Changes In CH4 Emissions Research Articles

Effect ofCarex rostrataon seasonal and interannual variability in peatland methane emissions

[1] Peatlands are a large natural source of atmospheric methane (CH4), and the sedge Carex rostrata plays a critical role in the production, oxidation, and transport of CH4 in these systems. This 4 year clipping experiment examined the changes in CH4 emissions from a temperate peatland after removing all aboveground C. rostrata biomass. Methane fluxes, dissolved CH4, and environmental variables were measured during spring, summer, and fall from 2008 to 2011. Clipping and removing the C. rostrata leaves and stems caused an immediate decrease in CH4 emissions that persisted over 4 years of this study. There was a strong seasonal trend in CH4 flux, with the largest treatment effects occurring during the fall months when the sedges were senescing. As expected, there was a strong positive correlation between C. rostrata green-leaf area and CH4 flux, implying that the presence of C. rostrata increases CH4 emissions from this peatland. Large interannual variability in vegetation distribution and biomass, water table depth, and temperature was observed in this study, indicating the importance of multiyear studies for understanding the interactions among these factors to determine how they could be incorporated into biogeochemical models to predict CH4 emissions under changing environmental conditions.

Methane emissions associated with the conversion of marshland to cropland and climate change on the Sanjiang Plain of northeast China from 1950 to 2100

Abstract. Wetland loss and climate change are known to alter regional and global methane (CH4) budgets. Over the last six decades, an extensive area of marshland has been converted to cropland on the Sanjiang Plain in northeast China, and a significant increase in air temperature has also been observed there, while the impacts on regional CH4 budgets remain uncertain. Through model simulation, we estimated the changes in CH4 emissions associated with the conversion of marshland to cropland and climate change in this area. Model simulations indicated a significant reduction of 1.1 Tg yr−1 (0.7–1.8 Tg yr−1) from the 1950s to the 2000s in regional CH4 emissions. The cumulative reduction of CH4 from 1960 to 2009 was estimated to be ~36 Tg (24–57 Tg) relative to the 1950s, and marshland conversion and the climate contributed 86% and 14% of this change, respectively. Interannual variation in precipitation (linear trend with P > 0.2) contributed to yearly fluctuations in CH4 emissions, but the relatively lower amount of precipitation over the period 1960–2009 (47 mm yr−1 lower on average than in the 1950s) contributed ~91% of the reduction in the area-weighted CH4 flux. Global warming at a rate of 0.3 ° per decade (P < 0.001) has increased CH4 emissions significantly since the 1990s. Relative to the mean of the 1950s, the warming-induced increase in the CH4 flux has averaged 19 kg ha−1 yr−1 over the last two decades. In the RCP (Representative Concentration Pathway) 2.6, RCP 4.5, RCP 6.0 and RCP 8.5 scenarios of the fifth IPCC assessment report (AR5), the CH4 fluxes are predicted to increase by 36%, 52%, 78% and 95%, respectively, by the 2080s compared to 1961–1990 in response to climate warming and wetting.

Microbial community composition controls the effects of climate change on methane emission from rice paddies

Rice paddies are one of the most important sources of CH4 emission from the terrestrial ecosystem. A Free-air CO2 Enrichment (FACE) experiment, which included a soil warming treatment, was conducted in a rice paddy at Shizukuishi, Japan. In this study, the changes in CH4 emission from a rice paddy, caused by global climate change, were explored in relation to the structural changes that have occurred in the methanogenic archaeal communities found in the soil and roots. The composition of the archaeal community was examined by terminal restriction fragment length polymorphism (T-RFLP) using the 16S rRNA gene, while its abundance was measured by real-time PCR using the methyl coenzyme M reductase (mcrA) gene. The archaeal community in the roots showed considerable change, characterized by the dominance of hydrogenotrophic methanogens and a corresponding decrease in acetoclastic methanogens. Seasonal changes in CH4 flux were closely related to the changes in methanogen abundance in the roots. Elevated CO2 caused an increase in root mass, which increased the abundance of methanogens leading to a rise in CH4 emissions. However, soil warming stimulated CH4 emissions by increasing CH4 production per individual methanogen. These results demonstrated that climate warming stimulates CH4 emission in a rice paddy by altering the abundance and activity of methanogenic archaea in the roots.

The impact of logging residue on soil GHG fluxes in a drained peatland forest

Northern peatlands contain substantial reservoirs of carbon (C). Forestry activities endanger the C storages in some of these areas. While the initial impacts of forestry drainage on peatland greenhouse gas (GHG) balance have been studied, the impacts of other silvicultural practices, e.g. logging residue (LR) retention or removal, are not known. We measured the CH4, N2O and CO2 fluxes between peat soil and atmosphere with and without decomposing LR over three (2002–2004) seasons (May–Oct) following clearfelling in a drained peatland forest, along with the mass loss of LR. Seasonal average CO2 efflux from plots with LR (3070g CO2 m−2 season−1) was twice as high as that from plots without LR (1447g CO2 m−2 season−1). Less than 40% of this difference was accounted for by the decay of logging residues (530g CO2 m−2 season−1), so the majority of the increased CO2 efflux was caused by increased soil organic matter decomposition under the LR. Furthermore LR increased soil N2O fluxes over 3-fold (0.70g N2O m−2 season−1), compared to plots without LR (0.19g N2O m−2 season−1), while no change in CH4 emissions was observed. Our results indicate that LR retention in clearfelled peatland sites may significantly increase GHG emissions and C release from the soil organic matter C storage. This would make the harvesting of LR for biofuel more beneficial, in the form of avoided emissions. Further investigations of the sources of CO2 under logging residues are, however, needed to confirm this finding.

Decreasing anthropogenic methane emissions in Europe and Siberia inferred from continuous carbon dioxide and methane observations at Alert, Canada

The rate of increase in global atmospheric methane (CH4) abundance has steadily declined since the late 1980s with near zero increase from 1999 through 2006. At the Canadian Baseline Observatory at Alert, Canada (82°28′N, 62°30′W), continuous measurements of methane (CH4) and carbon dioxide (CO2) have been made since 1987. During winter, both gases are frequently highly correlated during well‐defined episodes lasting from 2 to 5 days. We observe a gradual decrease in the ratios of CH4/CO2 during these episodes from ∼16 ppb CH4 (ppm CO2)−1 to ∼12 ppb CH4 (ppm of CO2)−1 over the entire record. An atmospheric transport model with prescribed CO2 and CH4 source distributions is used to partition simulated CH4 events into contributions by region. We show that anthropogenic emissions from Europe and Siberia account for more than 85% of the CO2 and CH4 enhancements simulated at Alert, but without a change in CH4 emissions, modeled CH4/CO2 ratios remain constant. To reproduce the observed trend in the ratio of CH4/CO2, the model requires a reduction in emissions of CH4 on the order of 30 Tg (13.6 to 33.4 Tg) in Europe and Siberia over the observational period. This is about twice the drop reported by the Emission Database for Global Atmospheric Research (EDGAR) emissions inventory and large enough to account for the leveling off of the global atmospheric CH4 burden observed over the past 20 years.

Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality

Reducing methane (CH4) emissions is an attractive option for jointly addressing climate and ozone (O3) air quality goals. With multidecadal full‐chemistry transient simulations in the MOZART‐2 tropospheric chemistry model, we show that tropospheric O3 responds approximately linearly to changes in CH4 emissions over a range of anthropogenic emissions from 0–430 Tg CH4 a−1 (0.11–0.16 Tg tropospheric O3 or ∼11–15 ppt global mean surface O3 decrease per Tg a−1 CH4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH4 emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost‐effective CH4 controls would offset the positive climate forcing from CH4 and O3 that would otherwise occur (from increases in NOx and CH4 emissions in the baseline scenario) and improve O3 air quality. We estimate that anthropogenic CH4 contributes 0.7 Wm−2 to climate forcing and ∼4 ppb to surface O3 in 2030 under the baseline scenario. Although the response of surface O3 to CH4 is relatively uniform spatially compared to that from other O3 precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O3 formation regime is NOx‐saturated. In the model, CH4 oxidation within the boundary layer (below ∼2.5 km) contributes more to surface O3 than CH4 oxidation in the free troposphere. In NOx‐saturated regions, the surface O3 sensitivity to CH4 can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH4. Accurately representing the NOx distribution is thus crucial for quantifying the O3 sensitivity to CH4.

How northern peatlands influence the Earth's radiative budget: Sustained methane emission versus sustained carbon sequestration

Northern peatlands sequester carbon and emit methane, and thus have both cooling and warming impacts on the climate system through their influence on atmospheric burdens of CO2 and CH4. These competing impacts are usually compared by the global warming potential (GWP) methodology, which determines the equivalent CO2 annual emission that would have the same integrated radiative forcing impact over a chosen time horizon as the annual CH4 emission. We use a simple model of CH4 and CO2 pools in the atmosphere to extend this analysis to quantify the dynamics, over years to millennia, of the net radiative forcing impact of a peatland that continuously emits CH4 and sequesters C. We find that for observed ratios of CH4 emission to C sequestration (roughly 0.1–2 mol mol−1), the radiative forcing impact of a northern peatland begins, at peatland formation, as a net warming that peaks after about 50 years, remains a diminishing net warming for the next several hundred to several thousand years, depending on the rate of C sequestration, and thereafter is or will be an ever increasing net cooling impact. We then use the model to evaluate the radiative forcing impact of various changes in CH4 and/or CO2 emissions. In all cases, the impact of a change in CH4 emissions dominates the radiative forcing impact in the first few decades, and then the impact of the change in CO2 emissions slowly exerts its influence.

The effects of ammonium sulfate application on methane emission and soil carbon content of a paddy field in Japan

Abstract Decreases in methane (CH4) emission from paddy fields are important for mitigating global warming. Some substrates such as soil carbon are used by methanogens for CH4 production. Thus, changes in CH4 emission may be accompanied by changes in soil carbon content. This study was carried out at an experimental paddy field in Agricultural and Forestry Research Center, University of Tsukuba, Japan. The experimental variables were rates of ammonium sulfate application (AS; 0, 45, 90, and 135 kg N ha−1) and levels of rice (Oryza sativa L., cultivar: Koshihikari) plant density (PD; 18.5 and 30.3 hill m−2). During the rice growing period, we measured CH4 emission by a closed-chamber method, and soil carbon content. Then, we considered the effects of the variables on CH4 emission and soil carbon content, and analyzed the quantitative relationship between them. There were no effects of PD level on CH4 emission and soil carbon content. This was likely due to the compensatory effect of rice plants. The CH4 emission decreased with increases in AS application rate, caused by competition for substrates between sulfate-reducing bacteria and methanogens. Decreases in soil redox potential (Eh) were delayed with increases in AS application rate. This process was another cause for the decreased CH4 emission at higher rates of AS application, but the reason for the higher soil Eh is not clear. Soil carbon content decreased with increases in AS application rate. Thus, there was a positive linear correlation between CH4 emission and soil carbon content (R2 = 0.977). Consequently, although AS application was effective to decrease CH4 emission, it simultaneously stimulated soil carbon decomposition. Carbon is stored in the soil in huge quantities, and is partly decomposed to carbon dioxide, also a greenhouse gas. Thus, it is necessary to consider not only CH4 emission but also soil carbon storage for mitigating global warming.

Influences of indigenous phototrophs on methane emissions from a straw-amended paddy soil

A pot experiment was conducted to investigate the influences of indigenous phototrophs on methane (CH4) emissions from a paddy soil where rice straw was incorporated or was surface-applied. During the cultiva- tion, half of the pots were covered with aluminum foil, except for the minimum space for rice plants, to prevent ambient light reaching the floodwater or the soil surface. Growth of oxygen-producing phototrophs was hardly observed in the unilluminated plots, whereas intensive growth of algae, duckweed and hydrophytes was found in the illuminated ones. Plant growth was not affected by the different treatments. Seasonal changes in CH4 emission determined by a closed chamber method indicated that illumination had no or only minor effects on CH4 emissions when rice straw was incorporated or was not applied, but significantly reduced CH4 emissions when rice straw was surface-applied. Methanogenesis occurring in the soil-floodwater interface was further investigated in two lab-scale model experiments measuring methanogenic activity. As a result, more activated methanogenesis was found in the surface-applied rice straw and the soil around the straw compared with the soil incubated without rice straw. The magnitude of the methanogenic activity in the rice straw incubated under illuminated conditions was significantly lower than that incubated in the dark. Consequently, this study demonstrates that methanogen- esis in paddy soil occurs even in the soil-floodwater interface if plant residues like rice straw exist, and such methanogenesis is likely to be suppressed by growth of indigenous phototrophs under illumination.

Short‐ and long‐term greenhouse gas and radiative forcing impacts of changing water management in Asian rice paddies

AbstractFertilized rice paddy soils emit methane while flooded, emit nitrous oxide during flooding and draining transitions, and can be a source or sink of carbon dioxide. Changing water management of rice paddies can affect net emissions of all three of these greenhouse gases. We used denitrification–decomposition (DNDC), a process‐based biogeochemistry model, to evaluate the annual emissions of CH4, N2O, and CO2 for continuously flooded, single‐, double‐, and triple‐cropped rice (three baseline scenarios), and in further simulations, the change in emissions with changing water management to midseason draining of the paddies, and to alternating crops of midseason drained rice and upland crops (two alternatives for each baseline scenario). We used a set of first‐order atmospheric models to track the atmospheric burden of each gas over 500 years. We evaluated the dynamics of the radiative forcing due to the changes in emissions of CH4, N2O, and CO2 (alternative minus baseline), and compared these with standard calculations of CO2‐equivalent emissions using global warming potentials (GWPs). All alternative scenarios had lower CH4 emissions and higher N2O emissions than their corresponding baseline cases, and all but one sequestered carbon in the soil more slowly. Because of differences in emissions, in radiative forcing per molecule, and in atmospheric time constants (lifetimes), the relative radiative impacts of CH4, N2O, and CO2 varied over the 500‐year simulations. In three of the six cases, the initial change in radiative forcing was dominated by reduced CH4 emissions (i.e. a cooling for the first few decades); in five of the six cases, the long‐term radiative forcing was dominated by increased N2O emissions (i.e. a warming over several centuries). The overall complexity of the radiative forcing response to changing water management could not easily be captured with conventional GWP calculations.

Influence of vascular plant photosynthetic rate on CH4emission from peat monoliths from southern boreal Sweden

Peat monoliths taken from a boreal peatland system were incubated at two different light intensities to investigate the effect of the photosynthetic rate of vascular plants (Eriophorum angustifolium) on net CH4 emission. The experimental set-up consisted of six replicate monoliths as controls and six where the photosynthetic active radiation (PAR) was reduced by 60%. NEP and total system respiration decreased significantly in response to reduced PAR. No significant changes in CH4 emission were found, but two different trends were noted. Methane emissions from the shaded monoliths initially seemed to be higher than emissions from the controls. After approximately four weeks the trend was reversed. The pattern may have been caused by “leakage” of organic compounds from inactivated roots that fueled CH4 production. It is suggested that a new balanced exchange of potential substrate carbon between the plants and the surrounding peat was established. Comparably less easily degradable carbon compounds would then become available for CH4 production. The fact that there appeared to be an effect of decreased carbon flow on CH4 emission is further supported by a tendency for lower concentrations of organic acids in porewater in the shaded monoliths at the end of the experiment. These results indicate a possible lagtime on the order of weeks before changes in photosynthesis rates and NEP have an effect onCH4 emission rates. Nevertheless it confirms the linkage between CO2 and CH4 cycling in wetland ecosystems.

Methane fluxes on boreal peatlands of different fertility and the effect of long‐term experimental lowering of the water table on flux rates

Methane (CH4) fluxes were measured at 17 peatland sites with different nutritional and hydrological characteristics in the southern and middle boreal zones in Finland by a static chamber technique. Many of the natural peatlands also had counterparts drained for forestry 30–50 years ago. The mean emissions from May to September were 8.0 g CH4 m−2 for natural ombrogenous bogs and 19.0 g CH4 m−2 for natural minerogenous fens thus being higher than the 2 g CH4 m−2 yr−1 estimated for the Canadian peatlands. Change in the mean water table level had greater effect on CH4 fluxes on natural fens than on natural bogs. The mean CH4 emissions on drained bogs and fens were 3.9 g CH4 m−2 and 0.3 g CH4m−2, respectively. Some drained fens even had CH4 uptake from the atmosphere. The change in the mean water table had the lowest effect on CH4 fluxes on drained peatlands. The CH4 fluxes on peatlands (natural fens and bogs and drained peatlands) are associated with peat aeration, nutrient level, vegetation cover, peat compaction, peat temperature, and finally with microbial processes responsible for the net release of CH4. We could explain 67% of the variation in mean CH4 fluxes on Finnish peatlands by measuring the mean water table, peat bulk density, and peat pH. The present results can be used to predict the possible changes in CH4 emissions on peatlands if the climate is drying in north. For example, lowering of the present water table by 10 cm would induce a 70% reduction in the CH4 emissions from fens and a 45% reduction from bogs.

Sensitivity of the CH4 growth rate to changes in CH4 emissions from natural gas and coal

The observed rate of increase in atmospheric CH4 declined gradually during the late 1980s [Steele et al., 1992] and then fell dramatically by ∼4 to 7 parts per billion by volume (ppbv) yr−1 from 1991 to 1992 [Dlugokencky et al., 1994a]. Post‐1993 the CH4 growth rate has recovered to pre‐1991 levels. In this study, we assess the sensitivity of the CH4 growth rate to changes in CH4 emissions from the coal and gas industries over the last 15 years using a global two‐dimensional (2‐D) model of the atmosphere. Emissions are estimated from available production data, and in the case of natural gas, a spectrum of leakage rates is assumed. Changes in the former Soviet Union (FSU) have been assessed separately from the rest of the world (ROW). Prior to 1989, increased coal and natural gas production led to an increase in the CH4 growth rate (i.e., opposite to that which was observed). During the period 1989 to 1993 the modelled CH4 growth rate decreased by ∼1–2 ppbv yr−2 at northern mid‐latitudes (assuming constant gas leakage rates), showing that declining emissions, primarily in the form of natural gas from the FSU, may have contributed to the general reduction in the CH4 growth rate from 1989 to 1991. However, this does not explain the large reduction which was observed between 1991 and 1992. A model run with FSU gas leakage rates decreasing by 1% yr−1 from 5% in 1990 to 2% in 1993 shows a decrease in the CH4 growth rate of ∼9–12 ppbv yr−1 (or ∼3–5 ppbv yr−2) over this period. This is similar in magnitude to the observed decline over the same period. This sensitivity study illustrates the large potential for significantly lowering the growth rate of atmospheric CH4 by reducing fossil fuel emissions from coal and, in particular, by reducing the leakage rates from natural gas installations.