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Effect of Soil Freezing on Physical and Microbiological Properties of Permafrost-Affected Soils

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Effect of Soil Freezing on Physical and Microbiological Properties of Permafrost-Affected Soils Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Effect of soil freezing on physical and microbiological properties of permafrost-affected soils W. M üller-Lupp & M. Bölter Institute for Polar Ecology, Kiel, Germany ABSTRACT: Soil systems consist of solid materials, liquids, and gases. In permafrost-affected soils ice causes sig- nificant changes in physico-chemical and microbial properties of the soil. The soil contains variable amounts of water, gas or ice, depending on ground temperature. Gas flux measurements were carried out in combination with other monitoring studies on Samoylov Island, Lena Delta, Siberia. Bi-directional freezing tests were conducted to observe freezing induced changes of soil properties and gas conductivity. The thermal regime, changes of unfrozen water content, and the radial stress state of the soil are logged automatically. The stresses recorded mainly depend on soil water/ice content and soil composition. A fine-grained soil reacts on cooling with cryosuction and contraction which leads to aggregation and the building of microfissures. Coarse-grained soils behave differently, because high pressures develop as a function of soil water content and freezing rate but without larger changes in soil structure. 1 INTRODUCTION spite of this fact, winter CO2-emission is reported by several authors (Soulides & Allison 1961, Coyne & The temperature amplitude in polar regions is respon- Kelley 1971, Zimov et al. 1993, Oechel et al. 1997, sible for cyclic freezing and thawing of the top layers Mast et al. 1998, Jones et al. 1999, Rivkina et al. 2000, of soils. Effects of subzero temperatures on soils influ- Skogland et al. 1988). Responsible processes are dis- ence a wide range of physical, chemical, and biologi- cussed controversially: as possible CO2-production cal processes. Deterioration of mineral particles and below freezing point or release of trapped soil gases water movement towards the freezing fronts have great through frost induced microfissures. influence on soil structure and its pore system. Soil Laboratory tests with cyclic freezing and thawing composition, soil water content, and the freezing rate soil cores and measurements of in situ potential CO2- are main factors influencing the freezing and thawing production at the investigation sites are carried out for behaviour of soil substrates. Soil composition deter- a better understanding of freezing induced changes mines the hydraulic conductivity of the substrate and of various soil structures and possible effects for soil the unfrozen water content below 0°C. The soil water gas flux. content sets the amount of transportable water as well as conductivity properties. The freezing rate deter- mines the time flow of accompanied processes. 2 MATERIAL AND METHODS Freezing in permafrost soils takes place bi- directional. On the one hand a freezing front advances 2.1 Study area from the soil surface downwards in the profile and on the other hand a freezing front moves upwards from In 1998 and 1999 field studies were carried out within the permafrost table towards the soil surface. the Russian-German cooperation project “Laptev Sea Cryostatic pressure may arise in the unfrozen part System 2000” on Samoylov Island in the Lena Delta, between the two advancing freezing fronts (for more East Siberia. The Lena Delta consists of ca. 1500 details see: Washburn 1979). islands and Samoylov Island is located in one of the A lot of research work is focused on freezing effects main river channels (Olenyok Channel) in the southern in soils on aggregation or aggregate stability (e.g., part of the delta (N 72° 22.22Ј; E 126° 28.54Ј). Hinman & Bisal 1968, Bisal & Nielsen 1976, Samoylov Island has a size of 1200 ha and can be Chamberlain & Gow 1979, Van Vliet-Lanoë et al. regarded as representative for the landscape in the 1984, Yong et al. 1985, Lehrsch et al. 1991, Lehrsch southwestern part of the Lena Delta. The island con- 1998). sists of two geomorphologic patterns, an erosion site in Cyclic freezing and thawing processes influence not the eastern part and an accumulation site in the western only physical properties of the soil, but also soil part. In the eastern part abrasion and erosion formed microorganisms are affected by the phase change from cliffs up to 8 m high and narrow beaches. Changing water to ice. As water turns into ice, water becomes the river water levels are responsible for different periods limiting factor for metabolism of microorganisms. In of sedimentation and delivery of sediments. Results are 801 strongly stratified soils. Soil texture is dominated by 2.3 Laboratory work sand, any coarser fractions are missing, which inhibits stronger frost sorting. The age of the oldest areas with Freezing tests were carried out with a new experimen- huge turf accumulation is estimated between 8000 and tal set-up. Soil cores (706 cm3, two replicates) were 9000 years (Grigoriev, pers. comm). These parts of the placed between two metal plates, covered with island are dominated by polygon structures (low-centre coolant cycles and fixed with PVC-plates to guaran- polygon). The hydrological situation with a restricted tee volume constance. 7 Thermistors (FF-U-V5-0, drainage of the shallow active layer (20–80 cm), low Grant) and 3 TDR-probes (Easy Test, Poland) were winter temperature (down to Ϫ40°C), and a thin snow installed in the middle of the probe, an uniaxial stress cover due to strong winds support frost cracking which transducer (Watzau, Germany) was placed on the out- results in polygonally patterned grounds with ice- side in the middle of the probe. Logging (Delta-T wedges. The active layer thickness varies with vegeta- Logger) intervals are 10 minutes. Freezing tempera- tion cover, exposition, substrate, soil temperature and ture varies between Ϫ5°C and Ϫ10°C. Figure 1 soil water content between 20 cm and 90 cm below soil shows a schema of the set-up. surface. Freezing tests were carried out with soil probes from the investigation sites. Table 1 represents the grain size distribution and site characteristics of the 2.2 Field work investigation plots. Values arise from a composite sample of the mineral soil (0–25 cm). Soil investigations were carried out at two representa- The troughs of the low-centre polygon (Glacic Aqui- tive sites, 1. a low-centre polygon (pedon of the trough: turbel) and the soil-wedge polygon (Typic Psammo- Glacic Aquiturbel) and 2. a soil-wedge polygon (Typic turbel) show only slight differences in their silt and Psammoturbel). The determination of carbon dioxide sand content. More distinct are the varieties in soil evaluation was performed in the field with the dynamic bulk density [g/cm3] and carbon content [% d. wt.]. CO gas exchange system (Walz & Co, Germany). The 2 Huge peat accumulations, due to restricted decompo- CO -analyser (Binos, Rosemount, Germany) consists 2 sition in the low-centre polygon lead to low soil bulk of an absolute CO -channel (0–2500 ppm) and a dif- 2 densities and higher values of carbon content. The ferential CO -channel (Ϫ50 ppm to ϩ50 ppm). A 2 fine-grained substrate of the Glacic Haploturbel closer description is given in Müller-Lupp (2002). originates from a centre of a mud boil and is used for Soil samples (10–20 g) from discrete soil layers comparison. (0–50 cm) were incubated at different temperatures, Data of in situ soil water content and soil bulk density related to ambient values (0°C to 20°C). Soil respira- Ϫ2 Ϫ1 are used for the setting of the experiments. tion data [␮gCO2 m d ] from Glacic Aquiturbel layers were combined with soil temperature measure- ments of a comparable tundra environment near Tiksi (GAME-Siberia Project). These data are split into temperature intervals which are regarded as different levels for microbial activity. Appropiate time spans for these temperature ranges were calculated for each month. By converting these data to area-related fig- ures we get potential data for CO2-production rates. Using the dynamic flow chamber method causes certain disturbances in the soil probe. Better aeration and drainage lead to a more favourable habitat for microorganisms. Thus obtained data show therefore the maximal potential CO2-production in defined soil parts (Bölter et al. in press). Figure 1. Experimental set-up. Table 1. Site characteristics and grain size distribution (data represent mixed probe (0–25 cm) of mineral soil. Clay [%] Silt[%] Sand [%] Soil bulk Site Ͻ2 ␮m2–63 ␮m63–2000 ␮m density [g/cm3] C-org [%] Glacic Aquiturbel 2.97 10.89 86.14 0.9–1.34 1.86 Typic Psammoturbel 3.91 19.59 76.50 1.48–1.58 0.79 Glacic Haploturbel 31.60 55.10 13.30 1.52–1.55 1.26 802 3 RESULTS increases in each freezing cycle. Pressure maximum, maintained only for a very short period of time, is fol- 3.1 Freezing experiments lowed by a complete pressure decrease. The freezing temperature of Ϫ3°C leads only to a slight pressure Figures 2 and 3 illustrate results of the experiments increase. After obtaining maximum values of 8 kpa, with different soil substrates. Figure 2 shows three pressure drops down to values around the starting freeze-thaw cycles of the coarse-grained Glacic Aqui- point. turbel. Soil water content is 39% w/w. Freezing tem- This is noticeable for each freezing cycle. Freezing perature in the sample is Ϫ3°C in the first cycle and temperatures of the second and third cycles corre- Ϫ6°C in the second and third cycles. Thawing tem- spond to each other approximately, but pressure max- perature is ϩ2°C. A decrease in temperature below ima vary. Figure 4 represents the soil sample after the the freezing point leads to an increase of pressure in freezing tests.
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