferentially bound to carbonates, followed by labile organic carbon, whereas Pb and Cd were easily bound to labile organic carbon. Environ Toxicol Chem 2019;38:2688-2697. © 2019 SETAC.

Paleoclimatic fingerprints of ferromanganese nodules in subtropical Chinese soils identified by synchrotron radiation-based microprobes

Ferromanganese nodules are important pedogenic indicators in soil. They are usually characterized by clearly expressed ring structures. Although these features are believed to be significant environmental fingerprints, the reasons for their formation still await interpretation. Here, we interpreted these fingerprints by the synergistic use of noninvasive synchrotron-radiation based techniques and advanced analytical methods. The results revealed that the soil nodules were formed due to the filling of the pore networks of the soil matrix that was mediated by both abiotic and biotic factors during alternative soil wetting-drying cycles. The ring structures are therefore the memories of the soil redox history. The variations in elemental compositions of the ring structures indicates the soil moisture conditions at the time of formation. Their porosity reflects the rate of change in soil redox potential and the soil redox potential is mainly affected by precipitation.

Biouptake Responses of Trace Metals to Long-Term Irrigation with Diverse Wastewater in the Wheat Rhizosphere Microenvironment.

Wastewater irrigation is widely practiced and may cause serious environmental problems. However, current knowledge on the effects of long-term irrigation with wastewater from different sources on the biouptake of trace metals (TMs) in the rhizosphere zone by plants in farmlands is limited. Here, we analyzed wheat rhizosphere soil and wheat roots collected from a typical wastewater irrigation area in North China to evaluate the influence of wastewater irrigation from different sources on the bioavailability of trace metals in soils. Results showed that irrigation with tanning and domestic wastewater helped enhance the bioavailability of trace metals in rhizosphere soil by increasing the active organic carbon content, soil redox potential, and catalase activity, thus enhancing the proportion of the potentially bioavailable part of trace metal speciation. Conversely, irrigation with pharmaceutical wastewater can reduce the bioavailability of trace metals in rhizosphere soil by increasing total soil antibiotics and thus decreasing the proportions of bioavailable and potentially bioavailable parts of trace metal speciation. These findings can provide insights into the migration and transformation of trace metal speciation in soil rhizosphere microenvironments under the context of wastewater irrigation.

Закономірності зміни висоти рослин та формування урожайності ячменю ярого під впливом мінеральних добрив і позакореневих підживлень у Західному Лісостепу

Закономірності зміни висоти рослин та формування урожайності ячменю ярого під впливом мінеральних добрив і позакореневих підживлень у Західному Лісостепу

Interpreting redox potential (Eh) and diffusive fluxes of phosphorus (P) and nitrate (NO3−) from commercial rice grown on histosols

In South Florida, approximately 11,000 ha of rice is grown every summer on highly organic histosols. With no added fertilizer inputs of phosphorus (P) or nitrogen (N) as part of the cultivation program, growing flooded rice has the potential to reduce nutrient fluxes from soils to surface water and limit its losses to the water column. Changes in soil redox potential (Eh) will play an important role in the availability of P and nitrate (NO3−) concentration within the groundwater, which ultimately drives the direction of diffusive fluxes across the soil–water interface. The objective of this study was to measure vertical Eh and groundwater concentrations of P and NO3− from commercial flooded rice fields and estimate diffusive fluxes across the soil–water interface. Results indicate that Eh was depth dependent, where deeper soils (30 cm deep) were slightly more anaerobic (mean − 162 mV) than shallow soils (15 cm deep) (mean − 144 mV). Groundwater P concentrations ranged between 0.38 and 1.09 mg L−1 in deep soils (up to 60 cm) and 0.018–0.608 mg L−1 in shallow soils (up to 40 cm). Groundwater NO3− concentrations ranged between 0.065 and 0.64 mg L−1 in deep soils (up to 60 cm) and 0.055–0.499 mg L−1 in shallow soils (up to 40 cm). Diffusive fluxes were mostly positive, which meant they flowed from shallow groundwater to the water column. Phosphorus fluxes were as high as 0.134 mg m−2 d−1 observed in 60-cm deep soils, whereas NO3− fluxes were as high as 0.12 mg m−2 d−1 observed in 40-cm shallow soils. Diffusive fluxes of P and NO3− from flooded rice fields were relatively lower compared to other hydrologic systems.

High soil redox potential contributes to iron deficiency in drip-irrigated rice grown in calcareous Fluvisol

Drip-irrigated rice (Oryza sativa L.) is susceptible to iron (Fe) deficiency. The major possible cause of Fe deficiency is the changes in the water regime, which mainly affects the redox potential (E<sub>h</sub>) of the soil dictating the solubility of Fe. However, how high soil E<sub>h</sub> affects soil available Fe and rice Fe uptake is unclear. In this paper, we investigated the effect of soil E<sub>h</sub> on rice Fe uptake under different water management strategies (drip irrigation (DI), flood irrigation (FI) and forced aeration of soil in flooding irrigation (FIO)). The results showed that the diethylenetriaminepentaacetic acid (DTPA)-extractable Fe and Fe<sup>(II)</sup> concentration in the soil, Fe concentration and chlorophyll contents of leaves and biomass of rice in FIO were greater than those in DI but significantly less than those in FI. The Fe uptake of the plant in DI was the lowest, but which in FI was the highest. Overall, FIO resulted in a significant reduction in Fe uptake of rice, but greater than that in DI. We concluded that both the decreased soil water content and the increased soil E<sub>h</sub> were important factors that caused Fe deficiency of drip-irrigated rice.

Kinetic characteristics and predictive models of methylmercury production in paddy soils

Understanding the mercury (Hg) methylation process is important for the management of paddy soils contaminated by Hg. In this work, samples of eighteen paddy soils with varying soil properties were spiked with inorganic Hg and subjected to a 90 d flooding period. Soil pH and redox potential (Eh) were measured in situ at intervals, and soils were sampled for the analysis of methylmercury (MeHg). The Hg methylation efficiency increased with flooding time and reached a relatively steady state at 30 d of incubation, ranging from 0.08% to 2.52%, and was significantly correlated with the in situ soil pH and Eh. The Elovich equation could adequately describe the kinetic production of MeHg. MeHg production was well predicted by the in situ soil pH and Eh of flooded soils, in addition to the organic matter content of air-dried soil samples and flooding time. The two predictive models explained 78% and 68% of the variability of the Hg methylation efficiency. The results suggested that the methylation of inorganic Hg in paddy soils after flooding can be predicted as a function of routinely measured soil properties and flooding time, a correlation that can be utilized to improve understanding of the extent of Hg methylation and the management of Hg-contaminated paddy soils.

Sulfate-reducing bacteria and methanogens are involved in arsenic methylation and demethylation in paddy soils

Microbial arsenic (As) methylation and demethylation are important components of the As biogeochemical cycle. Arsenic methylation is enhanced under flooded conditions in paddy soils, producing mainly phytotoxic dimethylarsenate (DMAs) that can cause rice straighthead disease, a physiological disorder occurring widely in some rice growing regions. The key microbial groups responsible for As methylation and demethylation in paddy soils are unknown. Three paddy soils were incubated under flooded conditions. DMAs initially accumulated in the soil porewater, followed by a rapid disappearance coinciding with the production of methane. The soil from a rice straighthead disease paddy field produced a much larger amount of DMAs than the other two soils. Using metabolic inhibition, quantification of functional gene transcripts, microbial enrichment cultures and 13C-labeled DMAs, we show that sulfate-reducing bacteria (SRB) and methanogenic archaea are involved in As methylation and demethylation, respectively, controlling the dynamics of DMAs in paddy soils. We present a model of As biogeochemical cycle in paddy soils, linking the dynamics of changing soil redox potential with arsenite mobilization, arsenite methylation and subsequent demethylation driven by different microbial groups. The model provides a basis for controlling DMAs accumulation and incidence of straighthead disease in rice.

日本海側積雪寒冷地の稲わら施用水田におけるメタン発生量の年次変動と中干し期間の延長によるメタン発生量の低減効果 第1報 同一試験地における17年間21事例のメタン発生量の年次変動に及ぼす気温，土壌Ehの影響

日本海側積雪寒冷地の稲わら施用水田におけるメタン発生量の年次変動と中干し期間の延長によるメタン発生量の低減効果 第1報 同一試験地における17年間21事例のメタン発生量の年次変動に及ぼす気温，土壌Ehの影響

The effect of the ferrihydrite dissolution/transformation process on mobility of arsenic in soils: Investigated by coupling a two-step sequential extraction with the diffusive gradient in the thin films (DGT) technique

Ferrihydrite has been prevalently used in soil remediation as an effective amendment for in situ immobilization of arsenic (As). However, its poorly crystalline structure is unstable and can pose the risk of re-releasing As into soil. In this study, sequential extraction was coupled with a newly-established method, chelex-metsorb diffusive gradient in thin films (DGT), to study the ferrihydrite transformation/dissolution process and its effect on the mobility of arsenic in soil. Experiments in this work found that high soil moisture (70% SWHC) with a low soil redox potential (Eh) can significantly increase the rate of the ferrihydrite transformation/dissolution process compared to 30% SWHC. Soils with low pH and high available iron (Fe) content may also accelerate ferrihydrite transformation/dissolution, while soils with high clay fraction and high soil total organic matter (STOM) may inhibit the process. The amount of arsenic adsorption can also affect ferrihydrite transformation, even exceeding the effect of soil pH and dissolved Fe. Arsenic release was clearly observed in all three soils across all treatments, and it was also affected by changes in soil redox potential. More arsenic was released at high soil moisture (70% SWHC) roughly 7–15 d after the release of Fe. In addition, a partial arsenic fraction was transformed, along with ferrihydrite, from a combined As (F1As) amorphous phase to combined As (F2As) well-crystallized phase. These results suggested that ferrihydrite transformation/dissolution can affect the mobility of arsenic and that this phenomenon is more extreme at higher soil moisture levels.

Effects of soil redox potential (Eh) and pH on growth of sunflower and wheat

ABSTRACTSoil redox potential (Eh) and pH are fundamentals parameters for plants growth. Measuring soil Eh is essential but complex due to the lack of measurement reliability resulting from high temporal variability and metrological challenges. This paper proposes practical advancements for measuring Eh in aerobic soils using combined electrodes (improvements in methodology specific to cleaning electrodes, measurement time, cleaning the dataset). The study of soil Eh and pH on sunflower and wheat in pot experiments has highlighted the relationship between Ehcumul (Eh associated with a dimension of time) and the portion of porosity that is accessible to air. For reduced soil conditions, sunflower positively reacts to better aeration. Strong correlations exist between the duration of each potential range and the growth of sunflower. The study of sunflower growth in soil reveals extremely harmful impact resulting from too high and/or too low Eh values.

Estimating Greenhouse Gas Emissions from Irrigated Paddy Fields in Indonesia under Various Water Managements

Methane (CH4) and nitrous oxide (N2O) are two main greenhouse gasses emitted from paddy irrigated paddy fields. Their fluxes are commonly affected by water managements in the fields. However, the main problem in the study of greenhouse gas emissions in paddy fields is the instrumentation for measuring emissions. Measurements of greenhouse gas emissions are costly and complicated. The current study proposes estimating method to quantify greenhouse gas emissions by an artificial neural network (ANN) model. They are estimated based on easily measurable parameters such as soil moisture, soil temperature, soil electrical conductivity (EC), soil redox potential (Eh) and soil pH. The model was verified based on field experiments that were conducted in Bogor, West Java, Indonesia during 26 March – 24 June 2015. Here, three regimes of water management, i.e. continuous flooded (FL), moderate (MR) and dry (DR) regimes, were performed in the field. The DR regime released the lowest total greenhouse gas emissions; however, it reduced grain yield by 58% and 12% compared to the FL and MR regimes respectively. The developed model showed high accuracies for both greenhouse gasses estimation where the coefficients of determination (R2) values were 0.84 and 0.76 for CH4 and N2O prediction respectively.

Effects of Integrated Rice-Frog Farming on Paddy Field Greenhouse Gas Emissions.

Integrated rice-frog farming (IRFF), as a mode of ecological farming, is fundamental in realizing sustainable development in agriculture. Yet its production of greenhouse gas (GHG) emissions remains unclear. Here, a randomized plot field experiment was performed to study the GHG emissions for various farming systems during the rice growing season. The farming systems included: conventional farming (CF), green integrated rice-frog farming (GIRF), and organic integrated rice-frog farming (OIRF). Results indicate that the cumulative methane (CH4) emissions from the whole growth period were divergent for the three farming systems, with OIRF having the highest value and CF having the lowest. For nitrous oxide (N2O) emissions, the order is reversed. IRFF significantly increased the dissolved oxygen (DO), soil redox potential (Eh), total organic carbon (TOC) content, and soil C:N ratio, which is closely related to GHG emissions in rice fields. Additionally, the average emissions of carbon dioxide (CO2) from soils during rice growing seasons ranged from 2312.27 to 2589.62 kg ha−1 and showed no significant difference in the three treatments. Rice yield in the GIRF and OIRF were lower (2.0% and 16.7%) than the control. The CH4 emissions contributed to 83.0–96.8% of global warming potential (GWP). Compared to CF, the treatment of GIRF and OIRF increased the GWP by 41.3% and 98.2% during the whole growing period of rice, respectively. IRFF significantly increased greenhouse gas intensity (GHGI, 0.79 kg CO2-eq ha−1 grain yield), by 91.1% over the control. Compared to the OIRF, GIRF decreased the GHGI by approximately 39.4% (0.59 kg CO2-eq ha−1 grain yield), which was 44.2% higher than that of the control. The results of structural equation model showed that the contribution of fertilization to CH4 emissions in paddy fields was much greater than that of frog activity. Moreover, frog activity could decrease GWP by reducing CH4 emissions from rice fields. And while GIRF showed a slight increase in GHG emissions, it could still be considered as a good strategy for providing an environmentally-friendly option in maintaining crop yield in paddy fields.

Methane emissions responding to Azolla inoculation combined with midseason aeration and N fertilization in a double-rice cropping system.

Methane (CH4) is an important greenhouse gas (GHG), and paddy fields are major sources of CH4 emissions. This pot experiment was conducted to investigate the integrated effects of Azolla inoculation combined with water management and N fertilization on CH4 emissions in a double-rice cropping system of Southern China. Results indicated that midseason aeration reduced total CH4 emissions by 46.9%, 38.6%, and 42.4%, followed by N fertilization with 32.5%, 17.0%, and 29.5% and Azolla inoculation with 32.5%, 17.0%, and 29.5%, on average, during the early, late, and annual rice growing seasons, respectively. The CH4 flux peaks and total CH4 emissions observed in the late rice growing season were significantly higher than those in the early rice growing season. Additionally, CH4 fluxes correlated negatively to soil redox potential (Eh) and dissolved oxygen (DO) concentration. Azolla inoculation and N fertilization greatly increased the rice grain yields, whereas midseason aeration had distinct effects on grain yields in both rice seasons. The highest annual rice grain yields of approximately 110gpot-1 were obtained in the Azolla inoculation and N fertilization treatments. In terms of yield-scaled CH4 emission, Azolla inoculation combined with midseason aeration and N fertilization generated the lowest yield-scaled CH4 emissions both in the early and in the late rice growing seasons, as well as during the annual rice cycle. In contrast, the highest yield-scaled CH4 emission was obtained in the treatment employed continuous flooding, without Azolla and no N application. Our results demonstrated that Azolla inoculation, midseason aeration, and N fertilization practices mitigated total CH4 emissions by 18.5-42.4% during the annual rice cycle. We recommend that the combination of Azolla inoculation, midseason aeration, and appropriate N fertilization can achieve lower CH4 emissions and yield-scaled CH4 emissions in the double-rice growing system.