Total CH4 Flux Research Articles

Comparison of Net Global Warming Potential and Greenhouse Gas Intensity Affected by Management Practices in Two Dryland Cropping Sites

Little is known about the effect of management practices on net global warming potential (GWP) and greenhouse gas intensity (GHGI) that account for all sources and sinks of greenhouse gas (GHG) emissions in dryland cropping systems. The objective of this study was to compare the effect of a combination of tillage, cropping system, and N fertilization on GWP and GHGI under dryland cropping systems with various soil and climatic conditions from 2008 to 2011 in western North Dakota and eastern Montana, USA. Treatments in western North Dakota with sandy loam soil and 373 mm annual precipitation were conventional till malt barley (Hordeum vulgarie L.) with 67 kg N ha-1 (CTB/N1), conventional till malt barley with 0 kg N ha-1 (CTB/N0), no-till malt barley-pea (Pisum sativum L.) with 67 kg N ha-1 (NTB-P/N1), no-till malt barley with 67 kg N ha-1 (NTB/N1), and no-till malt barley with 0 kg N ha-1 (NTB/N0). In eastern Montana with loam soil and 350 mm annual precipitation, treatments were conventional till malt barley-fallow with 80 kg N ha-1 (CTB-F/N1), conventional till malt barley-fallow with 0 kg N ha-1 (CTB-F/N0), no-till malt barley-pea with 80 kg N ha-1 (NTB-P/N1), no-till malt barley with 80 kg N ha-1 (NTB/N1), and no-till malt barley with 0 kg N ha-1 (NTB/N0). Carbon dioxide sink as soil C sequestration rate at the 0 - 10 cm depth was greater in NTB-P/N1 and NTB/N1 than the other treatments at both sites and greater in eastern Montana than western North Dakota. Carbon dioxide sources were greater with N fertilization than without and greater with conventional till than no-till. Soil total annual N2O and CH4 fluxes varied among treatments, years, and locations. Net GWP and GHGI were lower in NTB-P/N1 than the other treatments in western North Dakota and lower in NTB-P/N1 and NTB/N1 than the other treatments in eastern Montana. Net GWP across similar treatments was lower in eastern Montana than western North Dakota, but GHGI was similar. Annualized crop yield was greater in the treatments with N fertilization than without. Because of greater grain yield but lower GWP and GHGI, no-till malt barley-pea rotation with adequate N fertilization can be used as a robust management practice to mitigate net GHG emissions while sustaining dryland crop yields, regardless of soil and climatic conditions. Loam soil reduced GWP and crop yields compared with sandy loam soil.

Summer methane ebullition from a headwater catchment in Northeastern Siberia

Streams and rivers are active processors of terrestrial carbon and significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Recent studies suggest that ebullition may represent a sizable yet overlooked component of the total CH4 flux from these systems; however, there are no published CH4 ebullition estimates for streams or rivers in subarctic or arctic biomes, regions that store vast quantities of vulnerable, old organic carbon in permafrost soils. We quantified CH4 ebullition from headwater streams in a small arctic watershed in Northeastern Siberia. Ebullitive emissions were 0.64 mmol m−2 d−1, which is lower than the global average but approximately 2 times greater than the pan-arctic diffusive CH4 flux estimate reported in a recent synthesis of global freshwater CH4 emissions. The high CO2:CH4 of sediment bubbles (0.52) suggests that methane emissions may currently be constrained by resource competition between methanogens and microbes using more efficient metabolic strategies. Furthermore, the magnitude and frequency of ebullition events were greater as temperatures increased, suggesting that ebullition from streams could become a more prominent component of the regional CH4 flux in a warmer future.

Methane on Mars

In 2004, the first reports of methane (CH4) in Mars’ atmosphere, by both Mars Express and ground-based observations, stirred up excitement in the scientific community [1,2]. These detections (at 10 ppbv level) immediately raised questions regarding the origin of methane. On Mars, oxidants and UV radiation destroy atmospheric methane in approximately 300 yrs [3]. Therefore, the presence of atmospheric methane requires ongoing or recent methane emission. On Earth, methane is almost entirely of (live and fossil) biological origin, while abiotic sources, believed to be predominantly volcanic and/or hydrothermal, account for the rest (a few percent) of the total CH4 flux into the atmosphere

Dynamics of methane ebullition from a peat monolith revealed from a dynamic flux chamber system

Methane (CH4) ebullition in northern peatlands is poorly quantified in part due to its high spatiotemporal variability. In this study, a dynamic flux chamber (DFC) system was used to continuously measure CH4 fluxes from a monolith of near-surface Sphagnum peat at the laboratory scale to understand the complex behavior of CH4 ebullition. Coincident transmission ground penetrating radar measurements of gas content were also acquired at three depths within the monolith. A graphical method was developed to separate diffusion, steady ebullition, and episodic ebullition fluxes from the total CH4 flux recorded and to identify the timing and CH4 content of individual ebullition events. The results show that the application of the DFC had minimal disturbance on air-peat CH4 exchange and estimated ebullition fluxes were not sensitive to the uncertainties associated with the graphical model. Steady and episodic ebullition fluxes were estimated to be averagely 36 ± 24% and 38 ± 24% of the total fluxes over the study period, respectively. The coupling between episodic CH4 ebullition and gas content within the three layers supports the existence of a threshold gas content regulating CH4 ebullition. However, the threshold at which active ebullition commenced varied between peat layers with a larger threshold (0.14 m3 m−3) observed in the deeper layers, suggesting that the peat physical structure controls gas bubble dynamics in peat. Temperature variation (23°C to 27°C) was likely only responsible for small episodic ebullition events from the upper peat layer, while large ebullition events from the deeper layers were most likely triggered by drops in atmospheric pressure.

Five crop seasons' records of greenhouse gas fluxes from upland fields with repetitive applications of biochar and cattle manure

The application of char to agricultural land is recognized as a potential way to sequester atmospheric carbon (C) assimilated by plants in soil, thus decelerating global warming. Such a process would also be expected to improve plant growth and the physical and chemical properties of soil. However, field investigations of the effects of continuous char application have not been reported. In the present study, the effects of repetitive bamboo char application on CO2, CH4, and N2O flux from soil, soil C content, and crop yield were investigated at two upland fields over five crop seasons. Three treatments: chemical fertilizer (CF) applied plots (Control plot); cattle manure (CM) (10 t ha−1) and CF applied plot (CM plot); and bamboo char (20 t ha−1), cattle manure (10 t ha−1), and CF applied plot (Char/CM plot), were arranged in each field. After three crop seasons, the fourth treatment with char was applied without CF (Char plot) was given to one of the fields. CM and/or char were applied every crop season. Gas fluxes were measured using the static chamber method. Seasonal variations in CO2 flux and total CO2 emissions were consistently similar between the CM and Char/CM plots and between the Char and Control plots. As such, the decomposition rate of bamboo char was quite small, and the positive or negative effect of char on CM decomposition was not significant in the fields. Soil C analysis provided confirmation of this. CM application enhanced N2O emission mainly in the summer crop season. The differences in total N2O emission between the Char/CM and CM plots as well as between the Char and Control plots were insignificant in most cases. Total CH4 flux was negligibly small in all cases. Although the yield of winter crop (broccoli) in the Char/CM plots was twice observed to be higher than that in the Control and CM plots at one of the fields, in general, the char application had no effect on overall crop yield. Thus, the repeated application of bamboo char had no significant influence on greenhouse gas emissions and crop yields, but a high C accumulating function was found.

Methane and carbon dioxide emissions from inland waters in India – implications for large scale greenhouse gas balances

Inland waters were recently recognized to be important sources of methane (CH4 ) and carbon dioxide (CO2 ) to the atmosphere, and including inland water emissions in large scale greenhouse gas (GHG) budgets may potentially offset the estimated carbon sink in many areas. However, the lack of GHG flux measurements and well-defined inland water areas for extrapolation, make the magnitude of the potential offset unclear. This study presents coordinated flux measurements of CH4 and CO2 in multiple lakes, ponds, rivers, open wells, reservoirs, springs, and canals in India. All these inland water types, representative of common aquatic ecosystems in India, emitted substantial amounts of CH4 and a major fraction also emitted CO2 . The total CH4 flux (including ebullition and diffusion) from all the 45 systems ranged from 0.01 to 52.1mmolm(-2) d(-1) , with a mean of 7.8±12.7 (mean±1 SD) mmolm(-2) d(-1) . The mean surface water CH4 concentration was 3.8±14.5μm (range 0.03-92.1μm). The CO2 fluxes ranged from -28.2 to 262.4mmolm(-2) d(-1) and the mean flux was 51.9±71.1mmolm(-2) d(-1) . The mean partial pressure of CO2 was 2927±3269μatm (range: 400-11467μatm). Conservative extrapolation to whole India, considering the specific area of the different water types studied, yielded average emissions of 2.1 Tg CH4 yr(-1) and 22.0 Tg CO2 yr(-1) from India's inland waters. When expressed as CO2 equivalents, this amounts to 75 Tg CO2 equivalentsyr(-1) (53-98 Tg CO2 equivalentsyr(-1) ; ±1 SD), with CH4 contributing 71%. Hence, average inland water GHG emissions, which were not previously considered, correspond to 42% (30-55%) of the estimated land carbon sink of India. Thereby this study illustrates the importance of considering inland water GHG exchange in large scale assessments.

간척지 논 토양의 염 농도가 메탄 배출에 미치는 영향

BACKGROUND: Salt accumulation in coastal reclaimed soil can decrease plant growth and productivity, which could lead to considerable variation of methane(CH4) emission in a rice paddy. The objective of this study was to evaluate the effect of salt concentration on CH4 emission in a coastal reclaimed soil. METHODS AND RESULTS: The effect of salt concentration on CH4 emission and rice growth characteristics was studied by pot test, which packed by reclaimed paddy soils collected from Galsa, Hadong, Gyeongnam province. Electrical conductivity(EC) of each treatment was controlled by 0.98, 2.25, 5.05 and 9.48 dS/m and CH4 emission was characterized a week interval by closed chamber method during rice cultivation. The CH4 emission rate was significantly decreased with increase of salt accumulation, but total CH4 flux in EC 5.50 dS/m treatment was lower than those of EC 9.48 dS/m treatment. It seems because of higher content of water soluble SO4 2- in EC 5.50 dS/m treatment than those of EC 9.48 dS/m treatment. Rice growth and grain yield were significantly decreased with increase of salt accumulation. Soil properties, especially EC and pH were negatively correlated with CH4 flux, while rice growth characteristics like plant height and tiller number show significantly positive correlation with CH4 flux. CONCLUSION(S): Conclusively, salt accumulation significantly decreased CH4 flux in a rice paddy, which could be useful information for evaluating CH4 flux in reclaimed area in Korea.

Net Greenhouse Gas Emissions Affected by Sheep Grazing in Dryland Cropping Systems

Sheep (Ovis aries L.) grazing is an inexpensive method of weed control in dryland cropping systems, but little is known about its effect on net greenhouse gas (GHG) emissions. We evaluated the effect of sheep grazing compared with herbicide application for weed control on GHG (CO2, N2O, and CH4) emissions from May to October 2010 and 2011, net global warming potential (GWP), and greenhouse gas intensity (GHGI) in a silt loam under dryland cropping systems in western Montana. Treatments were two fallow management practices (sheep grazing [GRAZ] and herbicide application [CHEM]) and three cropping sequences (continuous alfalfa [Medicago sativa L.] [CA], continuous spring wheat [Triticum aestivum L.] [CSW], and spring wheat–pea [Pisum sativum L.]/barley [Hordeum vulgaris L.] hay–fallow [W‐P/B‐F]). Gas fluxes were measured at 3‐ to 14‐d intervals with a vented, static chamber. Regardless of treatments, GHG fluxes peaked immediately following substantial precipitation (>12 mm) and N fertilization mostly from May to August. Total CO2 flux from May to October was greater under GRAZ with CA, but total N2O flux was greater under CHEM and GRAZ with CSW than other treatments. Total CH4 flux was greater with CA than W‐P/B‐F. Net GWP and GHGI were greater under GRAZ with W‐P/B‐F than most other treatments. Greater CH4 flux due to increased enteric fermentation as a result of longer duration of grazing during fallow, followed by reduced crop residue returned to the soil and/or C sequestration rate probably increased net GHG flux under GRAZ with W‐P/B‐F. Sheep grazing on a cropping sequence containing fallow may not reduce net GHG emissions compared with herbicide application for weed control on continuous crops.

Effect of rice cultivar on CH4 emissions and productivity in Korean paddy soil

Cultivar selection management is important for increasing rice (Oryza sativa L.) productivity and regulating methane (CH4) emissions in paddy soil. However, the underlying causes of the differences in CH4 emission among the cultivars have not been thoroughly evaluated. The main objective of this study was to assess the effect of eight Japonica cultivars on CH4 emission and productivity in typical mono-rice paddy soil classified as fine-silty, mixed, nonacid, mesic Typic Endoaquepts in the southern Korean peninsula in 2010–2011. We hypothesised that the CH4 flux may differ significantly among rice cultivars because each rice cultivar has different physiological and anatomical characteristics and may differently affect the microbial abundance involved in the CH4 dynamics in paddy soils. In particular, the selection of an early-maturing cultivar with fewer flooding days (97 flooding days of 111 cultivation days) over a late-maturing cultivar (121 flooding days of 135 cultivation days) could be a good countermeasure to decrease CH4 emissions from paddy fields. In our results, the rice grain yield differed significantly among the cultivars but not between the late- and early-maturing groups. The mean CH4 emission rates (0.15–0.37gm−2day−1) and total CH4 fluxes (20.0–50.0gm−2) varied significantly among the cultivars (P<0.05). However, the shortening of the flooding period by ca. 24 days by selecting the early-maturing cultivars did not decrease the total CH4 fluxes because the CH4 emission rates at the ripening stage were very low, barely affecting the total CH4 flux in the late-maturing group. Methane fluxes were correlated in highly positive and negative manner with methanogens and methanotrophs abundances, respectively (P<0.05), but not with any of the apparent plant growth parameters; these suggest that CH4 emissions may be directly affected by the substrate-producing potential and gas transport capacity of each cultivar rather than the external plant growth variables. In conclusion, the selection and development of a cultivar with a lower CH4 flux and higher productivity may be an effective countermeasure for decreasing the CH4 emissions from rice paddy soil, whereas the selection of a short-term cultivar is not.

The importance of episodic ebullition methane losses from three peatland microhabitats: a controlled‐environment study

Ebullition and episodic ebullition in particular, may be an important pathway for methane (CH4) losses from northern peatlands. We quantified the importance of episodic ebullition using controlled environment laboratory incubations of samples of near‐surface bog peat, focusing on how ebullition can be measured effectively and assessing the variation in CH4 losses between microhabitats and seasons. The peat samples were collected from hollow and lawn microhabitats at two raised bogs: Longbridgemuir, southwest Scotland, and Cors Fochno, west Wales. We found that CH4 fluxes excluding episodic ebullition differed between peatland microhabitats but not between summer and early autumn conditions. Conversely, episodic ebullition did not differ between microhabitat types but virtually stopped after the onset of early autumn conditions. Most strikingly, episodic ebullition was less than 3.3% of total CH4 fluxes, and was therefore an insignificant mechanism of CH4 loss from our peat samples.

Optimum application level of winter cover crop biomass as green manure under considering methane emission and rice productivity in paddy soil

The combined seeding and cropping of non-leguminous and leguminous cover crops during the cold fallow season is recommended as an important agronomic practice to improve total biomass productivity and soil fertility in mono-rice (Oryza sativa L.) cultivation system. However, application of plant residues as green manure can increase methane (CH4) emission during rice cultivation and affect rice quality and productivity, but its effects are not well examined. In this field study, the mixture of barley (Hordeum vulgare R.) and hairy vetch (Vicia villosa R., hereafter, vetch) seeds with 75 % recommended dose (RD 140 kg ha−1) and 25 % RD (90 kg ha−1), respectively, were seeded after rice harvesting in late November, 2010, and harvested before rice transplanting in early June 2011. Total aboveground biomass was 36 Mg ha−1 (fresh weight basis with 68 % moisture content), which was composed with 12 Mg ha−1 of barley and 24 Mg ha−1 of vetch. In order to determine the optimum recycling ratio of biomass application that can minimize CH4 emission without affecting rice productivity, different recycling ratios of 0, 25, 50, 75, and 100 % of the total harvested biomass were incorporated as green manure 1 week before rice transplanting in a typical temperate paddy soil. The same rates of chemical fertilizers (N–P2O5–K2O = 90–45–58 kg ha−1) were applied in all treatments. Daily mean CH4 emission rates and total CH4 fluxes were significantly (p < 0.05) increased with increasing application rates of cover crop biomass. Rice productivity also significantly (p < 0.05) increased with biomass application, but the highest grain yield (53 % increase over the control) was observed for 25 % recycling. However, grain quality significantly (p < 0.05) decreased with increasing cover crop application rates above 25 % recycling ratio, mainly due to extended vegetative growth periods of rice plants. Total CH4 flux per unit grain yield, an indicator used to simultaneously compare CH4 emission impact with rice production, was not statistically different between 25 % biomass recycling ratio and the control but significantly increased with increasing application rates. Conclusively, the biomass recycling ratio at 25 % of combined barley and vetch cover crops as green manure might be suitable to sustain rice productivity without increasing CH4 emission impact in mono-rice cultivation system.

Considering winter cover crop selection as green manure to control methane emission during rice cultivation in paddy soil

Cover crop cultivation during the cold fallow season has been strongly recommended in mono-rice cultivation system to improve soil quality. Its addition as green manure could stimulate methane (CH4) emission in the submerged paddy soil during rice cultivation, but its effect has not been evaluated well. Leguminous Chinese milk vetch (hereafter milk vetch) and non-leguminous rye were seeded after rice harvesting in 2008 and 2009, and total above-ground biomasses were incorporated with the recommended mineral fertilizers (NPK+milk vetch, and NPK+rye, respectively) before rice transplanting in 2009 and 2010. No fertilization (control) and mineral fertilization alone (NPK) plots were installed for comparison. Methane flux which was investigated by the closed chamber method during rice cultivation was significantly increased by chemical fertilization to ca. 80–250% over the control (138–141kgCH4ha−1), but increased more markedly by cover crop biomass addition to ca. 28–121% over the NPK treatment. Milk vetch was more effective on stimulating less CH4 emission (28–61% increase over NPK) and improving rice productivity (18–31% increase over NPK) than rye (86–121% increase of CH4 emission and −3–6% of yield increase over NPK). As a result, the total CH4 flux per grain yield was significantly increased by rye addition, but were similar between the NPK+milk vetch and NPK treatments. Conclusively, low C/N ratio cover crop like milk vetch might be more recommendable green manure to minimize CH4 emission impact and increase rice productivity than high C/N ratio cover crop like rye in paddy soil.

Influence of Soil Amendments on Mitigating Methane Emissions and Sustaining Rice Productivity in Paddy Soil Ecosystems of Bangladesh

Two field experiments were conducted at two different rice ecosystems, one in the upland rice field of Bangladesh Agricultural University farm, Mymensingh and the another one in the low lying area of Bhaluka, Mymensingh to investigate the effects of soil amendments on mitigation of methane emissions and sustaining rice productivity. The experimental treatments were urea (250 kg ha-1), urea plus coal ash (1t ha-1), urea plus phosphogypsum (90 kg ha-1), urea plus silicate fertilizer (150 kg ha-1), ammonium sulphate 400 kg ha-1, ammonium sulphate plus silicate fertilizer (150 kg ha-1), urea (25% less than the recommended doze) plus cyanobacteria plus azolla (1t ha-1). In case of BAU upland rice field, the total seasonal CH4 emission was decreased by 12-21% and rice grain yield was increased by 4.0- 18.0% respectively, whereas 11.0-26.0% reduction in total CH4 emission and 4.5-24.0% increase in rice grain yield was recorded from the low lying rice field of Bhaluka with the application of soil amendments. Among the amendments silicate fertilization with urea and silicate in combination with ammonium sulphate reduced total CH4 flux by 18- 23% and 21-26% respectively, whereas rice grain yield was increased by 18-24% and 16-18%, respectively in both ecosystems. Although maximum reduction in total seasonal CH4 flux was recorded with silicate and sulfate of ammonia amendment in paddy soil, however soil acidity was developed which might affect soil fertility and rice productivity in the future. Therefore, silicate fertilizer could be introduced with the nitrogenous fertilizer sources, preferably with 50% urea plus 50% ammonium sulphate for reducing CH4 emissions and increasing rice productivity under both irrigated upland and lowland rice field ecosystems. DOI: http://dx.doi.org/10.3329/jesnr.v5i1.11574 J. Environ. Sci. &amp; Natural Resources, 5(1): 179 - 185, 2012

Mass fluxes and isofluxes of methane (CH4) at a New Hampshire fen measured by a continuous wave quantum cascade laser spectrometer

We have developed a mid‐infrared continuous‐wave quantum cascade laser direct‐absorption spectrometer (QCLS) capable of high frequency (≥1 Hz) measurements of 12CH4 and 13CH4 isotopologues of methane (CH4) with in situ 1‐s RMS precision of 1.5 ‰ and Allan‐minimum precision of 0.2 ‰. We deployed this QCLS in a well‐studied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 ± 0.7 mg CH4 m−2 hr−1 over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 ± 0.8 mgCH4 m−2 hr−1) over the same period. Mean composition of emitted CH4 calculated using EC isoflux methods was −71 ± 8 ‰ (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of −64.5 ± 0.8 ‰. Ebullitive fluxes, representing ∼10% of total CH4 fluxes at this site, were on average 1.2 ‰ enriched in 13C compared to diffusive fluxes. CH4 isoflux time series have the potential to improve process‐based understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis.

Greenhouse Gas Fluxes from Dutch Peatland Water Bodies: Importance of the Surrounding Landscape

Methane and carbon dioxide fluxes were quantified for 14 ditches and larger water bodies in Dutch peatlands, surrounded by different habitat types, using concentration changes in floating flux chambers. Average fluxes from these waters were 11 ± 2 (mean ± SE) mg CH4 m−2 h−1 (n = 66) and 86 ± 21 mg CO2 m−2 h−1 (n = 55). Variability among water bodies was substantial in both gases and could be explained by water depth and surrounding habitat. Ditches in intensively used dairy land or rough pastures had significantly higher CH4 emission rates (17 ± 6 and 18 ± 8 mg CH4 m−2 h−1) than those in reed and sedge beds or open water (7 ± 1 and 5 ± 2 mg CH4 m−2 h−1). Bubbles contributed between 34 and 69% to the total CH4 flux, with higher proportions observed in ditches sheltered by tall vegetation. Observed day-time CO2 fluxes were either positive (shaded ditches in reed and sedge stands or rough pasture; 120–150 mg CO2 m−2 h−1) or did not differ from zero (open water, ditches in intensively managed pasture). Since these peatlands have substantial areas of permanent surface water (6–43%), landscape-scale carbon flux estimates are improved by incorporating specific flux estimates for these waters.

An attempt to quantify the impact of changes in wetland extent on methane emissions on the seasonal and interannual time scales

Climate variability impacts CH4 wetland sources as changes in flux density per unit area and via expansion or contraction of wetland areas in response to surface hydrological processes. This paper is a first attempt to isolate the role of varying wetland area on the seasonal and interannual variability of CH4 wetland emissions over the past decade. Wetland area extent at monthly intervals was provided over the period 1993–2000 by a suite of satellite observations from multiple sensors. The regionally variable fraction of wetland area was optimized using satellite observations of flooded area as a first estimate and further adjusted to match the seasonal cycle of CH4 fluxes retrieved from a global atmospheric inversion. Wetland flux densities of CH4 were calculated by coupling the ORCHIDEE global vegetation model with a process‐based wetland CH4 emission model, calibrated by optimizing its parameters at the site level against representative CH4 flux time series. For boreal bogs north of 50°N, we found that variations in area contributed about 30% to the annual flux. For temperate and tropical wetlands, the variations in area has almost no influence on the annual CH4 emissions but contributes significantly to the seasonal behavior, accounting for 40% and 66% of the seasonal amplitude of fluxes, respectively. In contrast, the interannual variability of wetland area appears to be the dominant cause of interannual variations in regional CH4 emissions from wetlands at all latitudes (largest in the tropics), with up to 90% of annual flux anomalies explained by wetland area anomalies in some years. For example, in 1998, boreal wetlands north of 50°N contributed to approximately 80% of the positive anomaly according to our calculations. We also found that climate anomalies can lead to both increased emitting areas and decreased flux densities at the same time, with opposite effects on the total CH4 flux entering the atmosphere. With a view to forecasting the future trajectory of atmospheric methane content, our results point to the absolute necessity to be able to predict the variations in wetland extent, a hydrological problem, in order to affirm the reliability of simulations of changing methane emissions perturbed by climate.

Silicate fertilization in no-tillage rice farming for mitigation of methane emission and increasing rice productivity

Agricultural practices mostly influence methane (CH4) emissions from rice field, which must be controlled for maintaining the ecosystem balance. No-tillage farming with chemical amendments having electron acceptors could be an effective mitigation strategy in CH4 emissions from irrigated rice (Oryza sativa L.) field. An experiment was conducted in Korean paddy field under tillage and no-tillage farming practices with silicate iron slag amendments (1–4Mgha−1) for suppressing CH4 emissions and maintaining rice productivity. It was found that CH4 emissions from the no-tillage rice field significantly decreased as compared to that of tilled field, irrespective of silicate amendments. The total seasonal CH4 flux from the control tillage and control no-tillage plots were recorded 38.1 and 27.9gm−2, respectively, which were decreased by 20% and 36% with 4Mgha−1 silicate amendment. Silicate fertilization (4Mgha−1) with no-tillage system decreased total seasonal CH4 flux by 54% as compared to that of control tillage plot. This is most likely due to the higher concentrations of active iron and free iron oxides in the no-tilled rice field as compared to that of tilled field under silicate fertilization, which acted as electron acceptors and contributed to decrease CH4 emission. In addition, the improved soil porosity and redox potential, rice plant growth parameters such as active tillering rate, root volume and porosity, etc. in combination increased the rhizosphere oxygen concentrations and eventually suppressed CH4 emission during the rice growing season. The leaf photosynthetic rate was significantly increased with 4Mgha−1 silicate amendment, which ultimately increased grain yield by 18% and 13% in the tilled and no-tilled rice field, respectively. CH4 flux showed a strong positive correlation with the availability of soil organic carbon, while there were negative correlations with soil porosity, soil pH, soil Eh, and the content of active iron and free iron oxides in soil.

Effect of silicate fertilizer on reducing methane emission during rice cultivation

Slag-type silicate fertilizer, which contains high amount of active iron oxide, a potential source of electron acceptor, was applied at the rate of 0, 2, 6, 10, and 20 Mg ha−1 to reduce methane (CH4) emission from rice planted in potted soils. Methane emission rates measured by closed chamber method decreased significantly with increasing levels of silicate fertilizer application during rice cultivation. Soil redox potential (Eh) decreased rapidly after flooding, but floodwater pH and soil pH increased significantly with increasing levels of silicate fertilizer application. Iron concentrations in potted soils and in percolated water significantly increased with the increasing levels of silicate fertilizer application, which acted as oxidizing agents and electron acceptors, and thereby suppressed CH4 emissions. Silicate fertilization significantly decreased CH4 production activity, while it increased carbon dioxide (CO2) production activity. Rice plant growth, yield parameters, and grain yield were positively influenced by silicate application levels. The maximum increase in grain yield (17% yield increase over the control) was found with 10 Mg ha−1 silicate application along with 28% reduction in total CH4 flux during rice cultivation. It is, therefore, concluded that slag-type silicate fertilizer could be a suitable soil amendment for reducing CH4 emissions as well as sustaining rice productivity and restoring the soil nutrient balance in rice paddy soil.

Falling atmospheric pressure as a trigger for methane ebullition from peatland

Peatlands are widely regarded as a significant source of atmospheric CH4, a potent greenhouse gas. At present, most of the information on environmental emissions of CH4 comes from infrequent, temporally discontinuous ground‐based flux measurements. Enormous efforts have been made to extrapolate measured emission rates to establish seasonal or annual averages using relevant biogeochemical factors, such as water table positions or peat temperatures, by assuming that the flux was stationary during a substantial nonsampling period. However, this assumption has not been explicitly verified, and little is known about the continuous variation of the CH4 flux in a timescale of individual flux measurement. In this study, we show an abrupt change in the CH4 emission rate associated with falling atmospheric pressure. We found that the CH4 flux can change by 2 orders of magnitude within a matter of tens of minutes owing to the release of free‐phase CH4 triggered by a drop in air pressure. The contribution of the ebullition to the total CH4 flux during the measurements was significant (50–64%). These results clearly indicated that field campaigns must be designed to cover this rapid temporal variability caused by ebullition, which may be especially important in intemperate weather. Process‐based CH4 emission models should also be modified to include air pressure as a key factor for the control of ebullient CH4 release from peatland.

Carbon dioxide and methane emissions and the carbon budget of a 10‐year old tropical reservoir (Petit Saut, French Guiana)

The emissions of carbon dioxide (CO2) and methane (CH4) from the Petit Saut hydroelectric reservoir (Sinnamary River, French Guiana) to the atmosphere were quantified for 10 years since impounding in 1994. Diffusive emissions from the reservoir surface were computed from direct flux measurements in 1994, 1995, and 2003 and from surface concentrations monitoring. Bubbling emissions, which occur only at water depths lower than 10 m, were interpolated from funnel measurements in 1994, 1997, and 2003. Degassing at the outlet of the dam downstream of the turbines was calculated from the difference in gas concentrations upstream and downstream of the dam and the turbined discharge. Diffusive emissions from the Sinnamary tidal river and estuary were quantified from direct flux measurements in 2003 and concentrations monitoring. Total carbon emissions were 0.37 ± 0.01 Mt yr−1 C (CO2 emissions, 0.30 ± 0.02; CH4 emissions, 0.07 ± 0.01) the first 3 years after impounding (1994–1996) and then decreased to 0.12 ± 0.01 Mt yr−1 C (CO2, 0.10 ± 0.01; CH4, 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO2 emissions occurred by diffusion from the reservoir surface, 31% from the estuary, 7% by degassing at the outlet of the dam, and a negligible fraction by bubbling. CH4 diffusion and bubbling from the reservoir surface were predominant (40% and 44%, respectively) only the first year after impounding. Since 1995, degassing at an aerating weir downstream of the turbines has become the major pathway for CH4 emissions, reaching 70% of the total CH4 flux. In 2003, river carbon inputs were balanced by carbon outputs to the ocean and were about 3 times lower than the atmospheric flux, which suggests that 10 years after impounding, the flooded terrestrial carbon is still the predominant contributor to the gaseous emissions. In 10 years, about 22% of the 10 Mt C flooded was lost to the atmosphere. Our results confirm the significance of greenhouse gas emissions from tropical reservoir but stress the importance of: (1) considering all the gas pathways upstream and downstream of the dams and (2) taking into account the reservoir age when upscaling emissions rates at the global scale.