Measuring aerobic methane emission by plants
Introduction
Methane is a very potent greenhouse gas, thought to be partially responsible for global warming. Recent findings suggest that terrestrial plants may also emit substantial amounts of methane under aerobic conditions by a yet unknown physiological process (Keppler et al, 2006). The high emission rates might even account for the plumes of methane observed above tropical forests (Figure 1; Frankeneng et al, 2005). The Keppler paper sparked a discussion in the scientific community and their data are being used in global methane modelling (Bousquet et al, 2006). However, the discussions are based on short-term experiments in one single laboratory which have been criticised for the experimental set-up (Kirschbaum et al, 2006). Therefore, our aim was to re-examine their findings that terrestrial plants are able to emit methane under aerobic conditions. In general, a major problem in measuring methane emission is the high natural background concentration of about 2000 ppbv. Uniform labelling of the plants with 13C helps to solve this problem.
Figure 1. Difference between SCIAMACHY measurements from ESA’s ENVISAT satellite and TM3 model results. The largest discrepancies can be seen over tropical broadleaf evergreen forests. The model clearly underestimates the observed methane concentrations caused by hitherto unknown methane sources (Frankenberg et al, 2006).
Stable Isotope Solution: Uniformly 13C-labelled plants
Although the natural background of methane is as high as 2000 ppbv, the natural 13C-methane background is only 22 ppbv (i.e. 1.1 atom %). We therefore decided to cultivate 4 plant species (wheat, maize/corn, basil, tomato, common evening primrose, and sage) from seed for nine weeks in a 99%-enriched 13CO2 atmosphere (Dueck et al, 2007). In a first experiment, plants of basil, sage, wheat, and maize were transferred after 7 and 8 weeks to continuous flow gas exchange cuvettes for methane measurements using a laser-based detection technique. To increase the sensitivity of the measurements, we performed an experiment in our ESPAS 13C-labelling facility. Total plant biomass in the facility was about 30 times higher than in the exchange cuvettes and the incubation period was increased from 2 hours to 6 days. The facility was briefly vented after 7 and 9 weeks with ambient air to remove any possible accumulated methane. Subsequently, the facility was closed again and air samples were taken at two-day intervals during a six-day period for 13C-methane accumulation.
Results
In the first experiment with plant cuvettes containing 4- 12 g (dry weight) plant material, we measured methane emission rates for the four species varying from -10 to 42 ng-1 h-1, with an overall mean of 21 ng g-1 h-1. The emission rates were not statistically significant from zero. The detectable emission rate with the continuous flow system was 6-18 times lower than the average methane emission rates given by Keppler et al . (2006), i.e. 119 and 374 ng-1 h-1. The second, even much more sensitive experiment, showed a methane emission between -0.9 and 0.4 ng-1 h-1(Figure 2) which is only 0.1% and 0.3% of the emissions that would have been expected on the basis of the emission rates under ‘sunlight’ and ‘no-sun’ conditions respectively, reported by Keppler et al. (2006).
Figure 2. Long term steady state methane emissions.
A. Measured 13C-methane emissions by 13C-enriched plants in the ESPAS 13C-labeling facility. Plant biomass increased from 289 (day 0) to 374 (day 6) g dry weight.
B. Measured (solid line) and predicted (dashed lines) accumulation of methane by 13C-enriched plants in the ESPAS 13C-labelling facility. Measured methane concentrations (solid line, solid squares), and methane concentrations predicted from our continuous flow experiment (21 ng-1 h-1, dashed line open triangles), or from Keppler et al. (2006; sunlight, 374 ng-1 h-1, dashed line solid diamond, and no-sun, 119 ng-1 h-1, dashed line open squares).
We concluded that there is no evidence for a substantial emission of methane by terrestrial plants under aerobic conditions (Dueck et al, 2007). The methane emission source in tropical forest ecosystems thus remains to be a mystery.
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