Floodplains are one of the most dynamic and youngest areas of the Earth's Quaternary surface. They are located in transitional conditions (land-ocean) of the permafrost zone of present and of particular interest for ongoing geochemical processes and soil/water balance. The soil thermal and water regimes of polar soils are crucial for the development of vegetation cover as well as production, accumulation and redistribution of organic matter. This work characterizes the hydrological properties of soils formed in Russian Arctic. The data showed differences in water holding capacity between soils formed in conditions of seasonal flooding (soil stratification, redistribution of organic and mineral matter through the soil profile) and those not influenced by flooding in Lena River Delta (gradual decreasing of water holding capacity as a function of depth). Both of the soil profiles from the Yamal Peninsula are characterized by a gradually decreasing water-holding capacity with depth. The hydrological regime characteristics were strongly related to the depth of the active layer. The intensity and rate of the thawing/freezing processes depends on the features of the hydrological regime. In this study, significant differences were noted in the soil characteristics of the two study areas. That is why the profile values of water-holding capacity differed among the study sites. The predicted global climate change and high sensitivity of Arctic ecosystems may lead to significant changes in permafrost-affected landscapes and may alter their water regime in a very prominent way, as permafrost degrades and lateral and vertical water flow in the basins of large arctic rivers changes.

The large amounts of soil organic matter (SOM) in permafrost-affected soils are prone to increased microbial decomposition in a warming climate. The environmental parameters regulating the production of carbon dioxide (CO2) and methane (CH4), however, are insufficiently understood to confidently predict the feedback of thawing permafrost to global warming. Therefore, the effects of oxygen availability, freezing and thawing, temperature, and labile organic matter (OM) additions on greenhouse gas production were studied in northeast Siberian polygonal tundra soils, including the seasonally thawed active layer and upper perennially frozen permafrost. Soils were incubated at constant temperatures of 1 degrees C, 4 degrees C, or 8 degrees C for up to 150 days. CO2 production in surface layers was three times higher than in the deeper soil. Under anaerobic conditions, SOM decomposition was 2-6 times lower than under aerobic conditions and more CO2 than CH4 was produced. CH4 contributed less than 2% to anaerobic decomposition in thawed permafrost but more than 20% in the active layer. A freeze-thaw cycle caused a short-lived pulse of CO2 production directly after re-thawing. Q(10), values, calculated via the equal-carbon method, increased with soil depth from 3.4 +/- 1.6 in surface layers to 6.1 +/- 2.8 in the permafrost. The addition of plant-derived labile OM (C-13-labelled Carex aquatilis leaves) resulted in an increase in SOM decomposition only in permafrost (positive priming). The current results indicate that the decomposition of permafrost SOM will be more strongly influenced by rising temperatures and the availability of labile OM than active layer material. The obtained data can be used to inform process-based models to improve simulations of greenhouse gas production potentials from thawing permafrost landscapes. (C) 2017 The Authors. Published by Elsevier Ltd.