Increased CO2 fluxes under warming tests and soil solution chemistry in Histic and Turbic Cryosols, Salluit, Nunavik, Canada

Cryosols in tundra ecosystems contain large stocks of organic carbon as peat and as organic cryoturbated layers. Increased organic mater decomposition rate in those Arctic soils due to increasing soil temperatures and to permafrost thawing can lead to the release of greenhouse gases, thus potentially creating a positive feedback on global warming. Instrumentation was installed on permafrost terrain in Salluit (Nunavik, Canada; 62°14′N, 75°38′W) to monitor respiration of two Cryosols under both natural and experimental warmed conditions and to simultaneously monitor the soil solution composition in the active layer throughout a thawing season. Two experimental sites under tussock tundra vegetation were set up: one is on a Histic Cryosol (H site) in a polygonal peatland; the other one is on a Turbic Cryosol reductaquic (T site) on post-glacial marine clays. At each site an open top chamber was installed from mid-July to the end of August 2010 to warm the soil surface. Thermistors and soil moisture probes were installed both in natural (N), or non-modified, surface thermal conditions and in warmed (W) stations, i.e. under an open top chamber. At each station, ecosystem respiration (ER) was measured three times per day every second day with an opaque closed chamber linked to a portable IRGA. Soil solutions were also sampled every alternate day at 10, 20 and 30 cm depths and analysed for dissolved organic C (DOC), total dissolved nitrogen (TDN) and major elements. The experimental warming thickened the active layer in the Histic soil while it did not in the Turbic soil. In natural conditions, average ER at the HN station (1.27 ± 0.32 μmol CO2 m−2 s−1) was lower than at the TN station (1.96 ± 0.41 μmol CO2 m−2 s−1). A soil surface warming of 2.4 °C lead to a ∼64% increase in ER at the HW station. At the TW station a ∼2.1 °C increase induced an average ER increase of ∼48%. Temperature sensitivity of ER, expressed by a Q10 of 2.7 in the Histic soil and 3.9 in the Turbic Cryosol in natural conditions, decreased with increasing temperatures. There was no difference in soil solution composition between the N and W conditions for a given site. Mean DOC and TDN contents were higher at the H site. The H site soil solutions were more acidic and poorer in major solutes than the T ones, except for NO3-. The induced warming increased CO2 fluxes in both soils; this impact was however more striking in the Histic Cryosol even if ER was lower than in the Turbic Cryosol. In the Histic Cryosol, the thickening of the active layer would made available for decomposition new organic matter that was previously frozen into permafrost; due to acidic conditions, CO2 would be directly emitted to atmosphere. In contrast, the smaller increase in ER in the Turbic Cryosol may indicate the lack of organic matter input and carbon stabilization because of cold, non-acidic and more concentrated soil solutions; at this site warming mainly stimulates plant-derived respiration without decomposing a newly available carbon pool.