Spatiotemporal variability of methane emissions of tundra landscapes in the Lena River Delta, Siberia

Increased methane (CH₄) release from a warming Arctic is expected to be a major feedback on the global climate. However, due to the complex effects of climate change on arctic geoecosystems, projections of future CH₄ emissions are highly uncertain. CH₄ emissions from complex tundra landscapes will be controlled not only by direct climatic effects on production, oxidation and transport of CH₄ but, importantly, also by geomorphology and hydrology changes caused by gradual or abrupt permafrost degradation. Therefore, improving our understanding of both the temporal dynamics and the spatial heterogeneity of CH4 fluxes on multiple scales is still necessary.

Here, we present pedon- and landscape-scale CH₄ flux measurements at two widespread tundra landscapes (active floodplains and late-holocene river terraces) of the Lena River Delta in the Siberian Arctic (72.4° N, 126.5° E). The dominating scales of spatial variability of soil, vegetation and CH₄ fluxes differ between the two landscapes of different geological development stage. The active floodplains are characterized by sandy beaches and ridges, and backswamp depressions, forming a mesorelief with height differences of several meters on horizontal scales of 10-1000 m. On the other hand, the river terraces are characterized by the formation of ice-wedge polygons, which lead to a regular microrelief with height differences of several decimeters on horizontal scales of 1 to 10 meters. CH₄ fluxes were investigated on the landscape scale with the eddy covariance method (15 campaigns during 2002-2018 at the river terrace, 2 campaigns 2014-2015 at the floodplain) and on the pedon scale with chamber methods (campaigns at different sites in 2002, 2006, 2013, 2014, 2015).

Average growing season (June-September) CH₄ flux for the floodplain was 166 ± 4 mmol m^-2 (n=2) and for the river terrace 100 ± 25 mmol m^-2 (n=15). There was pronounced spatial variability of CH₄ fluxes within both tundra landscapes types. On the river terrace, growing season CH₄ flux was only 20-40 mmol m^-2 at elevated polygon rims and polygon high centers, respectively, and up to 300 mmol m^-2 at polygon low centers. On the floodplain, CH₄ flux was as low as 5 mmol m^-2 at sandy ridges and above 400 mmol m^-2 in backswamp depressions. Mean growing season CH₄ fluxes at the river terrace were positively linearly correlated (r² = 0.9, n=15) to growing-degree-days (base temperature of 5 °C). Our findings suggest that a warmer climate stimulates the production of CH₄, which is directly reflected in increased CH₄ emissions. On the other hand, warming effects on CH₄ oxidation appear limited because transport processes that bypass the soil oxidation zone, i.e. plant-mediated transport and ebullition, dominate CH₄ emission from wet tundra landscapes. However, since CH₄ emissions strongly vary with (micro-)topographical situation within tundra landscapes, the changes of geomorphology and hydrology due to permafrost degradation will probably be the dominating driver of future CH₄ emissions from arctic tundra landscapes.