Partial degradation of the soil complex of Quaternary deposits for the last 12-18 thousand years on the plains in Central Yakutia resulted in the formation of lots of thermokarst depressions called alasses. Here their number approximates 16000 at the occupied area over 4 400 sq.km. In places about 30% of the total area falls to the alasses and together with taiga environs they create a unique taiga-alas type of landscape. Alas meadows serve as the primary fodder reserve to develop the northern livestock breeding.
Since 1990 the study of the temperature and water regimes of soils on the typical taiga-alas landscapes is being conducted. Soil freezing conditions are fairly mosaic depending on the location, wettening rate and type of plant cover.The fastest chilling and strong freezing is observed in less moistened soils located on taiga cleaned up sites and xerophytic alas meadows. Soil freezing on wet meadows adjacent to lakes and on alasses near the slopes in the beginning of winter is slower due to the great amount of concealed heat in the soil water. Optimally moistured soils under forests and mesophytic meadows take an intermediate position. Soil thickness accumulates negative temperatures until mid February. Then the temperature rise in the nearground atmosphere layer causes a gradual expense of cold accumulated by soils during the winter period. Surface soil layers thaw out in the first 10-day period of May. Seasonal soil thawing may reach 1.2-3.0 m thick, lasting until the first frosts in early September.
For the period of observation the total amount of evaporation in the warm season of the year fluctuated from 133 to 192 mm in the larch forest averaging 163 per a season. For the same time the water loss from a ploughed field situated nearby on the cleaned up taiga site averaged 215 mm (from 170 to 260 mm), i.e by 132% more than in the larch. Still more water loss was registered on the alas meadows: xerophytic - 225 mm (mean), mesophytic - 250 mm and wet- 320 mm for a season that exceeds total evaporation in the forest by 138, 153, and 196%, accordingly.
R.V. Desyatkin Yakutsk Institute of Biology SD RAS Yakutsk 677891, Russia Tel.: 3 54 93, 4 32 67 fax: 095 230 29 19 ogai@nauka.yacc.yakutia.su