Flood effect on CH4 emission from the alas in Central Yakutia, East Siberia

Abstract

During four years (2006–2009), methane (CH4) emission was measured at different biomes (dry, wet grasslands, lake and lake vegetation) of mature thermokarst depression located at the most typical thermokarst terrain on the east bank of the Lena River, Central Yakutia, Russia (62°08′N, 130°30′E). To estimate total CH4 emission from the whole thermokarst depression ecosystem, CH4 emissions via plant bodies and ebullition were measured in addition to diffusive CH4 flux measurement. The lake area increased twice from 20.4 ha in 2006 to 43.3 ha in 2007 and then did not change significantly in 2008 and 2009 (46.5 and 44.4 ha, respectively). Ebullition was considered to be a minor source for CH4 emission from the lake in the studied thermokarst depression. CH4 emissions from the lake water surface and via the plant body of lake vegetation (hygrophyte and hydrophyte vegetation) were the main sources of CH4 and these increased by flooding both CH4 emission rate and area. Using spatial changes of these biomes, the annual emission of CH4 was calculated taking into account different sources of CH4. Total CH4 emission from the studied alas (63.7 ha) was 5.7, 5.2, 20.1 and 50.1 Mg carbon (C) in 2006–2009, respectively, and its difference during this period reached about 10 times. An extreme increase in CH4 emission from the lake occurred in the second year of continuous flooding (2008), which might have been caused by the decomposition of flooded organic C. So, the lake water ecosystem is the main source of CH4 in thermokarst depression controlled by the duration of flooding. Under future global climate change, thermokarst depressions in Central Yakutia have potential for lake expansion, causing significant increase in CH4 emission in the studied region.