The Content and Composition of Organic Matter in Soils of the Subpolar Urals

Soil organic matter at high latitudes is an important and sensitive indicator of climate change. This article describes the main morphological features, chemical properties, and composition of organic matter in the main types of soils along the altitudinal gradient of the Subpolar Urals. Soils formed in the mountain tundra zone (gleyic humus-illuvial podbur/Skeletiс Stagnic Entic Podzol (Turbic)), in the mountain subalpine zone (gray-humus soil/Skeletiс Umbrisol), in the mountain taiga zone (iron-illuvial podzol/Skeletiс Albic Podzol), and in the mountain tundra zone with permafrost (permafrost-affected gleyic humus-illuvial podbur/Skeletiс Folic Cryosol (Humic)) were studied. The method of densimetric fractionation was applied to study soil organic matter; it enabled us to distinguish its three fractions, differing in carbon participation in the biological turnover: free particulate organic matter (fPOM<1.6), occluded particulate organic matter (oPOM<1.6), and heavy organic matter bound with the mineral phase (MaOM>1.6). The latter fraction dominated in the upper mineral soil horizons and comprised 89–93% of the total organic carbon. The content of light fractions was significantly lower (0.6–4.7%). However, the content of organic carbon and nitrogen in the studied soils directly correlated with the contents of light fractions fPOM<1.6 (r = 0.40 and r = 0.79, p < 0.05) and oPOM<1.6 (r = 0.68 and r = 0.83, p < 0.05). Aliphatic fragments dominated in the composition of POM; their content varied from 74.5 to 80.5% for fPOM<1.6 and from 77.9 to 84.2% for oPOM<1.6. In addition, it was found that the organic matter of the oPOM<1.6 fraction was characterized by a higher decomposition rate (0.4–2.4) and hydrophobicity (34.7–66.5%).