Freeze-thaw processes of active layer soils in the Nanweng'he River National Reserve in the Yili'helishan Mountains, northeastern Northeast China

English Abstract:

Abstract: The Nanweng'he River National Reserve (125° 07′55″--125°50′05″E, 51°05′07″-- 51°39′24″N) is located at the southeastern margin of the Yile'hulishan Mountains, the joint of the Da and Xiao Xinganling (Hinggan) mountains. The freeze-thaw processes of the active layer soils are important for local ecosystems in a boreal environment, dominated by widespread needle-leaved forests and extensive wetlands. These processes have been substantially affected by the vegetation,climate warming and anthropogenic activities. Based on the analyses of data on the temperatures and unfrozen water content of active layer soils from 1 September 2011 to 1 September 2014 at two different permafrost sites(the active layer soils in shrub land and wetland), the freeze-thaw process of soils in the active layer were compared and contrasted. At two studied sites, The temperatures at the top of permafrost (TTOP) , i.e., the thaw penetration, were -4.2℃ at the wetland site, and -1.9℃ at the shrub land site. Difference in the surface condition of the vegetation cover and TTOPS may be responsible for the differentiations in the freeze-thaw processes of soils in the active layer. With rising TTOPs, ground thawing started earlier: shrub cover site on May 1, and wetland site on May 5, while the freezing began later: shurb cover site on October 5, and wetland site on September 19. With increasing TTOPs, the freeze-up periods active layer for permafrost sites were shortened: 243 days at wetland site and 202 days at shrub cover site, and the period of complete thaw was 137 days (active layer thickness or ALT : 100 cm) at wetland site and 120 days (ALT : 140 cm) at shrub cover site. At the two sites, the two-directional ground freezing(downwards from ground surface and upwards from the permafrost table) and one-directional thawing finished in the same year. At each studied sites, with rising TTOPs , the downward freezing accelerated in comparison with the upward freezing, and with an increasing proportion of downward frozen depth, and larger ratios of freezing to thawing duration. In summary, the pattern of freeze-thaw processes in the active layer in Nanweng'he differ markedly from those in other elevational permafrost regions in China, but they display a similar pattern to those in boreal or subarctic environment.