Plants do not emit methane

A recent study [*abstract] by Keppler and collegues in Nature suggested that terrestrial plants may be a global source of the potent greenhouse gas methane (CH4), making plants substantial contributors to the annual global methane budget. This controversial finding and the resulting commotion in the scientific community triggered a consortium of Dutch scientists to re-examine this in an independent study. Reporting in New Phytologist, Tom Dueck and colleagues present their results and conclude that methane emissions from plants are negligible and do not contribute to global climate change. The findings are crucial for those striving towards the creation of a global plant-based "bioeconomy" that substitites for petroleum products.

Being a potent greenhouse gas, methane emissions are controlled by humans and international rules, but there exists a natural methane cycle, driven by methanogenic microbes that thrive in anaerobic conditions (in the absence of air), such as in the stomachs of ruminants, the soils of swamps, in landfills, rice paddies or water reservoirs. A carbon cycle, based on one-carbon compounds, is taking place in the sediments and overlaying water of such freshwater environments. The anoxic sediments harbor archaea, which produce methane as a byproduct of their energy metabolism. The methane rises from the sediment and moves into the zone above it (image, click to enlarge). The vast bulk of methane enters the atmosphere because of this type of microbial action.

Terrestrial plants grow in an aerobic environment, that is in the open air. For this reason Keppler's finding that ordinary vegetation emits methane too came as a surprise to the scientific community. The Dutch consortium of researchers decided to revisit the issue, by bringing together a unique combination of expertise and facilities enabling the design and execution of a novel experiment. Plants were grown in a facility containing atmospheric carbon dioxide almost exclusively with a heavy form of carbon (13C). This makes the carbon released from the plants relatively easy to detect. Thus, if plants are able to emit methane, it will contain the heavy carbon isotope and can be detected against the background of lighter carbon molecules in the air:

Six plant species were grown in a 13C-carbon dioxide atmosphere, saturating the plants with heavy carbon: 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. 13C-Methane emission was measured under controlled, but natural conditions with a photo-acoustic laser technique. This technique is so sensitive that the scientists are able to measure the carbon dioxide in the breath of small insects like ants:
bioenergy :: biofuels :: energy :: sustainability :: greenhouse gas :: methane :: anaerobic :: aerobic :: vegetation :: biomass ::

Even with this state-of-the-art technique, the measured emission rates were so close to the detection limit that they did not statistically differ from zero (graph, click to enlarge). To our knowledge this is the first independent test which has been published since the controversy last year.

Conscious of the fact that a small amount of plant material might only result in small amounts of methane, the researchers sampled the ‘heavy’ methane in the air in which a large amount of plants were growing. Again, the measured methane emissions were neglible. Thus these plant specialists conclude that there is no reason to reassess the mitigation potential of plants. The researchers stress that questions still remain and that the gap in the global methane budget needs to be properly addressed.

The Dutch consortium included scientists from Plant Research International, IsoLife and Plant Dynamics in Wageningen, Utrecht University, and the Radboud University in Nijmegen.


Graph: Long-term steady-state methane emissions by vegetation. (a) Measured 13C-methane emissions (mean ± SE) by a mixture of 13C-enriched plants in the ESPAS (Experimental Soil Plant Atmosphere System) growth chamber under controlled steady-state conditions. Plant biomass increased from 289 (day 0) to 374 (day 6) g dry weight during the experiment (n = 3), and the emissions are given at the median of the time for accumulated emission. (b) Measured (solid line) and predicted (dashed lines) accumulation of methane by 13C-enriched plants in the ESPAS growth chamber. Measured methane concentrations (solid line, closed squares), and methane concentrations predicted from our continuous-flow experiment (Table 3; 21 ng g-1 h-1, dashed line, open triangles), or from Keppler et al. (2006: ‘sunlight’, 374 ng g-1 h-1, dot-dashed line, closed diamond; ‘no sun’, 119 ng g-1 h-1, dotted line, open squares). Courtesy: Nature.

More information:
Keppler F, Hamilton JT, Brass M, Rockmann T. "Methane emissions from terrestrial plants under aerobic conditions", Nature 439, 2006 Jan 12;439(7073):187-91.

Tom A. Dueck, Ries de Visser, Hendrik Poorter, Stefan Persijn, Antonie Gorissen, Willem de Visser, Ad Schapendonk, Jan Verhagen, Jan Snel, Frans J. M. Harren, Anthony K. Y. Ngai, Francel Verstappen, Harro Bouwmeester, Laurentius A. C. J. Voesenek and Adrie van der Werf, "No evidence for substantial aerobic methane emission by terrestrial plants: A 13C-labelling approach", New Phytologist. Article published online: 27-April-2007, doi:10.1111/j.1469-8137.2007.02103.x