Factors Affecting Summer Decomposition of Organic Matter in the Soil of Various Sites on the Subpolar Islands of Kandalaksha Bay (White Sea)

Subpolar northern ecosystems are considered to belong to climate-driven biomes where biota activities and decomposition of organic matter depend strongly on climate. According to Wall et al. (2008), in cold regions soil animals show neutral effects due to temperature constraints, and the suppression of macrofauna does not lead to a retardation of decomposition rates. In a field experiment performed on the subpolar islands of the White Sea, decomposition rates of a standard organic material (cotton strips) were compared among the patches of a boreal landscape in connection with local microclimatic conditions and abundance of soil invertebrates. Rates of cotton weight loss strongly differed ranging between 4 and 20% during the summer season. The lowest rates were recorded in turf cushions on bare rocks, in soils of pine forests and Sphagnum bog, the medium rates—in soils of spruce and beach forests, the highest rates—in soils of seashore meadow and crowberry sites. Variations in weather conditions (average temperature, sum of temperatures exceeding 10°C, range of daily temperature fluctuations, moisture content) did not show direct correlations with the decay rates. A certain dependence of the decay rates on hydrothermic conditions was only indicated by its correlation with the range of precipitation/evaporation index. In contrast, a strong significant correlation of cotton weight loss was recorded with the total biomass of macrofauna, and especially of Dendrobaena octaedra (the only earthworm species present); an even stronger correlation occurred with the biomass of total Oligochaeta (earthworms and enchytraeids together). The sites with faster decay rates were characterized by a dominance of macrofauna over mesofauna biomass and a higher overall biomass of saprophages. Hotspots of biological activity and decay rates (such as heaps of storm-blown algae on the supralittoral or ant mounds in a pine forest) were characterised by high concentrations of animal biomass. Overall, presence of macrofauna and especially of Oligochaeta should be considered as an important acceleration factor of decay rates in the soils of northern ecosystems studied, at least during the summer period.