Methane emission in the Arctic Coastal Plain, Alaska

Methane is an effective greenhouse gas that has a warming potential about 23 times that of CO2. The wetland regions of the world, especially the ones in the boreal and low Arctic, are major accumulation sites of organic materials under anaerobic conditions, and could be a significant source of global CH4 under a warming climate. The large majority of measurements of methane emission from the terrestrial biosphere have been made using flux chambers. Such measurements result in discrete samples in time and space and can cause a significant disturbance to surface integrity and transient soil-atmosphere pressure transients thereby impacting the apparent methane fluxes. In our experiment we utilized a new device, the LI_COR open-path methane test-bed instrument (TBI; LI-COR Biosciences, Lincoln, Nebraska) for eddy covariance flux measurements, and a closed-path TBI for gradient measurements,. These new instruments allowed continuous measurements, were non-invasive, and allowed more accurate estimation of methane fluxes including during the entire snow free period. The longer measurement period is particularly important because most of the past Arctic measurements were conducted during short periods in June or July (the peak growing season in the Alaskan arctic tundra) assuming that higher soil temperatures of summer would result in elevated methane emissions. Our results suggest that this is not necessarily the case. Continuous measurements revealed considerable daily and seasonal variation in methane emission rates from the Arctic tundra, and the air and soil temperature were sometimes high in the late season and/or during the “nighttime” period. Also observed were sudden and significant release of methane, which have not been captured by intermittent chamber measurements. In fact, our experiment showed high methane efflux at night that during July was on average 2.8 mg m-2 hr-1. We hypothesize that this high methane efflux at night is connected to low wind speeds since low windspeeds may result in a decrease in oxygen penetration in the soil and decreased methane oxidation leading, in tern, to higher methane effluxes. An inverse relationship between of wind speed and observed methane efflux is particularly relevant since traditional chambers and other non-atmospheric measurements (e.g., bubble traps) minimize wind effects and could lead to artificially high methane emission estimations. Also, lack of established diurnal pattern in methane flux could lead to a large error in methane estimation from non-continuous sampling, because the highest or lowest methane efflux may occur at an unpredictable time of the day, and thus, could be missed. Another major factor responsible of the high emissions at night could be soil temperature which, at -10cm depth,did not show any significant decrease at night. Warm temperatures at depth could result in continued methane production in the night, while cool surface temperatures at night might have resulted in reduced methane oxidation at the surface. This depth (-10 cm) has been reported to be the peak of methane production in a subarctic mire (Svensson and Rosswall, 1984). Our results suggest that the soil temperature (at -10 cm) and the wind speed are major controls responsible of methane emission from arctic tundra.

Bibliography

Svensson, B. H., T. Rosswall. 1984. In situ methane production from acid peat in plant communities with different moisture regimes. Oikos. 43:341–350