Spatial heterogeneity of soil properties in the zone of sporadic distribution of permafrost (Subpolar Urals)

Abstract

The structure of the soil cover of the Subpolar Urals is characterized by diversity due to the combination of contrasting environmental conditions: a mosaic of vegetation, character of soil-forming rocks and topography. The study of the spatial variation of soil properties will provide more accurate information about the features of the soil cover of the Subpolar Urals. The aim of this research was to study the spatial heterogeneity of the morphological and physical-chemical properties of soils of the polar Urals. We carried out studies on the territory of “Yugyd va” National Park (63°59ꞌ N, 59°13ꞌ E) in the northern part of the Subpolar Urals (See Fig. 1). We examined morphological and physicalchemical properties of soils of the mountain-forest and mountain-tundra belts of the Subpolar Urals using the example of two trenches (See Fig. 2). The first trench was located in the mountain-forest belt (See Fig. 3A). Coordinates: 65°08ꞌ12.5ꞌN, 60°51ꞌ24.0ꞌE. The second trench was located in the alpine-tundra belt (See Fig. 3B). Coordinates: 65°02ꞌ06.3ꞌꞌN, 60°35ꞌ19.2ꞌꞌE. The trenches were divided into three segments. Diagnostics and position classification of soils was carried out in accordance with “Field guide of Russian soils” (2008) and the World Reference Base of Soil Resources, version 2015 (IUSS Working Group WRB, 2015). For the studied soils, we determined the main physical-chemical parameters: acidity, total contents of C and N, exchangeable cations – Ca2+ and Mg2+, as well as the content of Feox, Alox and Fedith (Vorob’eva LA, 2006). The contents of carbon and nitrogen were determined using elemental analyzer EA 1110 (Carlo Erba, Italy). Granulometric composition was determined according to the method of Kachinsky (Shein EV and Karpachevskii LO, 2007). We showed that the heterogeneity of morphological properties may determine differences between soils within a few meters. We revealed that for the studied trenches the variation of morphological features determines major differences between soils. It is demonstrated that differences in the intensity of soil processes lead to a wide soil diversity. Thus, several types and subtypes of permafrost soils were identified. In the mountain forest belt are formed: Histic Cambi-Turbic Cryosol (Humic) in the first segment, Histic Cryosol (Turbic, Reductaquic)) in the second segment, and Histic Cryosol in the third segment. In the alpine tundra belt are formed: Stagnic Entic Podzol (Skeletic, Turbic, Reductaquic) in the first segment, Folic Cryosol (Skeletic. Humic) in the second segment, and Stagnic Entic Podzol (Turbic, Skeletic) in the third segment. The analysis of granulometric composition of the studied soils of the mountain-forest belt showed that the soils are characterized by a high content of fraction of physical clay. For soils of the mountain tundra belt, we revealed the predominance of large fractions of physical sand (See Table. 1). The soils of the investigated trenches have a slightly acidic reaction medium. Organogenic horizons have the highest acidity. In mineral horizons, there is a smooth decrease in acidity with depth until close to slightly acidic values. We established that soils of the mountain-forest belt are more acidic (рН 4.4-5.7) than soils in the mountain tundra belt (pH 5.1-5.9). The result of biogenic accumulation of the largest concentration of exchange forms of Ca2+ and Mg2+, like the trench of the mountain-forest and mountain tundra belt, was observed in organogenic horizons. In the soils of the mountain forest belt, the Ca2+ content ranged from 8.8 to 14.7 mmol/100 g with a decrease in sphagnum mosses in the vegetation cover and an increase in green mosses. In the soils of the tundra mountain belt, the Ca2+ content is from 14.7 to 23.2 mmol/100 g and correlates with an increase in lichens and a decrease in mosses in the vegetation cover. The analysis of iron compounds revealed that in the mountain forest belt zones the maximum Feox content increases from 0.84 in segment I to 1.44% in segment III. Fedith accumulation from the first to the third segment varies from 0.91 to 2.46% (See Fig. 4). For soils of the mountain-tundra belt, high concentrations of iron compounds are characteristic of horizons with signs of overmoistening. In horizon G of segment I, the content of oxalation-soluble forms was 1.59%, and that of dithioninsoluble forms was 2.59%. In the BFg horizon, it was 2.01 and 2.75%. As a result of the studies, we showed that the distribution of carbon and nitrogen in the studied soils of the Subpolar Urals gradually decreases along the profile (See Fig. 5). The carbon content in the soil litter of the mountain forest belt reaches 43.3%, and that of nitrogen - 1.5%. The litter of soils of the mountain tundra contains up to 42.3% carbon and up to 1.3% nitrogen, respectively. However, in the mineral horizons of soils of the mountain-tundra belt, the content of carbon and nitrogen is much higher than in soils of the mountain-forest belt, which can be explained by the low biological activity of soils of mountain-tundra plant communities. Thus, we illustrated that it is the morphological characters that primarily serve as the basis for determining the type of soil, both within the same trench and in different altitudinal zones, while the physicochemical parameters are more conservative and serve as a clarifying analytical characteristic. We found out that the increase in the content of Ca2+ and Mg2+ for both trenches is determined by the composition of the moss-lichen layer and a decrease in the moisture organic horizon. The content of C and N in the mineral horizons of soils of the mountain-tundra belt is much higher than in the soils of the mountain-forest belt. We showed that in the mineral horizons of soils of the mountain-tundra belt, due to more severe climatic conditions, a high proportion of bedrock fragments in the profile, and close occurrence of permafrost, organic carbon and nitrogen are better accumulated and preserved than in the soils of the mountainforest belt.