Temporal Variation of Ecosystem Scale Methane Emission From a Boreal Fen in Relation to Temperature, Water Table Position, and Carbon Dioxide Fluxes

• Published in: Global biogeochemical cycles Vol. 32; no. 7; pp. 1087 - 1106
• Main Authors: Rinne, Janne, Tuittila, Eeva‐Stiina, Peltola, Olli, Li, Xuefei, Raivonen, Maarit, Alekseychik, Pavel, Haapanala, Sami, Pihlatie, Mari, Aurela, Mika, Mammarella, Ivan, Vesala, Timo
• Format: Journal Article
• Language: English
• Published: 01.07.2018
• Subjects: carbon dioxide; flux; greenhouse gas; methane; peatland; wetland
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2017GB005747

We have analyzed decade‐long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and carbon dioxide exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July–August with average value of 73 nmol m−2 s−1. Wintertime fluxes were small but positive, with January–March average of 6.7 nmol m−2 s−1. Daily average methane emission correlated best with peat temperatures at 20–35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable‐slope generalized linear model (r2 = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process‐based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m−2 and was 2.7 ± 0.4% of annual GPP. Over 10‐year period methane emission was 18% of net ecosystem exchange as carbon. The weak relation of methane emission to water table position indicates that space‐to‐time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales. Plain Language Summary Methane emission from a boreal wetland was measured over one decade. Methane emission shows strong seasonal cycle, with highest emission in late summer and lowest emission during winter. No diel cycle was observed. The methane emission is an important part of the carbon balance of the wetland as 18% of carbon taken up as carbon dioxide was emitted back into atmosphere as methane. The seasonal cycle of the emission was controlled first by peat temperature and second by ecosystem photosynthesis. The interannual variability of methane emission was more related to photosynthesis. A large part of the interannual variability remained unexplained by the measured environmental parameters. Key Points Methane emission was insensitive to water table variations observed during decade‐long period Soil temperature was the dominant driver of methane emission during shorter periods of time (<1 year), while interannual variation was more related to variation in GPP Seventy‐nine percent of the variability of daily average methane emission is due to seasonal change of drivers, 7.0% is due to interannual variation of drivers, and 5.3% is due to functional change