Landscape controls of CH4 fluxes in a catchment of the forest tundra ecotone in northern Siberia

Abstract

Terrestrial ecosystems in northern high latitudes exchange large amounts of methane (CH4) with the atmosphere. Climate warming could have a great impact on CH4 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 CH4 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 CH4 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 CH4. The magnitude of annual CH4 uptake was higher for soils without permafrost (1.19 kg CH4 ha(-1) yr(-1)) than for soils with permafrost (0.37 kg CH4 ha(-1) yr(-1)). In well-drained soils, significant CH4 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 CH4 (0.38 kg CH4 ha(-1) yr(-1)). Thermokarst ponds, which developed from permafrost collapse in bog plateaux, were hot spots of CH4 emission (approximately 200 kg CH4 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 CH4 emission of 3.8 kg CH4 ha(-1) yr(-1). Export of dissolved CH4 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 CH4 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 CH4 emissions that act as positive feedback to climate warming.