Physical properties of Active layer Soil in Siberian wetland
Kunio WATANABE*, Masaru MIZOGUCHI*, Norihumi SATOU** and Yuji KODAMA***Faculty of Bioresources, Mie University**Institute of Low Temperature Science, Hokkaido University
INTRODUCTION
Soil and micro-landform of tundra wetland in Siberian permafrost area were surveyed during
the summer of 1997, and physical properties of soil horizons in active layer were measured. In thispaper, vegetation map and soil profiles have been reported, and soil genesis of the tundra wetland
was discussed on the basis of the survey.
STUDY SITES AND METHODS
The soil survey was carried out at a typical wetland located on a small river running through
Tikci, Sakha republic, Russia, from August 12 to September 10, 1997. Figure 1 shows avegetation and soil map of study area. Solid lines indicate the river and a divide, and arrows
indicate slopes. Sedges grew in the area where ground water level was low or the ground wasinundated, while mosses grew in the area where ground water level was a few centimeters.
Gravels spread at the elevation of higher than 150 m in stead of the vegetations.
Thickness of the active layer, ground temperature and vegetation type were measured along
the transit lines which were shown in figure 1 as dashed lines with 25 m intervals. On the transit
lines, 30 observation pits with size of 50×50 cm2 were dug to the frozen ground surface. Andhorizons, hydraulic conductivity, water content and soil structure were observed directly at the pits.
SOIL PROFILES
Soil of the active layer can be divided into four types according to vegetation and ground waterlevel, and each type of the soil was divided into upper and lower horizons.
Figure 2a shows the soil(a) which was seen in lowland with the water level of between -2 to
10 cm. Although the surface is covered by sedges, the covered rate was low. The upper horizon,
consisted of dark brown low moor, had thickness of 20 to 30 cm. A lot of plant residue remained
to be decomposed with low density and low viscosity in the upper horizon. On the other hand, thelower horizon consisted of gleyed heavy clay with very high viscosity. There was no clear
boundary between the upper and the lower horizons. In this season, the thickness of the active
layer was 40±10 cm. Frozen ground, strongly kept feature of mother rock, lay under the active
layer. Fine gravels were observed near thawing surface of the frozen ground.
Figure 2b shows the soil(b) which was seen in lowland without inundating. The groundwater level was 2 to 8 cm. Living plant layer consisted of mosses 1 to 10 cm in thick, and theupper horizon consisted of brown high moor 10 to 20 cm in thick with very low density, lowviscosity and rich humus. Some plant residue remained to be decomposed in the upper horizon.
The lower horizon consisted of gray heavy clay with a few board shaped gravels. There was clear

boundary between the upper and the lower horizons. The thickness of the active layer was 30±10cm.
Figure 2c shows soil(c) which was seen in middle-highlands with the ground water level of 2to 10 cm. Sedges covered the ground densely. The upper horizon was under a few centimetersplant residue layer and consisted of dark brown clayey soil with low density, middle viscosity andrich humus. There are some earthworms and moles. The lower horizon consisted of gray claywith gravels. There was clear boundary between the upper and the lower horizons. The thicknessof the active layer was 40±10 cm.
Figure 2d shows soil(d) which was seen in highlands where the elevation was higher than 150
m and the grand water level was lower than 50 cm. There were only piled up gravels under somelichens, and less horizontal soils were observed. The size of the gravels was decreased with depth.
The thickness of the active layer was more than 50 cm.
PHYSICAL PROPERTIES OF SOILS
Figure 3 shows profiles of temperature, water content, density and hydraulic conductivity insoil(c). The temperature in the living plant layer was about the air temperature. It decreased withdepth and reached 0 degree Celsius at frozen ground surface. Water contents were high in thesurface layer and the lower horizon while it was low in the upper horizon. On the other hand, bulk
densities were low in the surface layer and the lower horizons, while it was high in the upperhorizon. Hydraulic conductivity decreased with depth. Such profiles as soil(c) were also observedin soil(a) and soil(b), but did not observed in soil(d).
Except for soil(d), the surface layers had low densities and high hydraulic conductivity
because they had a lot of plant residues and high porosity. Since soil pores keep water, watercontents of the surface layers become high. The soil in the upper horizons was compacted andcontained rich humus, so that it had high density, low water content and low hydraulicconductivity. Because of disturbance by freeze-thaw effect, balk densities of the soil in the lower
horizons are lower than the upper horizons. Since, soil water is restricted to flow by the frozen
ground, the water contents of soil in the lower horizons become high. The hydraulic conductivity
in the lower horizons increased according to the amount of gravels.
DISCUSSIONS
Mother rocks were found isotropicaly in the study area. Assuming that climatic conditions
have been given isotropicaly, these soil feature would be related to only the ground water levels andthe vegetations. Upper schematic of figurer 4 indicates relations among elevation, ground waterlevel and thickness of active layers. It is supposed that the difference in the ground water levels
depends on the meter-scale difference of micro landform, and the active layer would be thick at thepoint which has low ground water level or has low elevation. It is also supposed that the thicknessof the active layer decreased with increasing the thickness of living plant layer due to adiabatic
effects by the plant layer.

Lower schematic of figure 4 indicates vegetations and soil cross section. The amount of peatincreased with decreasing the ground water level, and soil organisms were observed at the point
which had low ground water level. Soil horizons may be progress along the arrow as shown infigure 4. As a result of frost weathering of the mother rocks and freeze-thaw disturbance, clay
which has mother rock feature would spread all over the wetland and gravel-rich condition appear
near frozen ground surface.
CONCLUSION
Soil of active layer on tundra wetland was surveyed. Vegetation map and soil cross sections
have been reported in the present paper. As the result of the survey, four type of the soils wereclassified. On the basis of the soil classification, the vegetation and freeze-thaw action, soil
formation in tundra permafrost area has been discussed. It is important to understand these actual
soil conditions for make practical hydrological model.