Multi-year effect of wetting on CH 4 flux at taiga-Tundra boundary in northeastern Siberia deduced from stable isotope ratios of CH 4

Abstract

The response of CH 4 emission from natural wetlands due to meteorological conditions is important because of its strong greenhouse effect. To understand the relationship between CH 4 flux and wetting, we observed interannual variations in chamber CH 4 flux, as well as the concentration, δ 13 C, and δD of dissolved CH 4 during the summer from 2009 to 2013 at the taiga-tundra boundary in the vicinity of Chokurdakh (70°37′ N, 147°55 E), located on the lowlands of the Indigirka River in northeastern Siberia. We also conducted soil incubation experiments to interpret δ13C and δD of dissolved CH 4 and to investigate variations in CH 4 production and oxidation processes. Methane flux showed large interannual variations in wet areas of sphagnum mosses and sedges (36-140 mgCH 4 m -2 day -1 emitted). Increased CH 4 emission was recorded in the summer of 2011 when a wetting event with extreme precipitation occurred. Although water level decreased from 2011 to 2013, CH 4 emission remained relatively high in 2012, and increased further in 2013. Thaw depth became deeper from 2011 to 2013, which may partly explain the increase in CH 4 emission. Moreover, dissolved CH 4 concentration rose sharply by 1 order of magnitude from 2011 to 2012, and increased further from 2012 to 2013. Large variations in δ13C and δD of dissolved CH 4 were observed in 2011, and smaller variations were seen in 2012 and 2013, suggesting both enhancement of CH 4 production and less significance of CH 4 oxidation relative to the larger pool of dissolved CH 4 . These multi-year effects of wetting on CH 4 dynamics may have been caused by continued soil reduction across multiple years following the wetting. Delayed activation of acetoclastic methanogenesis following soil reduction could also have contributed to the enhancement of CH 4 production. These processes suggest that duration of water saturation in the active layer can be important for predicting CH 4 emission following a wetting event in the permafrost ecosystem.