Nature 439, 187-191 (12 January 2006) | doi:10.1038/nature04420
Abstract
Methane is an important greenhouse gas and its atmospheric concentration has almost tripled since pre-industrial times. It plays a central role in atmospheric oxidation chemistry and affects stratospheric ozone and water vapour levels. Most of the methane from natural sources in Earth’s atmosphere is thought to originate from biological processes in anoxic environments. Here we demonstrate using stable carbon isotopes that methane is readily formed in situ in terrestrial plants under oxic conditions by a hitherto unrecognized process. Significant methane emissions from both intact plants and detached leaves were observed during incubation experiments in the laboratory and in the field. If our measurements are typical for short-lived biomass and scaled on a global basis, we estimate a methane source strength of 62-236 Tg yr-1 for living plants and 1-7 Tg yr-1 for plant litter (1 Tg = 1012 g). We suggest that this newly identified source may have important implications for the global methane budget and may call for a reconsideration of the role of natural methane sources in past climate change.
Introduction
Methane (CH4) is the most abundant organic trace gas in the atmosphere (mixing ratio 1.8 p.p.m.) and is important to both tropospheric and stratospheric chemistry. Therefore, the atmospheric CH4 budget has been intensively studied over the past two decades using flux measurements on sources, global observation networks4 and global atmospheric models. In addition, stable carbon isotope ratios (13C/12C) have been applied to investigate sources and sinks of atmospheric CH4. Although uncertainties in the estimates of individual source strengths are large (50-100 Tg), it is generally thought that all major sources, including wetlands, animals, rice cultivation, biomass burning and fossil fuel production, have been identified and sum up to a global source strength of 600 Tg yr-1. However, significantly elevated CH4 mixing ratios were recently observed in tropical regions above evergreen forests indicating an additional tropical source of 30-40 Tg over the time period of the investigation (August-November), which could not be explained within the currently accepted global budget of CH4.
Following our observations of non-enzymic production of methyl halides from senescent plants and leaf litter, we investigated the possibility of methane formation by plant material. A large set of laboratory experiments using freshly collected and dried plant material-including tree and grass leaves from C3 and C4 plant categories-were conducted, in which CH4 release rates and stable carbon isotope composition (13C values) of emissions were measured under controlled conditions (see Methods). Whereas CH4 emissions were difficult to quantify for samples incubated in ambient air owing to the high atmospheric background levels of CH4, production was clearly evident when samples were incubated in CH4-free air. Emission rates typically ranged from 0.2 to 3 ng per g (dry weight) h-1 at 30 °C. Release of CH4 was very temperature sensitive-concentrations approximately doubled with every 10 °C increase over the range 30-70 °C, suggesting a non-enzymic rather than an enzyme-mediated process. 13C of the emitted CH4 ranged from -51.8 to -68.4 (mean = -58.2, n =
61) and -46.9 to -53.1 (mean = -49.5, n = 13) for C3 and C4 plants, respectively. The mean value determined for C3 plant emissions is comparable with the average 13C value for CH4 emitted from wetlands and rice paddies (approximately -60) and thus would be generally regarded as an indication for biological production by anaerobic bacteria. Even though this possibility was remote since most of our experiments were performed under aerobic conditions, we measured CH4 production by leaf tissue sterilized with -radiation. No significant difference, either in emission rates or 13C values of the emissions, was noted between sterilized and non-sterilized samples, thus further excluding microbial activity as the CH4 source and clearly indicating the existence of a hitherto unknown pathway for CH4 production in leaf tissue.
Having established CH4 production by detached leaf tissue, we investigated the possibility of CH4 formation by intact plants, using incubation chambers in the laboratory and in the field (see Methods). CH4 formation was observed for all plant species investigated, with release rates ranging from 12 to 370 ng per g (dry weight) h-1, thus one to two orders of magnitude higher than the emissions from detached leaf material. Furthermore, emission rates were found to increase dramatically, by a factor of 3-5 (up to 870 ng per g (dry weight) h-1), when chambers were exposed to natural sunlight, an effect also observed with detached leaf tissue…
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