Landscape controls of CH 4 fluxes in a catchment of the forest tundra ecotone in northern Siberia

Abstract

Terrestrial ecosystems in northern high latitudes exchange large amounts of methane (CH 4 ) with the atmosphere. Climate warming could have a great impact on CH 4 exchange, in particular in regions where degradation of permafrost is induced. In order to improve the understanding of the present and future methane dynamics in permafrost regions, we studied CH 4 fluxes of typical landscape structures in a small catchment in the forest tundra ecotone in northern Siberia. Gas fluxes were measured using a closed‐chamber technique from August to November 2003 and from August 2006 to July 2007 on tree‐covered mineral soils with and without permafrost, on a frozen bog plateau, and on a thermokarst pond. For areal integration of the CH 4 fluxes, we combined field observations and classification of functional landscape structures based on a high‐resolution Quickbird satellite image. All mineral soils were net sinks of atmospheric CH 4 . The magnitude of annual CH 4 uptake was higher for soils without permafrost (1.19 kg CH 4 ha −1 yr −1 ) than for soils with permafrost (0.37 kg CH 4 ha −1 yr −1 ). In well‐drained soils, significant CH 4 uptake occurred even after the onset of ground frost. Bog plateaux, which stored large amounts of frozen organic carbon, were also a net sink of atmospheric CH 4 (0.38 kg CH 4 ha −1 yr −1 ). Thermokarst ponds, which developed from permafrost collapse in bog plateaux, were hot spots of CH 4 emission (approximately 200 kg CH 4 ha −1 yr −1 ). Despite the low area coverage of thermokarst ponds (only 2.1% of the total catchment area), emissions from these sites resulted in a mean catchment CH 4 emission of 3.8 kg CH 4 ha −1 yr −1 . Export of dissolved CH 4 with stream water was insignificant. The results suggest that mineral soils and bog plateaux in this region will respond differently to increasing temperatures and associated permafrost degradation. Net uptake of atmospheric CH 4 in mineral soils is expected to gradually increase with increasing active layer depth and soil drainage. Changes in bog plateaux will probably be much more rapid and drastic. Permafrost collapse in frozen bog plateaux would result in high CH 4 emissions that act as positive feedback to climate warming.

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