Temperature regime of Fibrist Histosols of the Bakchar Mire (West Siberia)

Abstract

The aim of the study. Observations of the thermal regime of peat soils were carried out in three ecosystems of the complex Bakcharsky mire (56°50'N, 82°50'E): in an more drained raised bog with pine-shrub-Sphagnum community (ryam) and two lowered waterlogged ecosystems – sedge-Sphagnum poor fen (sedge mire) and shrub-cotton grass-Sphagnum marginal part of the poor fen adjacent to the ryam (cotton grass mire). Thermal regime of fibrist histosols was studied using temperature loggers – autonomous soil temperature meters AIPT (IMCES SB RAS, Tomsk). Loggers were installed into unflooded hummocks of 30–40 cm height in the ryam and sedge mire, and in the flooded leveled hollow of the cotton grass mire on June 26, 2012. The temperature was recorded until September 18, 2013, and in the cotton grass mire it was recorded until May 21, 2013, when the equipment failed due to high water table. The recording of temperature was carried out at depths of 2, 5, 10, 15, 20, 30, 40, 60, 80, 120, 160, 240 cm once per hour. To assess the weather conditions during the study period, we used data on air temperature, precipitation and snow depth of the Bakchar weather station (http://rp5.ru).

Main results. The average annual temperature in the peat soil of the ryam decreased from 3.9˚C in the surface layer to 2.7˚C at a depth of 160 cm and increased to 3.0˚C at a depth of 240 cm; in the sedge mire it increased from 3.4˚C in the surface layer to 6.7˚C at a depth of 160 cm and decreased to 6.2˚C at a depth of 240 cm. In the cotton grass mire, average temperature slightly increased from the surface to the deepest layers at 0.15˚C, averaging 4.7˚C. The annual temperature amplitude at a depth of 20 cm was slightly higher than 15˚C in the ryam and the cotton grass mire, which indicated soil climatic conditions as mild, and 21.6˚C in the sedge mire, which complied with the temperate continental soil climate. The average temperature during the warm period (June–September) in the sedge mire was 1.5–3.1˚C warmer than in the cotton grass mire. In the ryam the average temperature of the warm period in the upper 2–5 cm was the same as in the sedge mire, but sharply decreased with depth, unlike both poor fens. During the cold period (October–March), average temperature increased with depth from –1 ... –4 ˚С to 3 ... 6 ˚С, and the average value became positive at a depth of 25–30 cm in the sedge mire and the ryam, and at a depth of 13 cm in the cotton grass mire, which reflects the different water table depth at different elements of the microrelief. The delay of the heat wave was most pronounced in the ryam: at a depth of 240 cm, the maximum average monthly temperatures was in November, in poor fens it was in September–October. The minimum monthly temperatures was recorded in December 2012 in all ecosystems, whereas negative temperatures penetrated to a depth of 30 cm in the ryam, to a depth of 15 cm into the sedge mire, and to a depth of 5 cm into the cotton grass mire in accordance with the water table depth. Daily summer temperature fluctuations penetrated to a depth of 20 cm in the cotton grass mire, 30 cm in the ryam and 40 cm in the sedge mire. The highest temperature values were observed on July 20, 2012: at a depth of 2 cm it was 34.5˚C in the ryam, 30.0˚C in the sedge mire and 26.8˚C in the cotton grass mire, which reflects an increase in water table from the ryam to the cotton grass mire. According to the Bakchar weather station, on that date the maximum air temperature reached 35.0˚C. Minimum daily temperatures were recorded in November in all ecosystems at a depth of 2 cm: –18.0˚C in the sedge mire on November 26, –4.9˚C in the cotton grass mire on November 8, and –10.0˚C in a the ryam on November 9. The difference between ecosystems is associated with the retention of snow by trees and shrubs, its demolition into hollows of the cotton grass mire and blowing off from hummocks in the sedge mire. As snow cover increased during the winter, daily temperatures increased to daily average values of –5.3, –2.3, and –1.4˚C in the sedge mire, the ryam, and the cotton grass mire, respectively. The sum of the positive average daily temperatures for the hydrological year 2012/13 was higher in the sedge mire (2156˚C and 2280˚C at a depth of 2 and 20 cm) compared with the ryam (1906˚C and 1474˚C at a depth of 2 and 20 cm). The sum of negative temperatures at a depth of 2 and 20 cm was –485˚C and –188˚C in the ryam, –872˚C and –378˚C in the sedge mire, –225˚C and –67˚C in the cotton grass mire. The ratio of the positive temperature sums at the depth of 20 cm to the positive air temperature sums, characterizing the degree of soil heatability, amounted to 0.62 for the ryam and 0.96 for the sedge mire. Temperature gradients penetrate to the deepest layers of the soil profile in both poor fens and become equal to 0˚C·cm-1 below 60 cm in the ryam. Gradients were minimal in the ryam. Seasonal frost penetrated to a depth of 59 cm in the sedge mire, 44 cm in the cotton grass mire and 53 cm in the ryam, and the duration of stable soil frost in the surface layer was 171, 157 and 181 days, respectively. Thus in the peat soil of the cotton grass mire the difference between cold and warm seasons is less pronounced, while the sedge mire soil is characterized by the more contrasting and warmer climate, and the ryam soil is more contrasting and cold. The revealed difference in the soil thermal regimes is associated with the microrelief, which determined the water table depth and redistribution of snow cover. Hummocks and lower standing water table in the ryam and the sedge mires as compared with the cotton grass mire contributed to better warming in the summer and greater freezing in the winter.

Conclusion. The thermal regimes of ryams and poor fens of the Bakcharsky mire complex differ according to the results of this and other studies, which is due to the placement of temperature loggers on different elements of the microrelief and, possibly, mesoclimatic differences.