Modern Functioning of Surface Paleosols of the Russian Plain as Related to Lateral Redistribution of Heat Fluxes in the Upper Soil Layers.
Tatiana A. Arkhangelskaya, Moscow State Univ, Faculty of Soil Science, 119992 Leninskie Gori, Moscow, Russia
Surface paleosols of the Russian Plain formed on loess mantles covering high slightly undulating watersheds were studied on a gentle (1°) slope (125-127 m a.s.l.) 200 km east of Moscow. The terrain is a part of late Pleistocene periglacial area, so the soilscape is deeply influenced by paleocryogenic processes. The abundance of relic features allows regarding studied soils as surface paleosols. Soils with thick black second humus horizon (Pachic Argiborolls), that is a former surface horizon of a cryohydromorphic soil, have been formed within the thermokarst depressions. While soils with high (80-120 cm) carbonate horizon and without second humus horizons (Typic Eutroboralfs) have been formed on the interfluves. The relic soilscape is now passing through the agrogenic stage of its evolution that resulted in almost complete smoothing of soil surface and transition of the ancient relief into a buried form. Therefore, surface paleosols are now functioning under the conditions of quite a different surface topography as compared with the initial one. The modern surface provides similar insulation and precipitation for the areas with different soils. Nevertheless, the heterogeneity of the soilscape remains the same that requires a special consideration. The modern soil profiles include the Ap-Ah-AE-BEg-EB-B-C horizons in former depressions, the Ap-B-Bca-Cca horizons on former interfluves, and the Ap-EB-B-C-Cca horizons on transient slopes. The study of modern functioning of these soils permits to assume that existing heterogeneity of soilscape is partly ensured by differences in soil thermal properties that are responsible for the spatial variability of soil thermal regimes. The thermal diffusivities of horizons which occur under the arable layer in profiles of different soils are quite different. The extremely low density and a very high organic matter content of Ah horizon account for its lowest thermal diffusivity. The Ap, Ah, AE, BEg and EB horizons belong to the clay-depleted part of soil profile, and the low clay content in the EB horizon is the reason for its higher thermal diffusivity as compared with that of the B horizon, though the B and EB horizons are quite similar in density and organic matter content. Thus the B horizon demonstrates the medium thermal diffusivity, and the highest thermal diffusivity corresponds to the EB horizon. Hence the areas with high and low thermal diffusivities of subsurface horizons alternate in the soilscape, and this alternation results in development of lateral heterogeneities in soil temperature. The low thermal diffusivity makes the second humus horizon behave like thermo-insulating lenses distributed under the arable layer in the soils of former depressions. These soils are warmed and cooled slower; their temperature manifests a higher inertia and a less amplitude of seasonal fluctuations than those in the soils of the former inter-depression areas. In summer, the Ah horizon and the layers below it are cooler as compared with layers at the same depths in soils with subsurface EB or B horizons. The lowered temperature of soils with the second humus horizon results in slowing down the processes of mineralization, thus contributing to the maintenance of spatial heterogeneity of humus content in the upper soil layers. In winter, the soils with the second humus horizon cool and freeze with a lower rate and to a less depth than soils developed on former interfluves. It seems quite probable that the less thickness of frozen layer at the sites with the second humus horizon makes them the preferable ways for flowdown of melted water, which contributes to the existing pattern of alternation of leached soils with the second humus horizon and the soils with high carbonate layer. The dissimilarities in winter freezing of different soils also influence the intensities of cryogenic weathering. The analysis of mineralogical spectrum of coarse fractions showed that the Ah horizon is distinguished by the lowest intensity of winter weathering due to its frost-insulating properties and the cryoprotective role of soil organic matter. The Ah horizon is also biologically inert: the rate of decomposition of linen applications within this horizon is lower as compared with that in the arable layer and with that in the underlying BEg horizon. The high inertness of relic second humus horizons contributes to their good preservation and to maintenance of the inherited structure of soilscape. The modern functioning of surface paleosols is still determined by their relic differentiation in accordance with paleomicrotopography, despite agrogenic smoothing of modern surface. Indeed, the surface paleosols are the mirror of a landscape, but a mirror with good memory.