Mean CH4 Research Articles

Active and inactive oil and gas sites contribute to methane emissions in western Saskatchewan, Canada

The oil and gas industry is Canada’s largest contributor to national methane (CH4) emissions. To quantify the input of active and inactive (suspended and abandoned) oil and gas infrastructure to regional CH4 budgets, we conducted truck-based measurements (transect-based and OTM 33A) with a greenhouse gas analyzer, complimented with optical gas imaging at oil-producing sites of Saskatchewan, including understudied regions. We found that inactive sites regionally accounted for roughly 43% of total measured CH4 emissions in Lloydminster, 9% in Kindersley, and 0% in Swift Current. Thus, CH4 emissions from oil production in southwestern Saskatchewan are underestimated by almost 25% if emissions from inactive sites are ignored. Measured mean CH4 emissions of actively producing oil and gas infrastructure in Lloydminster were at least 50% lower (36 ± 7 m3/day) than found in previous studies potentially due to declines in production schemes, effective implementation of emission reduction approaches, or spatial differences between sampled sites. Unlike previous studies, measured emissions in Lloydminster were lower than reported values (147 ± 10 m3/day). In contrast, measured emissions in Kindersley (64 ± 17 m3/day) and Swift Current (23 ± 16 m3/day) were close to reported emissions despite observed tank vents and unlit flares. Unlit flares emitted at least 3 times more CH4 than other infrastructure types and were the “super-emitters” in this study. Currently, provincial and federal regulations target only active infrastructure, but regulators may consider extending regulations to inactive sites where data suggest significant emission reduction potential.

Sensing and Analysis of Greenhouse Gas Emissions from Rice Fields to the Near Field Atmosphere.

Greenhouse gas (GHG) emissions from rice fields have huge effects on climate change. Low-cost systems and management practices to quantify and reduce GHGs emission rates are needed to achieve a better climate. The typical GHGs estimation processes are expensive and mainly depend on high-cost laboratory equipment. This study introduces a low-cost sensor-based GHG sampling and estimation system for rice fields. For this, a fully automatic gas chamber with a sensor-integrated gas accumulator and quantifier unit was designed and implemented to study its performance in the estimation efficiency of greenhouse gases (CH4, N2O, and CO2) from rice fields for two crop seasons. For each crop season, three paddy plots were prepared at the experimental site and then subjected to different irrigation methods (continuous flooding (CF), intermittent flooding (IF), and controlled intermittent flooding (CIF)) and fertilizer treatments to study the production and emission rates of GHGs throughout the crop growing season at regular intervals. A weather station was installed on the site to record the seasonal temperature and rainfall events. The seasonal total CH4 emission was affected by the effects of irrigation treatments. The mean CH4 emission in the CIF field was smaller than in other treatments. CH4 and N2O emission peaks were high during the vegetative and reproductive phases of rice growth, respectively. The results indicated that CIF treatment is most suitable in terms of rice productivity and higher water use efficiency. The application of nitrogen fertilizers produced some peaks in N2O emissions. On the whole, the proposed low-cost GHGs estimation system performed well during both crop seasons and it was found that the adaption of CIF treatment in rice fields could significantly reduce GHG emissions and increase rice productivity. The research results also suggested some mitigation strategies that could reduce the production of GHGs from rice fields.

Effects of applying different carbon substrates on nutrient removal and greenhouse gas emissions by constructed wetlands treating carbon-depleted hydroponic wastewater

The addition of external carbon sources is crucial for effective biological treatment of nutrient-rich but carbon-depleted hydroponic wastewater using constructed wetlands. In this study, we examined the effects of applying three types of carbon substrates, namely sucrose, hydroponic kale residues, and common reed litter, on the nutrient removal efficiency and greenhouse gas emission rate of vertical flow constructed wetlands. The addition of sucrose and common reed litter was shown to perform equally well in enhancing the removal of total nitrogen (84.9–93.5%), nitrate (98.3–99.8%) and phosphate (53.8–55.2%) as compared to the control. Moreover, the application of common reed litter led to significantly lower mean CH4 and N2O emissions than that of kale residues. These findings suggested that Phragmites reed litter, which is easily found in wetlands worldwide, could be an effective, low-cost and climate-friendly carbon substrate to be applied in constructed wetlands for hydroponic wastewater treatment.

Methane emissions from US low production oil and natural gas well sites

Eighty percent of US oil and natural gas (O&G) production sites are low production well sites, with average site-level production ≤15 barrels of oil equivalent per day and producing only 6% of the nation’s O&G output in 2019. Here, we integrate national site-level O&G production data and previously reported site-level CH4 measurement data (n = 240) and find that low production well sites are a disproportionately large source of US O&G well site CH4 emissions, emitting more than 4 (95% confidence interval: 3—6) teragrams, 50% more than the total CH4 emissions from the Permian Basin, one of the world’s largest O&G producing regions. We estimate low production well sites represent roughly half (37—75%) of all O&G well site CH4 emissions, and a production-normalized CH4 loss rate of more than 10%—a factor of 6—12 times higher than the mean CH4 loss rate of 1.5% for all O&G well sites in the US. Our work suggests that achieving significant reductions in O&G CH4 emissions will require mitigation of emissions from low production well sites.

Bovine host genome acts on rumen microbiome function linked to methane emissions

Our study provides substantial evidence that the host genome affects the comprehensive function of the microbiome in the rumen of bovines. Of 1,107/225/1,141 rumen microbial genera/metagenome assembled uncultured genomes (RUGs)/genes identified from whole metagenomics sequencing, 194/14/337 had significant host genomic effects (heritabilities ranging from 0.13 to 0.61), revealing that substantial variation of the microbiome is under host genomic control. We found 29/22/115 microbial genera/RUGs/genes host-genomically correlated (|0.59| to |0.93|) with emissions of the potent greenhouse gas methane (CH4), highlighting the strength of a common host genomic control of specific microbial processes and CH4. Only one of these microbial genes was directly involved in methanogenesis (cofG), whereas others were involved in providing substrates for archaea (e.g. bcd and pccB), important microbial interspecies communication mechanisms (ABC.PE.P), host-microbiome interaction (TSTA3) and genetic information processes (RP-L35). In our population, selection based on abundances of the 30 most informative microbial genes provided a mitigation potential of 17% of mean CH4 emissions per generation, which is higher than for selection based on measured CH4 using respiration chambers (13%), indicating the high potential of microbiome-driven breeding to cumulatively reduce CH4 emissions and mitigate climate change.

Emission of Methane from a Key Lake in the Eastern Route of the South-to-North Water Transfer Project and the Corresponding Driving Factors

Lakes play an important role in the biogeochemical cycling of dissolved organic matter (DOM) and the emission of methane (CH4). We investigated the concentration and effluxes of CH4 and then analyzed the corresponding driving factors in Lake Luoma, a key lake along the South-to-North Water Transfer Project. Our results indicated that Lake Luoma was a hotspot of CH4 emissions with an annual mean concentration and efflux of (0.12±0.09) μmol·L-1 and (21.0±18.5) mmol·(m2·d)-1, respectively. We found higher mean CH4 levels in the wet season than those in the dry season and further higher levels than those in the wet-to-dry transition season. Spatially, the CH4 efflux was higher in the northwest inflowing regions and lower in the southeast outflow regions. The variability in annual CH4 efflux was affected by a combination of water temperature and hydrological conditions. Terrestrial input of dissolved organic carbon (DOC) and chromophoric DOM (CDOM) had fueled the production of CH4 by providing necessary carbon substrates, and four PARAFAC DOM components were identified including a microbial humic-like C1, a tryptophan-like C2, a terrestrial humic-like C3, and a tyrosine-like C4. The CH4 efflux from the lake was significantly promoted by the input and accumulation of terrestrial humic-like components, and Chl-a had no correlation with CH4 efflux, suggesting that algal degradation was not directly fueling the emission of CH4. Lake Luoma had been significantly disturbed by human activities, and terrestrial input of nutrient loading (TN and TP) into the lake not only improved the productivity and trophic level of the water body but also enhanced the production and release of CH4 from the surface water. We concluded that the CH4 emissions in Lake Luoma can be influenced by the combination of environmental factors, CDOM composition, and nutrient level. Long-term observation is needed for better evaluation of the driving factors in fueling the emission of CH4 so as to effectively reduce the emissions of CH4 and other greenhouse gases by taking corresponding countermeasures.

Electrochemical characterization of a microbial electrolysis cell during the bio-electrochemical conversion of CO2 to CH4

This study aims to assess the electrochemical performance of a Microbial Electrolysis Cell while bio-electrochemically converting CO2 to CH4 in the cathode and treating synthetic wastewater in the anode. The results indicate that CO2 is converted with a rate of 0.15 mmol/(gCOD_consumed*d). The COD consumption was 75% and the average produced current 0.04 mA/m2. This corresponds to an initial power density of 180 mW/m2 and a mean CH4 production of 1.6 mL/d. The electrochemical analyses indicated that charge transfer resistance and sluggish reaction kinetics are the main sources of energy losses, accounting for > 85% of the total resistance.

Study of internal and external temperatures and their influence on covered lagoon digester performance

There is an evident gap in terms of understanding the interactions among variables that directly affect anaerobic digestion in full-scale biodigesters. The objective of this study was to assess the behavior of temperatures inside and outside the biodigesters and their relationship with organic matter removal and CH4 yield. The experiment was carried out in a pig farm located in the municipality of Teixeiras, Minas Gerais, Brazil, which has two parallel covered lagoon digesters (CLDs). According to the results, air and biogas temperatures largely varied throughout the day (24-h period), in contrast to the soil and substrate temperatures. The CLD system presented a low removal efficiency in terms of chemical oxygen demand (COD) and volatile solids (VS). The mean CH4 content in the biogas was 67%, and the mean biogas yield was 740.2 m³.d−1. When assessing the correlation matrix between internal and external temperatures, and their correlation with treatment efficiency, in terms of COD and VS removal, biogas yield presented a linear correlation with VS removal efficiency (r = 0.60). All temperatures inside and outside the CLD were intercorrelated.

Genetic parameters for repeatedly recorded enteric methane concentrations of dairy cows

Animal breeding techniques offer potential to reduce enteric emissions of ruminants to lower the environmental impact of dairy farming. The aim of this study was to estimate the heritability and repeatability of methane (CH4) concentrations, using the largest data set from long-term repeatedly recorded CH4 on cows to date, and to evaluate (1) the accuracy of breeding values for different CH4 traits, including using visits or weekly means, and (2) recording strategies (with varying numbers of records and recorded daughters per sire). The data comprised of long-term recording of CH4 and carbon dioxide (CO2), from 1,746 Holstein Friesian cows, on 14 commercial dairy farms throughout the Netherlands. Emissions were recorded in 10- to 35-s intervals, between 64 and 436 d, depending on farms. From each robot visit, CH4 and CO2 concentrations were summarized into various traits, averaged per visit and per week: mean, median, mean log, and mean CH4/CO2 ratio. Genetic parameters were estimated with animal repeatability models, using a restricted maximum likelihood procedure, and a relationship matrix based on genotypes and pedigree. The heritability was equal for mean and median CH4 per visit (0.13) but lower for logCH4 and CH4/CO2 (0.07 and 0.01, respectively). Phenotypic and genetic correlations were high (≥0.78) between the CH4 traits, apart from the genetic correlations with the CH4/CO2 trait, which were negative. To achieve a minimum reliability of 50% for the estimated breeding value of a bull, 25 records on mean CH4, measured on 10 different daughters, were sufficient. Although the heritability and repeatability were higher for weekly (0.32 and 0.68, respectively) than for visit mean CH4 (0.13 and 0.30, respectively), the reliabilities of estimated breeding values from visit or weekly means were equal; thus, we found no advantage in averaging records to weekly means for genetic evaluations.

Seasonal agricultural wetlands act as potential source of N2O and CH4 emissions

Seasonal wetlands and depressions in agricultural fields that accumulate nutrients from runoff water might act as hotspots of nitrous oxide (N2O) and methane (CH4) emissions. However, the spatiotemporal pattern of these gases from different parts of agricultural fields had not been compared. A two-year field experiment was conducted in Ontario, Canada, to test whether field locations with higher wetness act as N2O and CH4 hotspots. The objectives included, i) to classify agricultural field using topographical wetness index (TWI), ii) to determine the spatiotemporal patterns of N2O, CH4, and soil properties from TWI classes; and iii) to determine the impact of soil variables on N2O and CH4 emission prediction. The field was divided into four classes based on TWI, i) low TWI (C1; steep slopes), ii) medium TWI and slopes (C2), iii) Higher TWI (C3; low slopes), and iv) very high TWI (C4; seasonal wetlands and depressions). Each TWI class had six sampling points as replicates to collect soil and gas samples during corn (2019) and soybean (2020) seasons. TWI classes had a significant impact on N2O and CH4 emissions. Mean maximum and minimum N2O emissions were observed during corn season from C4 (153 ± 34) and C1 (69 ± 35 µg N2O-N m-2h−1), respectively. However, during soybean season, C1 had higher (70 ± 1), and C4 had lower average (26.5 ± 8 µg N2O-N m-2h−1) emissions. Higher mean CH4 emissions were observed from C4 and C1, respectively, during corn (1264 ± 778) and soybean (60 ± 44 µg CH4-C m-2h−1) seasons. Regression analysis revealed that N2O emissions were mainly controlled by nitrate, electrical conductivity, and moisture during corn and ammonium, pH, and nitrate during soybean season. Similarly, CH4 emissions were regulated by moisture and clay during corn season and ammonium and pH during soybean season. These relationships could help upscale greenhouse gas emissions at regional levels and formulate mitigation strategies.

Increased annual methane uptake driven by warmer winters in an alpine meadow

Pronounced nongrowing season warming and changes in soil freeze-thaw (F-T) cycles can dramatically alter net methane (CH4 ) exchange rates between soils and the atmosphere. However, the magnitudes and drivers of warming impacts on CH4 uptake in different stages of the F-T cycle are poorly understood in cold alpine ecosystems, which have been found to be a net sink of atmospheric CH4 . Here, we reported a year-round ecosystem daily CH4 uptake in an alpine meadow on the Qinghai-Tibetan Plateau after a 5-year warming experiment that included a control, a low-level warming treatment (+2.4℃ at 5cm soil depth), and a high-level warming treatment (+4.5℃ at 5cm soil depth). We found that warming shortened the F-T cycle under the low-level warming and soils did not freeze under the high-level warming. Although both warming treatments increased the mean CH4 uptake rate, only the high-level warming significantly increased annual CH4 uptake compared to the control. The warming-induced stimulation of CH4 uptake mainly occurred in the cold season, which was mostly during spring thaw under low-level warming and during the frozen winter under high-level warming due to a longer period with thawed soil. We also found that warming significantly stimulated daily CH4 uptake mainly by reducing near-surface soil water content in the warm season, whereas both soil water content and temperature controlled daily CH4 uptake in different ways during the autumn freeze, frozen winter, and spring thaw periods of the control. Our study revealed a strong warming effect on CH4 uptake during the entire F-T cycle in the alpine meadow, especially the unfrozen winter. Our results also suggested the important roles of soil pH, available phosphorus, and methanotroph abundance in regulating annual CH4 uptake in response to warming, which should be incorporated into biogeochemical models for accurately forecasting CH4 fluxes under future climate scenarios.

Methane Contributions of Different Components of Kandelia candel–Soil System under Nitrogen Supplementation

Kandelia candel is the most widely distributed tree species on the southeast coast of China and is also the main afforestation tree species along the coastal wetland. In recent years, inorganic nitrogen pollution has become increasingly severe, and investigating the effects of nitrogen input on methane emissions in Kandelia candel–soil systems has become significant from a global change perspective. However, the effect of nitrogen input on methane emissions in coastal wetland systems is still uncertain. The field tidal environment is complex and varied, and thus it is difficult to accurately control the amount of nitrogen in the system. Therefore, in order to accurately assess the effects of different concentrations of foreign nitrogen input on methane emission fluxes in a Kandelia candel–soil system, we use indoor tidal simulation experimental devices and design two simulation systems with and without plant planting to explore the difference of methane emission flux in this system under five nitrogen input concentrations: N0 (0 g N·m−2·a−1), N1 (5 g N·m−2·a−1), N2 (10 g N·m−2·a−1), N3 (20 g N·m−2·a−1), and N4 (30 g N·m−2·a−1). The results showed that: (1) The introduction of Kandelia candel promoted methane emissions in coastal wetland ecosystem. Under each nitrogen application concentration, the mean CH4 emission flux in the planting group was 42.98%, 65.59%, 40.87%, 58.93% and 39.23% higher than that in the non-planting group, respectively. (2) Nitrogen input significantly promoted methane emissions in both planted and non-planted environments, and the promoting effect showed as follows: N4 > N3 > N2 > N1 > N0. (3) After the introduction of Kandelia candel, the contribution of Kandelia candel and soil microorganisms to methane emissions was different under different concentrations of nitrogen addition. The contribution rate of Kandelia candel to CH4 emission flux of Kandelia candel–soil system ranged from 10.74% to 60.25%, with an average contribution rate of 37.30%. The changed soil microbes contributed 39.75% to 89.26% to the CH4 emission flux in the Kandelia candel–soil system, with an average contribution rate of 62.60%. Under N3 nitrogen application concentration, the emission flux of plant was the largest, which was significantly higher than that of the soil microbial pathway; at other concentrations, the methane emissions from the soil microbial pathway were greater than that of the plant pathway, and the contribution rate to the plant–soil system reached 60.25%. The results of this study provide an important basis for improving the estimation accuracy of carbon emissions in coastal waters and formulating policies for the restoration and protection of coastal wetlands.

Impact of land use change on soil methane fluxes and diffusivity in Pampean plains, Argentina

Upland soils are the main methane (CH4) biological sink, and may be affected by land-use change. Changes in land uses and soil management affect soil properties that control diffusion of gases, which in combination with microbial activity, determine CH4 flux (fCH4) through the soil. Net CH4 fluxes and diffusivity -estimated by the CH4 diffusion coefficient- were measured in three common land uses typical from Pampean plains, South America (natural grassland NG; Eucalyptus globulus Labill. afforestation E; and agricultural land AL: oat, soybean and red clover in successive cultivation) during two years (March 2017–March 2019). Methane fluxes in the soil-atmosphere interface were measured using the static chamber technique, and a diffusion model was applied to estimate soil CH4 diffusivity from soil porosity. We aimed to quantify the effect of land use change (both E and AL vs. NG, the reference system) on fCH4 and gas diffusivity due to changes in the soil parameters. Soils were net sinks in the three land uses, with mean CH4 flux higher in the afforestation, intermediate in the natural grassland and lower in the agricultural land (− 10.99 ± 5.85, − 8.9 ± 5.32 and − 4.58 ± 4.19 ng CH4 m−2 s−1, respectively). CH4 fluxes varied significantly through seasons and space coinciding with variations in water-filled pore space and air-filled pore space variables (ρ > 0.7 and <−0.7 respectively; p < 0.05). Land-use change metric for methane flux ΔfCH4 was − 2.1 ± 3.7 and 4.4 ± 2.5 for NG-E and NG-AL, respectively, indicating a significant increment in net CH4 uptake when the natural grassland is afforested and a decrease when it was converted to agricultural use. This change was mainly explained by changes in soil physical properties (bulk density, soil water content, WFPS and air filled porosity). In relation to this, soil CH4 diffusion coefficient followed the same pattern as fCH4 (0.024 ± 0.011; 0.015 ± 0.007 and 0.008 ± 0.007 cm2 s−1 for E, NG and AL respectively); and allowed us to recalculate mean CH4 fluxes. Theoretical and in situ measured CH4 fluxes were similar and followed the same patterns across land uses, suggesting the possibility to determine CH4 fluxes by means of simple measures of soil properties (bulk density and soil water content) and soil CH4 gradient concentration.

Seasonal and annual variations of CO2 and CH4 at Shadnagar, a semi-urban site

Carbon dioxide (CO2) and methane (CH4) are the most important greenhouse gases (GHGs) due to their significant role in anthropogenic global climate change. The spatio-temporal variations of their concentration are characterized by the terrestrial biosphere, seasonal weather patterns and anthropogenic emissions. Hence, to understand the variability in regional surface GHG fluxes, high precision GHGs measurements were initiated by the National Remote Sensing Center (NRSC) of India. We report continuous CO2 and CH4measurements during 2014 to 2017 for the first time from Shadnagar, a suburban site in India. Annual mean CO2 and CH4 concentrations are 399.56 ± 5.46 ppm and 1.929 ± 0.09 ppm, respectively, for 2017. After the strong El Niño of 2015–2016, an abnormal rise in CO2 growth rate of 5.5 ppm year−1 was observed in 2017 at the study site, compared to 3.03 ppm year−1 at Mauna Loa. Thus, the repercussion of the El Niño effect diminishes the net uptake by the terrestrial biosphere accompanied by increased soil respiration. Seasonal tracer to tracer correlation between CO2 and CH4 was also analyzed to characterize the possible source-sink relationship between the species. We compared CO2 and CH4 concentrations to simulations from an atmospheric chemistry transport model (ACTM). The seasonal phases of CH4 were well captured by the ACTM, whereas the seasonal cycle amplitude of CO2 was underestimated by about 30%.

Response of CO2 and CH4 transport to damming: A case study of Yulin River in the Three Gorges Reservoir, China

The growing number of dams in the Three Gorges Reservoir (TGR) make tributaries of TGR into spatially complex and temporally dynamic systems. To assess the influence of damming on the carbon emission in the tributary of TGR, we investigated the spatial heterogeneity of CO2, CH4, organic carbon, inorganic carbon, and evaluated the transport mechanisms of CO2 and CH4 within water column during different TGR operation periods. We found that mean CO2 and CH4 concentrations in water downstream (44.04 and 0.44 μmol L−1 for CO2 and CH4, respectively) were lower than upstream (48.36 and 1.63 μmol L−1 for CO2 and CH4, respectively) in the impoundment period of TGR, which was consistent with the spatial variations of organic carbon. In the drainage period of TGR, the mean CO2 concentration of upstream (58.71 μmol L−1) was significantly lower than that of downstream (88.92 μmol L−1). The higher CO2 concentration downstream was attributed to terrestrial input and higher microbial diversity of the water column, while the lower CO2 concentration upstream was due to the photosynthesis of phytoplankton. Furthermore, low CH4 concentrations (less than 0.1 μmol L−1) of both upstream and downstream were detected in the drainage period. Based on results of 16s rRNA sequencing, quantitative PCR, and functional prediction, it was indicated that aerobic CH4 oxidation predominantly in the bottom water layer reduced CH4 of the water column in drainage period. Our results expand the theory of CO2 and CH4 transport within the water column in complex river systems and provide theoretical references for the distribution of carbon in the dammed tributaries of TGR.

Responses of soil CH4 fluxes to nitrogen addition in two tropical montane rainforests in southern China

Atmospheric nitrogen (N) deposition is projected to increase in the next few decades, which may have a marked impact on soil-atmosphere CH 4 fluxes. However, the impacts of increased atmospheric N depositions on soil CH 4 flux in tropical rainforests are still poorly understood. From January 2015 to December 2018, a field experiment was conducted in a primary tropical montane rainforest (PTMR) and a secondary tropical montane rainforest (STMR) in southern China to quantify the impact of N additions at four levels (N0: 0 ​kg N·ha −1 ·year −1 ; N25: 25 ​kg N·ha −1 ·year −1 ; N50: 50 ​kg N·ha −1 ·year −1 ; N100: 100 ​kg N·ha −1 ·year −1 ) on soil CH 4 flux. Four years of measurements showed clear seasonal variations in CH 4 flux in all treatment plots for both forest types (PTMR and STMR), with lower rates of soil CH 4 uptake during the wet season and higher rates of soil CH 4 uptake during the dry season. Soil CH 4 uptake rates were significantly and negatively correlated with both soil temperature and soil moisture for both forest types. Annual CH 4 uptake for the N0 plots from the PTMR and STMR soils were −2.20 and −1.98 ​kg N·ha −1 ·year −1 , respectively. At the PTMR site, mean CH 4 uptake compared with the N0 treatment was reduced by 19%, 29%, and 36% for the N25, N50, and N100 treatments, respectively. At the STMR site, mean CH 4 uptake compared with the N0 treatment was reduced by 15%, 18%, and 38% for the N25, N50, and N100 treatments, respectively. High level N addition had a stronger inhibitory impact on soil CH 4 uptake than did the low level N addition. Our data suggest that soil CH 4 uptake in tropical rainforests is sensitive to N deposition. If atmospheric N deposition continues to increase in the future, the soil CH 4 sink strength of tropical rainforests may weaken further.

Model-based evaluation of methane emissions from paddy fields in East Asia

Evaluating regional budgets of methane (CH4), a potent greenhouse gas and short-lived climate forcer, is an important task for future climate management. This study estimated historical CH4 emissions from paddy fields in East Asia by using a process-based terrestrial biogeochemical model driven by climate and land-use data. To capture the range of estimation uncertainty, this study used two CH4 emission schemes, four paddy field maps, and two seasonal inundation methods for a total of 16 simulations. The mean CH4 emission rate during 2000-2015 was estimated to be 5.7 Tg CH4 yr-1, which is similar to statistical inventories and other estimates. However, the large standard deviation (± 3.2 Tg CH4 yr-1) among the simulations implies that serious estimation uncertainties remain. Three factors - CH4 emission scheme, paddy field map, and inundation seasonality - were responsible for the disparity of the estimates. Because of the lack of historical management data, the model simulation did not show a decreasing trend in the agricultural CH4 emissions. A sensitivity analysis for temperature indicated that a 1-2 °C temperature rise (typical warming in mitigation-oriented scenarios) would substantially enhance CH4 emissions. However, a sensitivity analysis for water management indicated that a lower water-table depth would largely mitigate the emission increase. Additional studies to improve agricultural datasets and models for better paddy field management are still needed.

Oxic urban rivers as a potential source of atmospheric methane

Urban rivers play a vital role in global methane (CH4) emissions. Previous studies have mainly focused on CH4 concentrations in urban rivers with a large amount of organic sediment. However, to date, the CH4 concentration in gravel-bed urban rivers with very little organic sediment has not been well documented. Here, we collected water samples from an oxic urban river (Xin'an River, China; annual mean dissolved oxygen concentration was 9.91 ± 1.99 mg L−1) with a stony riverbed containing very little organic sediment. Dissolved CH4 concentrations were measured using a membrane inlet mass spectrometer to investigate whether such rivers potentially act as an important source of atmospheric CH4 and the corresponding potential drivers. The results showed that CH4 was supersaturated at all sampling sites in the five sampling months. The mean CH4 saturation ratio (ratio of river dissolved CH4 concentration to the corresponding CH4 concentration that is in equilibrium with the atmosphere) across all sampling sites in the five sampling months was 204 ± 257, suggesting that the Xin'an River had a large CH4 emission potential. The CH4 concentration was significantly higher in the downstream river than in the upstream river (p < 0.05), which suggested that human activities along the river greatly impacted the CH4 level. Statistical analyses and incubation experiments indicated that algae can produce CH4 under oxic conditions, which may contribute to the significantly higher CH4 concentration in August 2020 (p < 0.001) when a severe algal bloom occurred. Furthermore, other factors, such as heavy rainfall events, dissolved organic carbon concentration, and water temperature, may also be vital factors affecting CH4 concentration. Our study enhances the understanding of dissolved CH4 dynamics in oxic urban rivers with very little organic sediment and further proposes feasible measures to control the CH4 concentration in urban rivers.

Estimation of methane emission from an urban wastewater treatment plant applying inverse Gaussian model.

The methane (CH4) emissions from urban sources are increasing, and they depend on the processes and technologies applied in each one. Thus, studying them individually to quantify their emissions and understand their behavior to design CH4 mitigation strategies is meaningful. Although many studies have been carried out in different cities worldwide, the complex methodologies and technologies applied are not readily available in developing countries. The main objective of this work is to apply a simple and inexpensive methodology to collect air samples in urban areas using syringes with a three-way stopcock. Considering the baseline concentration in different urban zones, the WWTP contribution to atmospheric CH4 concentration was assessed. Moreover, it was possible to estimate the CH4 emission rate from the source by applying the inverse Gaussian model. The atmospheric CH4 concentrations inside and around the WWTP varied from 2.04 to 32.78ppm. Most of the highest concentrations were found inside the WWTP; however, high concentrations were found up to 500m from its center. The values in the urban zones were between 2.06 and 3.52ppm, consistently higher in the area with the highest population density. Finally, considering the WWTP as a single source and according to the operational and atmospheric conditions during the studied period, the mean CH4 emission rate from this source was 2.08E + 04μgs-1. The proposed sampling methodology could be applied to estimate CH4 emission rates from fixed sources in areas with overlapping sources.

Meta-analysis of bioenergy recovery and anaerobic digestion in integrated systems of anaerobic digestion and microbial electrolysis cell

In current study, a meta-analysis approach was used to identify and evaluate the impact of various factors on the performance of integrated systems of anaerobic digestion with microbial electrolysis cell. In this study, related articles on the topic were systematically identified and collected according to the considered criteria, and the effect size that refers to the value of the difference between variables mean (total chemical oxygen demand (TCOD) removal rate and CH4 yield) was estimated. According to the meta-analysis, fed-batch operation mode, the range of 20< temperature ≤30 °C, metal cathodes, the range of 500< anode surface area ≤5000 cm2, HRT (hydraulic retention time) >20 days, 0.6< applied voltage ≤0.8 V, utilization of buffer solution and lack of membrane have high positive impact on the TCOD removal rate. Also, batch operation mode, the range of 30< temperature ≤40 °C, metal cathodes with non-Pt catalyst, the range of 100< anode surface area cm2 ≤500, HRT < 2 days, 0.4< applied voltage ≤ 0.6 V, utilization of buffer solution and membrane have high positive impact on methane yield. Analysis of the results showed this approach is a scientific and reliable method for identifying and estimating the best range of factors affecting AD-MEC (anaerobic digester combined with MEC) performance.