Atmospheric methane

About: Atmospheric methane is a research topic. Over the lifetime, 2034 publications have been published within this topic receiving 119616 citations.

The Regulation of Methane Oxidation in Soil

Abstract: The atmospheric concentration of methane, a greenhouse gas, has more than doubled during the past 200 years. Consequently, identifying the factors influencing the flux of methane into the atmosphere is becoming increasingly important. Methanotrophs, microaerophilic organisms widespread in aerobic soils and sediments, oxidize methane to derive energy and carbon for biomass. In so doing, they play an important role in mitigating the flux of methane into the atmosphere. Several physico-chemical factors influence rates of methane oxidation in soil, including soil diffusivity; water potential; and levels of oxygen, methane, ammonium, nitrate, nitrite, and copper. Most of these factors exert their influence through interactions with methane monooxygenase (MMO), the enzyme that catalyzes the reaction converting methane to methanol, the first step in methane oxidation. Although biological factors such as competition and predation undoubtedly play a role in regulating the methanotroph population in soils, and thereby limit the amount of methane consumed by methanotrophs, the significance of these factors is unknown. Obtaining a better understanding of the ecology of methanotrophs will help elucidate the mechanisms that regulate soil methane oxidation.

Methane emission and methane oxidation in land-fill cover soil

Abstract: The vertical profiles of methane and oxygen concentrations were measured in the cover soil at four sites in a restored and covered landfill. At sites 2 and 3 within the landfill area methane was detectable even to the soil surface and emission of methane occured at these two sites. Measured methane emission rates varied seasonally and appeared to be most influenced by soil water content. On an annual basis methane emissions at these two sites were 495 and 909 mol methane m−2 y−1, respectively. At sites 1 and 4 methane was detected in the cover soil but was not present in the immediate subsurface layer, and emission of methane did not occur. Oxidation of methane by bacteria within the soil profile at these two sites appeared to prevent methane emission from the surface. A methane-oxidising microflora had been enriched in the soils of all four landfill sites, as shown by counts of methanotrophs and methylotrophs garden soil not subjected to elevated methane. Counts of methanotrophs and methylotrophs were generally higher in those soil strata where methane concentrations were greatest. Methane oxidation rates were maximum at soil depths where gradients of methane and oxygen overlapped, usually 10–30 cm depth. The depth integrated rates of methane oxidation were very high at sites 2 and 3, the sites also where methane was emitted from the soil surface. A maximum oxidation rate of 450 mmol CH4 m−2 d−1 was measured at site 3. The data suggested that the microflora in the soil above landfill adapted to the presence of elevated methane concentrations by selection of a more methanotrophic community which was able to rapidly oxidise methane. Optimisation of microbial oxidation of methane by bacteria in landfill cover soil may provide a cheap management strategy to minimise the emissions of methane to the atmosphere from landfill.

Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions.

Abstract: Atmospheric methane (CH4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH4 emissions, but the precise magnitude of these contributions is a subject of debate2,3. Carbon-14 in CH4 (14CH4) can be used to distinguish between fossil (14C-free) CH4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14CH4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century4,5. Moreover, the partitioning of total fossil CH4 emissions (presently 172 to 195 teragrams CH4 per year)2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH4 per year6,7. Geological emissions were less than 15.4 teragrams CH4 per year at the end of the Pleistocene, about 11,600 years ago8, but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14CH4 measurements to show that natural geological CH4 emissions to the atmosphere were about 1.6 teragrams CH4 per year, with a maximum of 5.4 teragrams CH4 per year (95 per cent confidence limit)—an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH4 emissions are underestimated by about 38 to 58 teragrams CH4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH4 budget, and will help to inform strategies for targeted emission reductions9,10. Isotopic evidence from ice cores indicates that preindustrial-era geological methane emissions were lower than previously thought, suggesting that present-day emissions of methane from fossil fuels are underestimated.

Response of peatland carbon dioxide and methane fluxes to a water table drawdown experiment

Abstract: [1] Northern peatlands play an important role in the global carbon cycle representing a significant stock of soil carbon and a substantial natural source of atmospheric methane (CH4). Peatland carbon cycling is affected by water table position which is predicted to be lowered by climate change. Therefore we compared carbon fluxes along a natural peatland microtopographic gradient (control) to an adjacent microtopographic gradient with an experimentally lowered water table (experimental) during three growing seasons to assess the impact of water table drawdown on peatland-atmosphere carbon exchange. Water table drawdown induced peat subsidence and a change in the vegetation community at the experimental site. This limited differences in carbon dioxide (CO2) exchange between the control and experimental sites resulting in no significant differences between sites after three seasons. However, there was a trend to higher respiration rates and increased productivity in low-lying zones (hollows) and this was coincident with increased vegetation cover at these plots. In general, CH4 efflux was reduced at the experimental site, although CH4 efflux from control and experimental hollows remained similar throughout the study. The differential response of carbon cycling to the water table drawdown along the microtopographic gradient resulted in local topographic high zones (hummocks) experiencing a relative increase in global warming potential (GWP) of 152%, while a 70% reduction in GWP was observed at hollows. Thus the distribution and composition of microtopographic elements, or microforms, within a peatland is important for determining how peatland carbon cycling will respond to climate change.

Methane-oxidizing bacteria in a California upland grassland soil: diversity and response to simulated global change.

Abstract: We investigated the diversity of methane-oxidizing bacteria (i.e., methanotrophs) in an annual upland grassland in northern California, using comparative sequence analysis of the pmoA gene. In addition to identifying type II methanotrophs commonly found in soils, we discovered three novel pmoA lineages for which no cultivated members have been previously reported. These novel pmoA clades clustered together either with clone sequences related to “RA 14” or “WB5FH-A,” which both represent clusters of environmentally retrieved sequences of putative atmospheric methane oxidizers. Conservation of amino acid residues and rates of nonsynonymous versus synonymous nucleotide substitution in these novel lineages suggests that the pmoA genes in these clades code for functionally active methane monooxygenases. The novel clades responded to simulated global changes differently than the type II methanotrophs. We observed that the relative abundance of type II methanotrophs declined in response to increased precipitation and increased atmospheric temperature, with a significant antagonistic interaction between these factors such that the effect of both together was less than that expected from their individual effects. Two of the novel clades were not observed to respond significantly to these environmental changes, while one of the novel clades had an opposite response, increasing in relative abundance in response to increased precipitation and atmospheric temperature, with a significant antagonistic interaction between these factors. Methane-oxidizing bacteria (methanotrophs) are a unique group of aerobic, gram-negative bacteria that use methane as their sole source of energy. They are ubiquitous in nature and, as the major biological sink for the greenhouse gas methane, they are involved in the mitigation of global warming. Methanotrophs are also of special interest to environmental microbiologists because of their capability to degrade various environmental contaminants, their potential for single cell protein production, and other novel aspects of their biochemistry (19). Based on physiological and biochemical characteristics, cultured members of the methanotrophs are traditionally divided into two main groups: type I methanotrophs, which are members of the class Gammaproteobacteria (e.g., Methylomonas, Methylococcus, Methylomicrobium, Methylothermus, Methylohalobium, Methylocaldum, and Methylobacter) and type II methanotrophs, which are in the class Alphaproteobacteria (e.g., Methylosinus, Methylocella, Methylocapsa, and Methylocystis) (14, 15, 19).