Spatial and Temporal Variability of Methane Mole Fractions and Exchanges in and Between Soil, Snow, and the Atmosphere in a Tundra System in Northern Alaska

Abstract:

An important global source of atmospheric methane (CH₄) is production in tundra soils (an important global source). To place constraints on the potential role that tundra soils play in global CH₄ cycling, we have been continuously measuring mole the air space in soils, snow, and the atmosphere as gradient-based surface-atmosphere fluxes for arctic tundra at Toolik Field Station (68° 38' N) starting in October 2014. We have found that atmospheric CH₄ mole fractions were, on average, relatively constant during the first 9 months of sampling (averaging 1.93 µmol mol⁻¹), with pronounced diel patterns starting in May and nighttime exceeding daytime mole fractions. However, gradients measured within the soil profile showed high variability in air withdrawn from different locations of these tundra soils (Typic Aquiturbels), with one soil profile indicating a CH₄ sink during fall until January; mole fractions were similar to the atmospheric measurements during winter indicating no source or sink (average 1.89 µmol mol⁻¹). A second soil profile 5 m away showed production of CH₄ (average 2.48 µmol mol⁻¹, two-times higher than atmospheric levels), even during mid-winter when soil temperatures were below −10 °C. Measurements of CH₄ in interstitial snowpack air also exhibited a similar combination of sources and sinks. We used micrometeorological gradient surface flux measurements to confirm that the area was a net source of CH₄ in fall, winter, and spring, with emissions averaging 26.6, 25.2, and 16.8 mg m⁻² d⁻¹, respectively. In the summer months, we saw strong diel flux patterns with deposition during day and emission at night, corresponding with observed diel variability in CH₄ snowpack mole fractions. Our results indicated a high variability of tundra landscape CH₄ fluxes, which locally shift from sources to sinks with high temporal variability. CH₄ oxidation by methanotrophic bacteria probably occurs in tundra soils, confirming observations in one soil, snowpack, and the atmosphere during spring, but oxidation ceased during the coldest months. In close proximity, we suggest a CH₄ production by methanogenic archaea, and surprisingly this production continued throughout winter even during the coldest periods showing little temperature dependence.