Much stronger tundra methane emissions during autumn freeze than spring thaw.

Warming in the Arctic has been more apparent in the non-growing season than in the typical growing season. In this context, methane (CH ₄ ) emissions in the non-growing season, particularly in the shoulder seasons, account for a substantial proportion of the annual budget. However, CH ₄ emissions in spring and autumn shoulders are often underestimated by land models and measurements due to limited data availability and unknown mechanisms. This study investigates CH ₄ emissions during spring thaw and autumn freeze using eddy covariance CH ₄ measurements from three Arctic sites with multi-year observations. We find that the shoulder seasons contribute to about a quarter (25.6 ± 2.3%, mean ± SD) of annual total CH ₄ emissions. Our study highlights the three to four times higher contribution of autumn freeze CH ₄ emission to total annual emission than that of spring thaw. Autumn freeze exhibits significantly higher CH ₄ flux (0.88 ± 0.03 mg m ^-2  hr ^-1 ) than spring thaw (0.48 ± 0.04 mg m ^-2  hr ^-1 ). The mean duration of autumn freeze (58.94 ± 26.39 days) is significantly longer than that of spring thaw (20.94 ± 7.79 days), which predominates the much higher cumulative CH ₄ emission during autumn freeze (1,212.31 ± 280.39 mg m ^-2  year ^-1 ) than that during spring thaw (307.39 ± 46.11 mg m ^-2  year ^-1 ). Near-surface soil temperatures cannot completely reflect the freeze-thaw processes in deeper soil layers and appears to have a hysteresis effect on CH ₄ emissions from early spring thaw to late autumn freeze. Therefore, it is necessary to consider commonalities and differences in CH ₄ emissions during spring thaw versus autumn freeze to accurately estimate CH ₄ source from tundra ecosystems for evaluating carbon-climate feedback in Arctic.
(© 2020 John Wiley & Sons Ltd.)