Rainfall can significantly reduce pond methane emissions by depressing ebullition.

Abstract. Shallow ponds can occur either in a clear-water state dominated by macrophytes or a turbid-water state dominated by phytoplankton, but it is unclear if and how these two alternative states affect the emission of greenhouse gases (GHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) to the atmosphere. We measured the dissolved concentration of CO2, CH4, and N2O from which the diffusive air–water fluxes were computed, in four urban ponds in the city of Brussels (Belgium): two clear-water macrophyte-dominated ponds (Silex and Tenreuken), and two turbid-water phytoplankton-dominated ponds (Leybeek and Pêcheries) on 46 occasions over 2.5 years (between June 2021 and December 2023). Ebullitive CH4 fluxes were measured with bubble traps in the four ponds during deployments in spring, summer, and autumn, totalling 48 d of measurements. Measured ancillary variables included water temperature, oxygen saturation level ( %O2), concentrations of chlorophyll-a (Chl-a), total suspended matter (TSM), soluble reactive phosphorus (SRP), nitrite (NO2-), nitrate (NO3-), and ammonium (NH4+). The turbid-water and clear-water ponds did not differ significantly in terms of diffusive emissions of CO2 and N2O. Clear-water ponds exhibited higher values of ebullitive CH4 emissions compared to turbid-water ponds, most probably in relation to the delivery of organic matter from macrophytes to sediments, but the diffusive CH4 emissions were not significantly different between clear- and turbid-water ponds. Across seasons, CH4 emissions increased with water temperature in all four ponds, with ebullitive CH4 fluxes having a stronger dependence on water temperature (Q10) than diffusive CH4 fluxes. The temperature sensitivity of ebullitive CH4 fluxes decreased with increasing water depth, implying that shallow sediments would respond more strongly to warming (e.g. heat waves). Total annual CH4 emissions (diffusive + ebullitive) in CO2 equivalents equalled those of CO2 in turbid-water ponds and exceeded those of CO2 in clear-water ponds, while N2O emissions were negligible compared to the other two GHGs. Total annual GHG emissions in CO2 equivalents from all four ponds increased from 2022 to 2023 due to higher CO2 diffusive fluxes, likely driven by higher annual precipitation in 2023 compared to 2022 (leading putatively to higher inputs for organic or inorganic carbon from run-off), possibly in response to the intense El Niño event of 2023. The findings of this work suggest that it might be necessary to account for the presence of submerged macrophytes when extrapolating ebullitive CH4 fluxes in ponds at a larger scale (regional or global) (particularly if Chl-a is used as a descriptor), although it might be less critical for the extrapolation of diffusive CH4, CO2, and N2O fluxes.

Methane emissions along biomethane and biogas supply chains are underestimated

High methane emissions from thermokarst lakes on the Tibetan Plateau are largely attributed to ebullition fluxes

Shallow aquatic systems exchange large amounts of carbon dioxide (CO2) and methane (CH4) with the atmosphere. The production and consumption of both gases is determined by the interplay between abiotic (such as oxygen availability) and biotic (such as community structure and trophic interactions) factors. Fish communities play a key role in driving carbon fluxes in benthic and pelagic habitats. Previous studies indicate that trophic interactions in the water column, as well as in the benthic zone can strongly affect aquatic CO2 and CH4 net emissions. However, the overall effect of fish on both pelagic and benthic processes remains largely unresolved, representing the main focus of our experimental study. We evaluated the effects of benthic and pelagic fish on zooplankton and macroinvertebrates; on CO2 and CH4 diffusion and ebullition, as well as on CH4 production and oxidation, using a full‐factorial aquarium experiment. We compared five treatments: absence of fish (control); permanent presence of benthivorous fish (common carps, benthic) or zooplanktivorous fish (sticklebacks, pelagic); and intermittent presence of carps or sticklebacks. We found trophic and non‐trophic effects of fish on CO2 and CH4 emissions. Intermittent presence of benthivorous fish promoted a short‐term increase in CH4 ebullition, probably due to the physical disturbance of the sediment. As CH4 ebullition was the major contributor to the total greenhouse gas (GHG) emissions, incidental bioturbation by benthivorous fish was a key factor triggering total carbon emissions from our aquariums. Trophic effects impacted GHG dynamics in different ways in the water column and the sediment. Fish predation on zooplankton led to a top‐down trophic cascade effect on methane‐oxidising bacteria. This effect was, however, not strong enough as to substantially alter CH4 diffusion rates. Top‐down trophic effects of zooplanktivorous and benthivorous fish on benthic macroinvertebrates, however, were more pronounced. Continuous fish predation reduced benthic macroinvertebrates biomass decreasing the oxygen penetration depth, which in turn strongly reduced water–atmosphere CO2 fluxes while it increased CH4 emission. Our work shows that fish can strongly impact GHG production and consumption processes as well as emission pathways, through trophic and non‐trophic effects. Furthermore, our findings suggest their impact on benthic organisms is an important factor regulating carbon (CO2 and CH4) emissions.

Streams and rivers are a well‐recognized source of methane (CH4), with high spatiotemporal variability in fluxes. However, CH4 release in form of bubbles (ebullition) is rarely included in current global CH4 emission estimates from lotic ecosystems, due to the lack of reliable models to upscale ebullition. Our study aimed to determine the importance of individual emission pathways (diffusion and ebullition) for total CH4 emissions from a lowland stream with low sediment heterogeneity and explore the relations of ebullition to environmental variables to build a stream ebullition model for this simplified system. We measured CH4 and carbon dioxide (CO2) diffusive emissions and ebullition from a temperate lowland stream in Czech Republic (Central Europe) during the ice‐free season 2021. The studied stream was a significant source of CH4 (mean 260 ± 107 mg CH4 m−2 day−1), with ebullition as a prevailing pathway of CH4 emission (mean 74 ± 7%, range 55%–85%) throughout the whole monitored period. CH4 ebullition showed a high spatiotemporal heterogeneity, with sediment temperature and water depth as the strongest predictors, followed by the interaction between flow velocity and sediment temperature. Our model explained 81% of total variance of CH4 ebullition and suggests that it is possible to model ebullitive fluxes in lowland streams with homogeneous sediments. Since CH4 was an important part of the total CO2‐equivalent emissions from the examined stream, accounting for mean (±SD) 35 ± 7.4%, and ebullition the majority of the CH4 emission, the ability to adequately model ebullition is pertinent for lowland streams.

Streams serve as open windows for carbon emissions to the atmosphere due to the frequent supersaturation of carbon dioxide (CO2) and methane (CH4) that originates from large carbon input during runoff and associated in-stream processes. Due to the high spatial and temporal variability of the underlying environmental drivers (e.g., concentrations of dissolved CO2 and CH4, turbulence, and temperature), it has remained difficult to address the importance and upscale the emissions to annual whole-system and regional values. In this study, we measured concentrations and calculated emissions of CO2 and CH4 at diel and seasonal scales at 15 stations in a 1.4 km2 stream network that drains a mixed lowland catchment consisting of agriculture (210 km2), forest (56 km2), and lakes, ponds, and wetlands (22 km2) in the upper River Odense, Denmark to evaluate environmental drivers behind the spatiotemporal variability. We used automatically venting floating chambers to calculate hourly diffusive fluxes of CO2 and CH4 and CH4 ebullition. We found: 1) highly supersaturated CO2 and CH4 concentrations (median: 175 and 0.33 µmol L−1, respectively) and high diffusive fluxes of CO2 and CH4 (median: 3,608 and 19 µmol m−2 h−1, respectively); 2) lower daytime than nighttime diffusive emissions of CO2 in spring and summer, but no diel variability of CH4; 3) higher concentrations and emissions of CH4 at higher temperatures; and 4) higher emissions of CH4 at stations located in sub-catchments with higher agricultural coverage. Ebullition of CH4 peaked at two stations with soft organic sediment and low summer flow, and their ebullition alone constituted 30% of total annual CH4 emissions from the stream network. Mean annual CO2 emissions from the hydrological network (37.15 mol CO2 m−2 y−1) exceeded CH4 emissions 100-fold (0.43 mol CH4 m−2 y−1), and their combined warming potential was 1.83 kg CO2e m−2 y−1. Overall, agricultural sub-catchments had higher CH4 emissions from streams, while lakes and ponds likely reduced downstream CH4 and CO2 emissions. Our findings demonstrate that CO2 and CH4 emissions data at high spatial and temporal resolution are essential to frame the heterogeneous stream conditions, understand gas emissions regulation, and upscale to annual values for hydrological networks and larger regions.

To determine how elevated night temperature interacts with carbon dioxide concentration ([CO2]) to affect methane (CH4) emission from rice paddy soil, we conducted a pot experiment using four controlled‐environment chambers and imposed a combination of two [CO2] levels (ambient: 380 ppm; elevated: 680 ppm) and two night temperatures (22 and 32 °C). The day temperature was maintained at 32 °C. Rice (cv. IR72) plants were grown outside until the early‐reproductive growth stage and then transferred to the chambers. After onset of the treatment, day and night CH4 fluxes were measured every week. The CH4 fluxes changed significantly with the growth stage, with the largest fluxes occurring around the heading stage in all treatments. The total CH4 emission during the treatment period was significantly increased by both elevated [CO2] (P=0.03) and elevated night temperature (P<0.01). Elevated [CO2] increased CH4 emission by 3.5% and 32.2% under high and low night temperature conditions, respectively. Elevated [CO2] increased the net dry weight of rice plants by 12.7% and 38.4% under high and low night temperature conditions, respectively. These results imply that increasing night temperature reduces the stimulatory effect of elevated [CO2] on both CH4 emission and rice growth. The CH4 emission during the day was larger than at night even under the high‐night‐temperature treatment (i.e. a constant temperature all day). This difference became larger after the heading stage. We observed significant correlations between the night respiration and daily CH4 flux (P<0.01). These results suggest that net plant photosynthesis contributes greatly to CH4 emission and that increasing night temperature reduces the stimulatory effect of elevated [CO2] on CH4 emission from rice paddy soil.

Methane (CH4) emissions were compared for an intensively and extensively managed agricultural area on peat soils in the Netherlands to evaluate the effect of reduced management on the CH4 balance. Chamber measurements (photoacoustic methods) for CH4 were performed for a period of three years in the contributing landscape elements in the research sites. Various factors influencing CH4 emissions were evaluated and temperature of water and soil was found to be the main driver in both sites. For upscaling of CH4 fluxes to landscape scale, regression models were used which were specific for each of the contributing landforms. Ditches and bordering edges were emission hotspots and emitted together between 60% and 70% of the total terrestrial CH4 emissions. Annual terrestrial CH4 fluxes were estimated to be 203 (±48%), 162 (±60%) and 146 (±60%) kg CH4 ha �1 and 157 (±63%), 180 (±54%) and 163 (±59%) kg CH4 ha �1 in the intensively managed site and extensively managed site, for 2006, 2007 and 2008 respectively. About 70% of the CH4 was emitted in the summer period. Farm based emissions caused per year an additional 257 kg CH4 ha �1 and 172 kg CH4 ha �1 for the intensively managed site and extensively managed site, respec- tively. To further evaluate the effect of agricultural activity on the CH4 balance, the annual CH4 fluxes of the two managed sites were also compared to the emissions of a natural peat site with no management and high ground water levels. By comparing the terrestrial and additional farm based emissions of the three sites, we finally concluded that transformation of intensively managed agricultural land to nature devel- opment will lead to an increase in terrestrial CH4 emission, but will not by definition lead to a significant increase in CH4 emission when farm based emissions are included.

Uncertainty in estimates of CH4 emissions from peatlands arise, in part, due to difficulties in quantifying the importance of ebullition. This is a particular concern in temperate lowland floodplain fens in which total CH4 emissions to the atmosphere (often measured as the sum of diffusive and plant‐mediated fluxes) are known to be high, but few direct measurements of CH4 ebullition fluxes have been made. Our study quantified CH4 fluxes (diffusion, plant‐mediated, and ebullition) from two temperate floodplain fens under conservation management (Norfolk, UK) over 176 days using funnels and static chambers. CH4 ebullition was a major component (>38%) of total CH4 emissions over spring and summer. Seasonal variations in quantifiable CH4 ebullition fluxes were marked, covering six orders of magnitude (5 × 10−5 to 62 mg·CH4·m−2·hr−1). This seasonal variability in CH4 ebullition fluxes arose from changes in both bubble volume flux and bubble CH4 concentration, highlighting the importance of regular measurements of the latter for accurate assessment of CH4 ebullition using funnels. Soil temperature was the primary control on CH4 ebullition fluxes. Elevated water level was also associated with increased CH4 ebullition fluxes, with a distinct increase in CH4 ebullition flux when water level rose to within 10 cm of the peat surface. In contrast, CH4 ebullition flux decreased steadily with increasing plant cover (measured as vascular green area). Ebullition was both steady and episodic in nature, and drops in air pressure during the two‐day funnel deployments were associated with higher fluxes.

Methane emissions from rice fields under continuous straw return in the middle-lower reaches of the Yangtze River

To understand methane (CH4) and nitrous oxide (N2O) emissions from permanently flooded rice paddy fields and to develop mitigation options, a field experiment was conducted in situ for two years (from late 2002 to early 2005) in three rice-based cultivation systems, which are a permanently flooded rice field cultivated with a single time and followed by a, non-rice season (PF), a rice-wheat rotation system (RW) and a rice-rapeseed rotation system (RR) in a hilly area in Southwest China. The results showed that the total CH4 emissions from PF were 646.3+/-52.1 and 215.0+/-45.4 kg CH4 hm(-2) during the rice-growing period and non-rice period, respectively. Both values were much lower than many previous reports from similar regions in Southwest China. The CH4 emissions in the rice-growing season were more intensive in PF, as compared to RW and RR. Only 33% of the total annual CH4 emission in PF occurred in the non-rice season, though the duration of this season is two times longer than the rice season. The annual mean N2O flux in PF was 4.5+/-0.6 kg N2O hm(-2) yr(-1). The N2O emission in the rice-growing season was also more intensive than in the non-rice season, with only 16% of the total annual emission occurring in the non-rice season. The amounts of N2O emission in PF were ignorable compared to the CH4 emission in terms of the global warming potential (GWP). Changing PF to RW or RR not only eliminated CH4 emissions in the non-rice season, but also substantially reduced the CH4 emission during the following rice-growing period (ca. 58%, P >RR approximate to RW. The GWP of PF is higher than that of RW and RR by a factor of 2.6 and 2.7, respectively. Of the total GWP of CH4 and N2O emissions, CH4 emission contributed to 93%, 65% and 59% in PF, RW and RR, respectively. These results suggest that changing PF to RW and RR can substantially reduce not only CH4 emission but also the total GWP of the CH4 and N2O emissions.

Ebullition was a major pathway of methane emissions from the aquaculture ponds in southeast China

Methane (CH4) emissions from aquatic systems should be coupled to CH4 production, and thus a temperature‐dependent process, yet recent evidence suggests that modeling CH4 emissions may be more complex due to the biotic and abiotic processes influencing emissions. We studied the magnitude and regulation of two CH4 pathways—ebullition and diffusion—from 10 shallow ponds and 3 lakes in Québec. Ebullitive fluxes in ponds averaged 4.6 ± 4.1 mmol CH4 m−2 d−1, contributing ∼56% to total (diffusive + ebullitive) CH4 emissions. In lakes, ebullition only occurred in waters < 3 m deep, averaging 1.1 ± 1.5 mmol CH4 m−2 d−1, and when integrated over the whole lake, contributed only 18% to 22% to total CH4 emissions. While pond CH4 fluxes were related to sediment temperature, with ebullition having a stronger dependence than diffusion (Q10, 13 vs. 10; activation energies, 168 kJ mol−1 vs. 151 kJ mol−1), the temperature dependency of CH4 fluxes from lakes was absent. Combining data from ponds and lakes shows that the temperature dependency of CH4 diffusion and ebullition is strongly modulated by system trophic status (as total phosphorus), suggesting that organic substrate limitation dampens the influence of temperature on CH4 fluxes from oligotrophic systems. Furthermore, a strong phosphorus‐temperature interaction determines the dominant emission pathway, with ebullition disproportionately enhanced. Our results suggest that aquatic CH4 ebullition is regulated by the interaction between ecosystem productivity and climate, and will constitute an increasingly important component of carbon emissions from northern aquatic systems under climate and environmental change.

New facility-level methane (CH4) emissions measurements obtained from 114 natural gas gathering facilities and 16 processing plants in 13 U.S. states were combined with facility counts obtained from state and national databases in a Monte Carlo simulation to estimate CH4 emissions from U.S. natural gas gathering and processing operations. Total annual CH4 emissions of 2421 (+245/-237) Gg were estimated for all U.S. gathering and processing operations, which represents a CH4 loss rate of 0.47% (±0.05%) when normalized by 2012 CH4 production. Over 90% of those emissions were attributed to normal operation of gathering facilities (1697 +189/-185 Gg) and processing plants (506 +55/-52 Gg), with the balance attributed to gathering pipelines and processing plant routine maintenance and upsets. The median CH4 emissions estimate for processing plants is a factor of 1.7 lower than the 2012 EPA Greenhouse Gas Inventory (GHGI) estimate, with the difference due largely to fewer reciprocating compressors, and a factor of 3.0 higher than that reported under the EPA Greenhouse Gas Reporting Program. Since gathering operations are currently embedded within the production segment of the EPA GHGI, direct comparison to our results is complicated. However, the study results suggest that CH4 emissions from gathering are substantially higher than the current EPA GHGI estimate and are equivalent to 30% of the total net CH4 emissions in the natural gas systems GHGI. Because CH4 emissions from most gathering facilities are not reported under the current rule and not all source categories are reported for processing plants, the total CH4 emissions from gathering and processing reported under the EPA GHGRP (180 Gg) represents only 14% of that tabulated in the EPA GHGI and 7% of that predicted from this study.

During four years (2006–2009), methane (CH4) emission was measured at different biomes (dry, wet grasslands, lake and lake vegetation) of mature thermokarst depression located at the most typical thermokarst terrain on the east bank of the Lena River, Central Yakutia, Russia (62°08′N, 130°30′E). To estimate total CH4 emission from the whole thermokarst depression ecosystem, CH4 emissions via plant bodies and ebullition were measured in addition to diffusive CH4 flux measurement. The lake area increased twice from 20.4 ha in 2006 to 43.3 ha in 2007 and then did not change significantly in 2008 and 2009 (46.5 and 44.4 ha, respectively). Ebullition was considered to be a minor source for CH4 emission from the lake in the studied thermokarst depression. CH4 emissions from the lake water surface and via the plant body of lake vegetation (hygrophyte and hydrophyte vegetation) were the main sources of CH4 and these increased by flooding both CH4 emission rate and area. Using spatial changes of these biomes, the annual emission of CH4 was calculated taking into account different sources of CH4. Total CH4 emission from the studied alas (63.7 ha) was 5.7, 5.2, 20.1 and 50.1 Mg carbon (C) in 2006–2009, respectively, and its difference during this period reached about 10 times. An extreme increase in CH4 emission from the lake occurred in the second year of continuous flooding (2008), which might have been caused by the decomposition of flooded organic C. So, the lake water ecosystem is the main source of CH4 in thermokarst depression controlled by the duration of flooding. Under future global climate change, thermokarst depressions in Central Yakutia have potential for lake expansion, causing significant increase in CH4 emission in the studied region.