Temperature regime of peat deposit of ombrotrophic bogs in the forest-steppe zone of Western Siberia (the example of the Nikolayevsky ryam)

Ombrotrophic raised bogs in the forest-steppe zone of Western Siberia are at the extreme southern limit of their distribution and exist under conditions of insuffcient moisture and are surrounded by saline soils. In the changing climate, it is especially important to carry out regime observations of various components of such ecosystems. The aim of this research was to reveal peculiarities of temperature regime of fbrist histosol at a pine - dwarf shrub - Sphagnum raised bog in the Baraba forest-steppe of Western Siberia. We carried out the study at a pine - dwarf shrub - Sphagnum raised bog, Nikolaevsky ryam (55°08'59''N, 79°02'59''E ). An autonomous soil temperature measurement system (IMCES SB RAS, Tomsk) was used. Temperature was registered at depths of 2, 5, 10, 15, 20, 30, 40, 60, 80, 120, 160, 240 and 320 cm once per hour, the duration of the study was 4.5 years (05.10.2012-22.05.2017). We used data of Barabinsk meteorological station on air temperature, precipitations and snow cover depth ( See Table 1 ). Average annual temperature of the fbrist histosol in 2-20 cm layer was 4.7-5.1°C and it decreased at a depth of 240 cm to 3.3-3.7°C. The highest average monthly temperatures of the upper peat layer were 18.4-20.2°C in summer months, and the lowest ones were -2.0...-5.1°C and were recorded from November to February. Simultaneously, monthly air temperatures were -15...-26°C ( See Fig. 1 ). Average daily temperatures of the peat deposit were always minimal in November, reaching -11...-17°C due to the low thickness of thermal insulating snow cover. The annual soil temperature amplitude in the upper 20 cm ranged from 19 to 23°C in different years, and it did not change over the years below 40 cm and decreased from 12°C at a depth of 60 cm to 3°C at a depth of 240 cm ( See Fig. 2 ). Soil temperatures were always positive below 60 cm. The sum of positive average daily temperatures in the surface layer of the bog ranged from 2200 to 2400°C, at a depth of 20 cm it decreased by 200°C, and at a depth of 2.4 m the sum of positive temperatures decreased by 1000°C in comparison with the surface layer. The sum of negative average daily temperatures was -1660 ... -2100°С in different years of observations ( See Fig. 3 ). The dynamics of changes in the sums of negative temperatures for air and for peat soil do not coincide due to the change in thickness and time of establishment of snow cover and the different water table depths that have a signifcant role in the temperature regime of the soil in winter. The ratio of the sums of positive and negative temperatures at a depth of 2 cm (under a layer of live Sphagnum mosses) and 20 cm to the corresponding sums of air temperatures characterizes the soil warm-accumulating ability. During several years, these indicators decreased for positive temperatures and increased for negative ones, indicating an increase in soil climate severity ( See Table 2 ). Temperature gradients are positive when the heat flux flows from the soil to the atmosphere, and the peat deposit cools down; and negative when the heat flows in the opposite direction and the soil is heated. We observed the maximum temperature gradients in the layer of 15–30 cm, it is the largest negative value typical of May when the soil is intensively warmed, and they gradually decrease during the summer ( See Fig. 4 ). Freezing of the soil took place in October-November; at this time the freezing depth reached 40 cm, and then slowly descended during the winter. High water table standing and warm weather in autumn 2013/14 caused a decrease in the depth of freezing to 20 cm at the beginning of winter. In the spring, the thawing of peat deposit began immediately after the snow had melted and for a few days covered the entire freeze layer ( See Fig. 5 ). The duration of the frostfree period in the moss carpet (at a 2 cm depth) varied from 132 to 170 days, while the number of days with positive average daily temperatures was more stable, 161±5 days ( See Table 3 ). During the observation period, average summer temperatures, the sum of average daily positive temperatures and the sums of negative average daily temperatures (absolute values) increased, as well as average winter temperatures decreased and the depth of freezing lowered. The weather conditions of November were crucial for soil temperature regime during the winter due to air temperature and time of onset of snow cover with 15 cm thickness or more as well as the water table depth in autumn. In comparison with the same soils of the southern taiga, fbrist histosols in the forest-steppe were distinguished by an enlargement in acrotelm thickness and an increase in continentality of soil climate, which were due to the deeper water table. This is expressed in higher average values of summer temperatures, a greater depth of attenuation of daily temperature fluctuation, a more substantial decrease in average annual temperatures with the depth, greater annual amplitude along the profle, greater depth of maximum temperature gradients, an earlier seasonal permafrost melting, as well as a longer duration of periods with negative temperatures and a greater depth of freezing. The paper contains 5 Figures, 3 Tables and 27 References.