Abstract
Abstract. Understanding the processes that influence and control carbon cycling in Arctic tundra ecosystems is essential for making accurate predictions about what role these ecosystems will play in potential future climate change scenarios. Particularly, air–surface fluxes of methane and carbon dioxide are of interest as recent observations suggest that the vast stores of soil carbon found in the Arctic tundra are becoming more available to release to the atmosphere in the form of these greenhouse gases. Further, harsh wintertime conditions and complex logistics have limited the number of year-round and cold-season studies and hence too our understanding of carbon cycle processes during these periods. We present here a two-year micrometeorological data set of methane and carbon dioxide fluxes, along with supporting soil pore gas profiles, that provide near-continuous data throughout the active summer and cold winter seasons. Net emission of methane and carbon dioxide in one of the study years totalled 3.7 and 89 g C m−2 a−1 respectively, with cold-season methane emission representing 54 % of the annual total. In the other year, net emission totals of methane and carbon dioxide were 4.9 and 485 g C m−2 a−1 respectively, with cold-season methane emission here representing 82 % of the annual total – a larger proportion than has been previously reported in the Arctic tundra. Regression tree analysis suggests that, due to relatively warmer air temperatures and deeper snow depths, deeper soil horizons – where most microbial methanogenic activity takes place – remained warm enough to maintain efficient methane production whilst surface soil temperatures were simultaneously cold enough to limit microbial methanotrophic activity. These results provide valuable insight into how a changing Arctic climate may impact methane emission, and highlight a need to focus on soil temperatures throughout the entire active soil profile, rather than rely on air temperature as a proxy for modelling temperature–methane flux dynamics.
Highlights
Active-layer soils and permafrost soils in the Arctic permafrost region contain significant stores of terrestrial organic carbon
To investigate the climatological influence of atmospheric forcing on soil cooling, Fig. S7 shows that 2014–2015 had the shortest cold season and the third-smallest freezing degree day (FDD) value on record
We further provided evidence that high cold-season methane emission occurs when deep soil horizons are insulated and temperatures remain above the point where methanogenesis is efficient, while cold surface soil temperatures simultaneously minimise methanotrophic activity
Summary
Active-layer soils and permafrost soils in the Arctic permafrost region contain significant stores of terrestrial organic carbon. Methanogenesis in peatlands is an obligate anaerobic process that takes place largely within deeper, anoxic layers of the soil, generally below the water table level (Le Mer and Roger, 2001). Methane production at these depths creates concentration gradients that lead to upward diffusion of methane through the soil to the surface (Preuss et al, 2013). Methane can be transported to the surface via ebullition and through aerenchymatous tissues within some vascular plants (Joabsson et al, 1999; Lai, 2009)
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