Physical and Thermal Properties of Coarse-Fragment Soil in the Moraine-Talus Zone of the Qilian Mountains

Abstract

Moraine-talus zone (hereafter referred to as MTZ) refers to the non-glacial area (including glacier mass supply area) above the upper boundary of alpine meadow in high mountains, where vegetation is sparsely distributed and coarse fragment (diameter > 2 mm) is widespread. The MTZ acts as the headwater region for many large river basins, and the freeze–thaw process of its coarse-fragment soil largely affects regional hydrological processes, carbon exchange, and ecosystem diversity. However, our knowledge of the physical and thermal properties of coarse-fragment soil in MTZs remains limited. Mainly distributed in the surroundings of Hala Lake, the area ratio of the MTZ in the Qilian Mountains is about 21%. On the basis of 170 samples collected from 22 soil profiles at a depth of 80 cm, coarse fragments dominated the compositions of soil textures in the MTZ, with relatively high volume proportion (about 63.3%) and mass proportion (about 75.0%). The mean volume and mass ratio of the coarse fragments tended to increase gradually from the surface to the deep soils and varied largely at different particle size ranges (i.e., 2–5, 5–10, 10–20, 20–40, 40–60, and >60 mm). Thermal conductivity measurements for the 24 samples collected from three soil profiles in dry (Kdry) and water-saturated (Ksat) conditions indicated that Kdry increased rapidly with temperatures from −20 °C to 25 °C (with a 5 °C interval), which appeared to be closely related with soil porosity. Ksat did not obviously change with temperature at the two designated temperature ranges (i.e., from −20 °C to −5 °C and from 5 °C to 25 °C), but fluctuated largely at 0 °C, possibly due to the drastic phase change. More detailed experimental designs combined with more influential elements should be considered in future research to fully understand the thermal properties of coarse-fragment soil in the MTZ.