Features of Organic Horizons in Different Types of Forests of the Middle Taiga in the European Northeast of Russia

Features of the structure and properties of organic horizons (O horizons) in different types of forests in the middle taiga subzone (the Lyal’skii State Natural Reserve, Knyazhpogostskii district, Komi Republic) are discussed. The authors have studied the morphological properties of O horizons (thickness, structure, and fractional composition), have determined their particular chemical characteristics (acidity and the C and N content), and have assesed the organic matter and Corg reserves in them. Median values of all parameters are calculated in Statistica 13.3 program. It is shown that O horizons of automorphic spruce and small-leaved forests of accumulative and trans-accumulative relief positions with not deep carbonate-containing parent rocks are characterized by relatively high pH (5.3–6.4) and N content (1.4–1.5%), C : N ratio 25–29, and increased content of microbial biomass (20–24 mg C/g of soil in the upper O horizon layer). The above-mentioned peculiarities favor the decomposition and formation of relatively thin (5–6 cm) O horizons in automorphic conditions, their reserves being 37–54 t/ha. The moisture content rise in sphagnum spruce forests ensures organic matter conservation and an increase in the thickness of O horizons to 15 cm and in organic matter reserves to 85 t/ha. In soils of green-moss and sphagnum pine forests on sandy deposits of eluvial positions, O horizons are characterized by acid reaction (pH 4.2–4.5), low N content (0.9–1.0%), wide C : N ratio (44–48), and low content of microbial biomass (13–14 mg C/g of soil in the upper O horizon layer). Unfavorable conditions of organic matter decomposition in green-moss pine forests contribute to the formation of O horizons with a thickness of 12 cm and total reserves of 73 t/ha. Excessive moisture of O horizons in sphagnum pine forests determines the increase in its thickness to 17 cm and in organic matter reserves to 105 t/ha. In automorphic forests, the O horizon thickness and reserves decrease in the sequence: tree-base mound (trunk) → under–crown space (crown) → inter–crown space (window). In sphagnum forest types, the trend is opposite, which is probably related to a draining effect of trees. Carbon stocks in O horizons of forests are 15–40% of its total stocks in the 1-m-thick soil layer in automorphic conditions and to 49% in semi-hydromorphic conditions.