Impact of hydrology and vegetation community structure on CO2 and CH4 flux patterns in a permafrost ecosystem in Northeast Siberia

Abstract:

A large fraction of organic carbon stored in Arctic permafrost soil is to be decomposed and released to the atmosphere under climate change. Among many drivers that influence decomposition, changes in hydrology play a pivotal role: Shifts in water table depth (WTD) often trigger modifications on soil and vegetation (e.g. soil temperature and vegetation community structure), which in turn alter carbon cycle processes. The presented study is focused on CO2 and CH4 fluxes measured with chambers in a floodplain of the Kolyma River near Cherskii, Northeast Siberia. Our study site is separated into two areas, one that has been drained since 2004, and a nearby reference site. Carbon flux (NEE, ER, methane) was measured for ~16 weeks during summer and early winter of 2013, and summer of 2014. In addition, plant-mediated CH4 transport was measured in 2014 to separate different CH4 emission pathways. Vegetation community structure was investigated in 2013 and 2014. After a decade of drainage history that lowered WTD by about 20cm in the drained area, Eriophorum (cotton grass) that previously dominated have been much replaced by Carex (tussock-forming sedge) and shrub species. ANCOVA analysis revealed that NEE, ER, and methane flux rates were all influenced by vegetation in both summer and winter, leading to different flux patterns between two sites. WTD also influenced NEE and methane, showing little less CO2 uptake and much less CH4 emission in drained site. Plant-mediated CH4 transport through cotton grasses was correlated with diameter and length of green leaves, implying bigger plants transport more CH4 because their roots reach deeper soil layers where methanogenesis occurs. This correlation was strong at the drained site, where CH4 oxidation was dominant in shallow depths. Summarizing all effects of vegetation and WTD, the drainage results in a stronger net sink for carbon (CO2 and CH4 fluxes combined) in the growing season, but a stronger source in early winter.