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Methane (Emissions to the Atmosphere: Natural Vegetation) -- Summary
What impact do global warming, the ongoing rise in the air's carbon dioxide (CO2) content and a number of other contemporary environmental trends have on the atmosphere's methane (CH4) concentration? The implications of this question are substantial, in light of the fact that methane is a more powerful greenhouse gas, molecule for molecule, than is carbon dioxide. We here consider this question as it applies to methane emissions associated with natural vegetation.

Beginning with a model-based study, Frolking and Roulet (2007) introduced their work by stating that "throughout the Holocene, northern peatlands have both accumulated carbon and emitted methane," so that "their impact on climate radiative forcing has been the net of cooling (persistent CO2 uptake) and warming (persistent CH4 emission)." Against this backdrop they then developed Holocene peatland carbon flux trajectories based on estimates of contemporary CH4 flux, total accumulated peat C, and peatland initiation dates, which they used as inputs to a simple atmospheric perturbation model to calculate the net radiative impetus for surface air temperature change. In doing so, Frolking and Roulet note that early in the Holocene the capture of CO2 and emission of CH4 by Earth's northern peatlands is likely to have produced a net warming impetus of up to +0.1 W m-2. Over the following eight to eleven thousand years, however, they say Earth's peatlands have been doing just the opposite, and that the current radiative forcing due to these atmospheric CO2 and CH4 perturbations represents a net cooling force on the order of -0.22 to -0.56 W m-2. It can thus be appreciated that the impetus for global cooling due to carbon sequestration by Earth's peatlands historically has been - and currently is - significantly greater than the global warming potential produced by their emissions of methane.

Writing as background for their work, Toet et al. (2001) state that "only three previous published studies have assessed the impacts of O3 on CH4 and CO2 fluxes in peatlands." Niemi et al. (2002), as they describe it, "reported that CH4 emissions more than doubled when peatland microcosms were exposed to 100 ppb O3 over 4-7 weeks during summer in controlled-environment chambers." In contrast, they state that "Rinnan et al. (2003) reported no significant effect on CH4 emissions of a 7-week exposure of peat microcosms to 100 or 200 ppb O3." Last of all, they indicate that "Morsky et al. (2008) reported that open-field exposure of boreal peatland microcosms in central Finland to a doubling of ambient O3 concentrations caused a decrease in CH4 emission at the end of the first growing season," but they note that the decrease "was lost in the three subsequent growing seasons." Thus, it is clear that prior work on the subject has not provided a definitive answer to the core question of whether rising O3 concentrations have a significant impact, one way or the other, on methane emissions from peatlands.

In a study that was more reflective of reality in terms of scale, Toet et al. moved up in size from microcosms to mesocosms, which they collected from a lowland raised bog on the northern shore of Morecambe Bay, Cumbria, UK (54°13'N, 3°1'W) and subsequently placed into open-top chambers situated on a level gravel base at Newcastle University's field station (54°59'N, 1°48'W). And there, for the next two years, they observed what happened in four ambient and four O3-enriched chambers, the latter of which had their atmospheric O3 concentrations raised by 50 ppb for eight hours of each day during the summer period (April-early October) and by 10 ppb for eight hours of each day throughout the winter.

Results indicated that "methane emissions were significantly reduced, by about 25%, by elevated ozone during midsummer periods of both years," but that "no significant effect of ozone was found during the winter periods." And after lengthy discussion of their findings, as well as those of other researchers they cite, Toet et al. concluded in the final sentence of their paper that "increased O3 could be a significant brake on the increased flux of CH4 that is expected as these northern peatlands warm."

In another paper, Davidson et al. (2004) reported that the climate of the Amazon Basin may become gradually drier due to the intensification of a number of different phenomena, including (1) less recirculation of water between the increasingly-deforested region and the atmosphere, (2) more rainfall inhibition by smoke caused by increased biomass burning, and (3) a warming-induced increase in the frequency and/or intensity of El Niņo events that have historically brought severe drought to the eastern Amazon Basin (Nepstad et al., 1999; but see Timmermann et al., 1999 as well). Driven by concern about these potential problems, they devised an experiment to determine the consequences of the drying of the soil of an Amazonian moist tropical forest for the net surface-to-air fluxes of two important greenhouse gases: nitrous oxide (N2O) and methane (CH4).

In the Tapajos National Forest near Santarem, Brazil, the researchers modified a one-hectare plot of land covered by mature evergreen trees so as to dramatically reduce the amount of rain that reached the forest floor (throughfall) while maintaining an otherwise similar one-hectare plot of land as a control for comparison. Prior to making this modification, they measured the gas exchange characteristics of the two plots for a period of 18 months; then, after initiating the throughfall-exclusion treatment, they continued their measurements for an additional three years. This protocol revealed, in their words, that the "drier soil conditions caused by throughfall exclusion inhibited N2O and CH4 production and promoted CH4 consumption." In fact, they say that "the exclusion manipulation lowered annual N2O emissions by >40% and increased rates of consumption of atmospheric CH4 by a factor of >4," which results they attributed to the "direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy."

As for the implications of their work, if global warming did indeed increase the frequency and/or intensity of El Niņo events - which real-world data suggest is highly debatable (see El Niņo - Relationship to Global Warming in our Subject Index) - the results of this study suggest that the anticipated drying of the Amazon Basin would initiate a strong negative feedback to warming via (1) large drying-induced reductions in the evolution of N2O and CH4 from its soils and (2) a huge drying-induced increase in the consumption of CH4 by its soils. Although Davidson et al. envisaged a more extreme second phase response, "in which drought-induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots" (a response that would be expected to increase N2O and CH4 emissions), it should be noted that the projected rise in the air's CO2 content would likely prohibit such extreme events from ever occurring, in light of the tendency for elevated levels of atmospheric CO2 to greatly increase the water use efficiency of essentially all plants (see Water Use Efficiency in our Subject Index, including the subsection Trees), which would enable the Amazon Basin's vegetation to continue to flourish under significantly drier conditions than those of the present.

In another paper, Strack et al. (2004) also reported that climate models predict increases in evapotranspiration that could lead to drying in a warming world and a subsequent lowering of water tables in high northern latitudes. This prediction stresses the importance of determining how lowered water tables will impact peatland emissions of CH4; and in a theoretical study of the subject, Roulet et al. (1992) calculated that for a decline of 14 cm in the water tables of northern Canadian peatlands, due to climate-model-derived increases in temperature (3°C) and precipitation (1mm/day) predicted for a doubling of the air's CO2 content, CH4 emissions would decline by 74-81%. Hence, in an attempt to obtain some experimental data on the subject, at various times over the period 2001-2003 Strack et al. measured CH4 fluxes to the atmosphere at different locations that varied in depth-to-water table within natural portions of a poor fen in central Quebec, Canada, as well as within control portions of the fen that had been drained eight years earlier. And what did they find?

At the conclusion of their study, the Canadian scientists reported that "methane emissions and storage were lower in the drained fen." The greatest reductions (up to 97%) were measured at the higher locations, while at the lower locations there was little change in CH4 flux. Averaged over all locations, they determined that the "growing season CH4 emissions at the drained site were 55% lower than the control site," indicative of the fact that the biosphere appears to be organized to resist warming influences that could push it into a thermal regime that might otherwise prove detrimental to its health.

In one final anaerobic-based study, Garnet et al. (2005) grew seedlings of three emergent aquatic macrophytes (Orontium aquaticum L., Peltandra virginica L. and Juncus effusus L.) plus one coniferous tree (Taxodium distichum L.), all of which are native to eastern North America, in a five-to-one mixture of well-fertilized mineral soil and peat moss in pots submerged in water in tubs located within controlled environment chambers for a period of eight weeks. Concomitantly, they measured the amount of CH4 emitted by the plant foliage, along with net CO2 assimilation rate and stomatal conductance, which were made to vary by changing the CO2 concentration of the air surrounding the plants and the density of the photosynthetic photon flux impinging on them.

In doing so, it was found that methane emissions from the four wetland species increased linearly with increases in both stomatal conductance and net CO2 assimilation rate; but the researchers report that changes in stomatal conductance affected foliage methane flux "three times more than equivalent changes in net CO2 assimilation," making stomatal conductance the more significant of the two CH4 emission-controllers. In addition, they note that evidence of stomatal control of CH4 emission has also been reported for Typha latifolia (Knapp and Yavitt, 1995) and Carex (Morrissey et al., 1993), two other important wetland plants. And, since atmospheric CO2 enrichment leads to approximately equivalent - but oppositely directed - changes in foliar net CO2 assimilation (which is increased) and stomatal conductance (which is reduced) in most herbaceous plants (which are the type that comprise most wetlands), it can be appreciated that the ongoing rise in the air's CO2 content should be acting to reduce methane emissions from Earth's wetland vegetation, because of the three-times-greater negative CH4 emission impact of the decrease in stomatal conductance compared to the positive CH4 emission impact of the equivalent increase in net CO2 assimilation.

Shifting to studies examining aerobic conditions, Dueck et al. (2007) introduced their work by stating that recent findings suggest that terrestrial plants may "emit methane under aerobic conditions by an as yet unknown physiological process (Keppler et al., 2006), and in this way may substantially contribute to the annual global methane budget (Bousquet et al., 2006)," resulting in "estimated values for methane emission by terrestrial plants varying between 10 and 260 Tg yr-1 (Houweling et al., 2006; Keppler et al., 2006; Kirschbaum et al., 2006)." To test the validity of this claim the fifteen Dutch researchers conducted two separate experiments involving six plant species - Ocimum basilicum L. (basil), Triticum aestivum L. (wheat), Zea mays L. (maize), Salvia officinalis L. (sage), Lycopersicon esculentum Miller (tomato), and Oenothera biennis L. (common evening primrose) - the first three of which were also used by Keppler et al. (2006) in their study.

The experiments were performed in "a unique hermetically sealed plant growth chamber with a volume of 3500 liters, specifically designed for atmospheric isotope labeling," where "plants were grown hydroponically to exclude any methane production derived from anaerobic soil pockets." When all was said and done, Dueck et al. report there was no evidence for substantial aerobic methane emission by the terrestrial plants they studied, stating that "maximally," it was only "0.3% of the previously published studies." Indeed, they say that methane concentrations in continuous-flow gas cuvettes with plants "were not significantly higher than those of control cuvettes without plants," stating that under both the short- and long-term, they "did not find any evidence of a substantial emission of methane."

Keppler et al.'s findings have been further debunked by other researchers. Beerling et al. (2008), for example, raised the C4 plant Zea mays and the C3 plant Nicotiana tabacum from seed for six weeks at an ambient CO2 concentration of 400 ppm and an ambient methane concentration of 1800 ppb, after which their leaves were studied in "a custom-built flowthrough cuvette with a sufficiently large area to allow the detection of methane emissions" via "a process gas chromatograph linked to a high-precision, high-accuracy flame ionization detector," all of which was done in a controlled-environment room. In describing their findings, the team of five UK researchers report that "well-illuminated actively photosynthesizing Z. mays leaves did not, in our experimental system, emit substantial quantities of methane during repeated three-hour high irradiance episodes," while adding that "neither did we detect methane emissions from actively respiring leaves during repeated three-hour dark periods." They additionally state that "measurements with leaves of the C3 species N. tabacum also failed to detect substantial aerobic methane emissions in the light when photosynthesizing with regular stomatal conductances, and in the dark when respiring."

For their part, Nisbet et al. (2009) "conducted further experiments on plants grown in controlled conditions" and "re-analyzed the previously published data" on the topic. Accordingly, the fourteen researchers (thirteen from the UK and one from Sweden) were able to demonstrate that "plants do not contain a biochemical mechanism for methanogenesis," and that they "cannot produce methane as an end-product or by-product of their metabolism." However, they determined that "when plants transpire, any methane that is already dissolved in the water derived from the soil will be released into the atmosphere," and that "under high stress conditions, such as high UV radiation, methane is released as part of the cellular breakdown process." In light of such findings, plus "a new analysis of global methane levels from satellite retrievals," Nisbet et al. concluded that "plants are not a major source of the global methane production." On the other hand, they acknowledge "the role of plants in moving methane about," and indicate their importance in the global cycling of methane, but not its production.

In one final study, Wang et al. (2009) concluded after their review of the scientific literature that "aerobic CH4 [methane] emissions from plants may be affected by O2 stress or any other stress leading to ROS [reactive oxygen species] production," leading the team of researchers to examine whether or not physical injury would also affect CH4 emissions from plants. In doing so their work revealed that "physical injury (cutting) stimulated CH4 emissions from fresh twigs of Artemisia species under aerobic conditions," and that "more cutting resulted in more CH4 emissions," as did hypoxia in both cut and uncut Artemisia frigida twigs.

In discussing their findings, and those of previous studies that suggest, in their words, "that a variety of environmental stresses stimulate CH4 emission from a wide variety of plant species," Wang et al. concluded that "global change processes, including climate change, depletion of stratospheric ozone, increasing ground-level ozone, spread of plant pests, and land-use changes, could cause more stress in plants on a global scale, potentially stimulating more CH4 emission globally," while further concluding that "the role of stress in plant CH4 production in the global CH4 cycle could be important in a changing world."

It is significant to note, however, that although lots of things "could" be important in this regard, the ongoing rise in the air's CO2 content is hard at work countering stress-induced CH4 emissions from plants. In the Antioxidants heading of our Subject Index, for example, it is seen that environmental stresses of all types do indeed generate highly-reactive oxygenated compounds that damage plants, but that atmospheric CO2 enrichment typically boosts the production of antioxidant enzymes that scavenge and detoxify those highly-reactive oxygenated compounds. Thus, it can be appreciated that the historical rise in the air's CO2 content should have gradually been alleviating the level of stress experienced by Earth's plants; and this phenomenon should have been gradually reducing the rate at which the planet's vegetation releases CH4 to the atmosphere. In addition, it should have been doing it at an accelerating rate commensurate with the accelerating rate of the upward trend in the air's CO2 content.

In closing, it would appear that current environmental trends that may impact methane emissions from natural vegetation, including the ongoing rise in the air's CO2 content, primarily tend to reduce this flux; and perhaps that is why the rate-of-rise of the atmosphere's methane concentration has changed little over the past couple of decades (see Methane (Atmospheric Concentrations) in our Subject Index).

References
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Davidson, E.A., Ishida, F.Y. and Nepstad, D.C. 2004. Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest. Global Change Biology 10: 718-730.

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Last updated 10 July 2013