Content of Humus Compounds in Soils of Secondary Forests and Sub-Mediterranean Steppes of the Northwestern Caucasus (by the Example of the Gruzinka Ridge)

Native ecosystems in the lower and middle mountains of the Northwestern Caucasus are mainly represented by secondary arboreous communities with a minor participation of steppe ecosystems. The soil cover of this area comprises a combination of native soil types: brown forest soils or burozems (Cambisols) and soddy-calcareous soils (rendzinas, Rendzic Leptosols). Cambisols predominate under forests of pubescent oak, while rendzinas, which were once formed under Mediterranean steppes, have undergone significant changes and are now found under oak shrubs, ash communities, and lime groves, as well as under preserved steppe vegetation. Consequently, there is a decrease in soil organic carbon (SOC) content in the soil-successional series: leached rendzinas under herbaceous communities, leached rendzinas under tree communities, and brown forest soils. The functioning of rendzinas under arboreous phytocenoses and their further transformation into brown forest soils reliably reduces the SOC content to the depth of 60 cm. In contrast, the SOC content in rendzinas under herbaceous phytocenoses is higher not only in the humus-accumulative horizon but throughout the entire soil profile. The most significant changes in the composition of humus affect its water-soluble fractions separated by cold and hot extractions. Brown forest soils and rendzinas of arboreous phytocenoses are characterized by a similar accumulation of the easily soluble fraction of water-soluble organic matter (cold extraction) in the surface horizons and its depthwise distribution. Meanwhile, the absolute values of water-soluble organic matter (hot extraction) in the profile of rendzinas under arboreous formations are almost halved as compared to soils under herbaceous phytocenoses. The intensity of humification in rendzinas under forest communities decreases due to the destruction of organic matter, which was once strongly associated with calcium, and undergoes maximum transformation because of the change in the composition of plant litter and microclimate under trees. Soil acidity affects the degree of extraction of water-soluble organic matter: the proportion of the hot fraction of water-soluble organic matter in SOC significantly increases with a decrease in pH of the soil solution.