Genetic features of forest-steppe soils with albic horizon: a case study in the Oksko-Don Lowland

In the closed depressions of the forest-steppe, unique soils form that differ significantly in their properties from the zonal Chernozems. These soils have a light-colored, acidic Albic horizon and the complete absence of carbonates. They resemble podbels, which are formed in the southern Far East of Russia, with their different soil-forming processes, regimes, and genesis. The aim of this study is to interpret the genesis of the soils in the center of the closed depression by analyzing their morphological, micromorphological, chemical, physical, and physico-chemical properties. This will help to determine the main diagnostic criteria for their classification. The depression is located in the Oka-Don Lowland, in Tambov Oblast (52.0398°N, 41.1839°E, See Fig. 1a). We collected a total of 39 samples, from three walls of soil pit, at 10-centimeter intervals, to a depth of 130 centimeters. For micromorphology, we took samples from all genetic horizons, in triplicate (12 samples). In the pre-dried and homogenized samples, we determined pH values, total dissolved salts and soluble salt content (conductivity), particle size distribution (laser granulometry, with preliminary ultrasonic treatment at an energy level of 450 joules per milliliter), total carbon (by dry combustion), cation exchange capacity, exchangeable cation composition (using Pfef-fer's method), and iron content in oxalate extracts (Tamm's method) and dithionite-citrate extracts (Mehra-Jackson method). We also calculated the Schwertmann ratio and and the degree of hydromorphism were calculated as indicators of soil overmoisture. The diagnostic properties of the studied soil (See Fig. 1b) include: (1) a thick (30 cm) mollic horizon, rich in organic carbon, with a loosely aggregated structure and varying degrees of organic matter decomposition; this horizon is overlain by a (2) light-colored albic horizon, which has signs of humus leaching and contains up to 5% of small-sized iron-rich concretions, and (3) an illuvial horizon that has a weaker structure and a higher concentration of concretions than the overlying horizon. This horizon also has cutans along the edges of ped. The soil demonstrates a sharp decrease in carbon and a gradual increase in pH from slightly acidic to neutral values with depth (See Fig. 2). The middle part of the profile has a slightly higher clay content compared to the mollic horizon. The top 10 cm of the soil contains soluble salts at a concentration of 106 mg/ L (0.14% of soil dry mass), classifying this layer as slightly saline and chloride-magnesium. However, at the lower boundary of the soil surface (20-30 cm), the salt content drops sharply to 27 mg/L (0.04% of the dry soil mass) and does not exceed 50 mg/L in deeper layers, indicating that the soil is not saline. The anion-cation composition of the soil in the depression remains consistent throughout the profile. There are two peaks in the cation exchange capacity, one in the humus layer and another at a depth of 100-130 centimeters. This latter peak is likely due to an increase in exchangeable Ca2+, which is probably caused by its influx from solutions in the underlying carbonate parent material. The relatively high content of soluble salts and exchangeable sodium in the topsoil can be attributed to lateral water flow from surrounding depressions with solonetz and meadow-chemozem soils. Based on the Schwertmann's and hydromorphic criteria, the upper part of the soil profile, particularly the mollic and albic horizons, experience prolonged surface overmoisture. These coefficients decrease sharply in the gleic horizon. Based on a comprehensive study of soil properties, we can conclude that the light-colored horizon of the depression soil we studied is predominantly eluvio-gleyic in nature, with concretions (See Fig. 3 d, f) and cutans (See Fig. 3 c, e). In the underlying horizon, Schwertmann and hydromorphic criteria indicate seasonal surface overmoistening and periods of drying. The additional factor contributing to the formation of the light-colored horizon may be the presence of sodium in the upper horizons of the soil-absorbing complex, ranging from 4 to 14%. The soil profile formula of the studied depression soil is AH- AHel-ELhi,g-BTg (according to WRB, Ah-Eg-Btg); in terms of the genetic horizons it is absent in the Russian Soil Classification, partially corresponding to dark-humus pseudo-gleyic (AH-EL-BTg-BCA-Q; Calcic Albic Mollic Planosol, A-Eg-Bt-Bk-Bgk), dark-humus podbel gleyic (AU-ELg-BTg; Mollic Albic Stagnosol, A-Eg-Btg). Unlike the dark-humus pseudo-gleyic soil, the studied soil does not contain carbonates, which are leached from the soil due to its large catchment area (55 hectares). Different from dark-humus podbel gleyic, the studied soil differs in the nature of the upper horizon and less pronounced processes of iron segregation in the lightened horizon (a lower quantity of concretions). We hypothesize that the water regime of the depression soils is less contrasting, with a smoother alternation of periods of overmoistening and drying compared to the podbels of the Far East, which may explain the lower quantity of concretions in the depression soil. Following the classical tradition of Russian soil science - reflection of the major soil-forming processes in the soil name - the studied depression soil can be termed as dark-humus elluvial gleyeic. With the expansion of soil studies in depressions and the identification of soils with a similar profile structure, introduction of an independent type of peat-dark-humus gleic soils into the Russian Soil Classification may be proposed. The article contains 3 Figures, 41 References. The authors express their gratitude to the head of the farm, Alexander Fedorovich Tolmachev, for his assistance in conducting field research. The Authors declare no conflict of interest.

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