Physico-mechanical properties of automorphic taiga soils of the Komi Republic (according to rheological studies)

We carried out rheological studies of automorphic taiga soils formed at silty-loamy rocks in spruce forests to assess their structural-mechanical features determining soil resistance to the mechanical impact. Soil pits were made at five model sites located in taiga and forest-tundra zones (Table 1 and Fig. 1). Rheological measurements were performed using a module rheometer MCR 302 "Anton Paar" (Austria) by the method of amplitude sweep (oscillating method) using measurement systems plate-by-plate. Rheological properties were assessed according to the following parameters (Fig. 2): the modulus of elasticity (G'), the elastic deformation range -LVE-range (Linear viscoelastic), the modulus of viscosity (G''), and plastic deformation range - CROSSOVER (Flow point) measured using the point of modules crossover (G'=G''). We also investigated the main physico-chemical properties of soils: рН, content of organic carbon, exchangeable bases, oxalate-and dinitro-soluble forms of ferrum, and granulometric composition. Temperature regime of soils was studied using loggers "HOBO-U12" (USA) set at the depth of 0-20-50-100 cm. The clearest rheological features of taiga soils due to their hydrological and climate environment were observed in the upper mineral horizons. In the lower part of the soil profile, rheological parameters determined by physico-chemical and lithological soil properties were clearer. The most stable interparticle contacts with G' over 1.00 10⁶ Ра were noticed in the horizons with a high content of labile humic substances of fulvic origin and organic-mineral alphehumic compounds (Table 3 and 4). At site CS-I with soil Folic Albic Retisol, this was humus accumulative horizon AY. In the podzolic soil with podzol microprofile texture-differentiated soil of the middle taiga (site CS-II Folic Albic Retisol), it was illuvial horizon of podzol microprofile EL[hf]. In illuvial ferrous svetlozems (Folic Albic Stagnosols) of the northern (CS-III), far northern taiga (CS-IV) and forest-tundra (CS-V), these were horizons of podzol microprofile (Eg-BHF-BF). An increased stability of the structure was due to the cementation of soil particles as a result of alphehumic compounds intake with the following formation of strong interparticle contacts. Another reason was the seasonal freezing. The impact of freezing on rheological properties of soils was the clearest in svetlozems (Folic Albic Stagnosols). An increased stability of interparticle contacts was found in the area of zero veil (0±0.1°С) at the depth of 3050 cm due to condensation sealing of the particles caused by freezing. Moving to the north (from southern taiga to forest tundra), the stability of soil microstructure against mechanical damages decreased due to the formation of strong but fragile interparticle contacts. The mean value of the modulus of elasticity (G') increased from 7.95 10⁵ Ра (southern taiga, soil L-2) to 1.69 10⁶ Ра (forest tundra, soil Р-4-1). LVE-range values decreased when moving to the north (less than 0.00328%). Plastic deformation range CROSSOVER was less than 3.48%, which indicated an increased fragility of strong interparticle contacts. These strong but fragile contacts have low structural stability and demonstrate rapid destruction and slow recovering under extreme mechanical influences.

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