Revisiting factors controlling methane emissions from high-Arctic tundra

摘要

The northern latitudes are experiencing disproportionate warming relative tothe mid-latitudes, and there is growing concern about feedbacks between thiswarming and methane production and release from high-latitude soils. Studiesof methane emissions carried out in the Arctic, particularly those withmeasurements made outside the growing season, are underrepresented in theliterature. Here we present results of 5 yr (2006–2010) of automaticchamber measurements at a high-Arctic location in Zackenberg, NE Greenland,covering both the growing seasons and two months of the following freeze-inperiods. The measurements show clear seasonal dynamics in methane emission.The start of the growing season and the increase in CH₄ fluxes werestrongly related to the date of snowmelt. Within each particular growingseason, CH₄ fluxes were highly correlated with the soil temperature(R2 > 0.75), which is probably explained by high seasonality of bothvariables, and weakly correlated with the water table. The greatestvariability in fluxes between the study years was observed during the firstpart of the growing season. Somewhat surprisingly, this variability could notbe explained by commonly known factors controlling methane emission, i.e.temperature and water table position. Late in the growing season CH₄emissions were found to be very similar between the study years (except theextremely dry 2010) despite large differences in climatic factors(temperature and water table). Late-season bursts of CH₄ coincidingwith soil freezing in the autumn were observed during at least three years.The cumulative emission during the freeze-in CH₄ bursts was comparablein size with the growing season emission for the year 2007, and about onethird of the growing season emissions for the years 2009 and 2010. In allthree cases the CH₄ burst was accompanied by a corresponding episodicincrease in CO₂ emission, which can compose a significant contributionto the annual CO₂ flux budget. The most probable mechanism of thelate-season CH₄ and CO₂ bursts is physical release of gasesaccumulated in the soil during the growing season. In this study we discusspossible links between growing season and autumn fluxes. Multiannual dynamicsof the subsurface CH₄ storage pool are hypothesized to be such a linkand an important driver of intearannual variations in the fluxes, capable ofoverruling the conventionally known short-term control factors (temperatureand water table). Our findings suggest the importance of multiyear studieswith a continued focus on shoulder seasons in Arctic ecosystems.