Properties of Mountainous Forest Soils and the Structure of Earthworm Taxocenes in the Central Caucasus

Mountainous dark gray forest soils (mountainous Greyic Phaeozems) and mountainous brown forest soils (mountainous Haplic Cambisols) are a valuable natural resource that supports functioning of mountainous ecosystems. Earthworms are soil engineers, playing a vital role in regulating soil processes and maintaining soil fertility. These two components are rarely studied together, although their interaction determines many ecosystem functions. The aim of this study was to establish the key physicochemical and biological parameters of natural and agrogenic mountainous dark gray forest soils (mountainous Greyic Phaeozems) and natural brown forest soils (mountainous Haplic Cambisols) in the Central Caucasus and to assess their influence on the distribution and structure of earthworm taxocenes. Using routine physicochemical and microbiological methods, it was demonstrated that the activity of hydrolytic enzymes indicates the intensity of carbon, nitrogen, and phosphorus metabolism in the soils of natural forest biogeocenoses. Agrocenoses show a trend toward alkalization of the soil suspension, increased bulk density, dehumification (a 57% decrease in humus content and a 42% decrease in humus reserves), and a decrease in the enzymatic activity of the upper soil horizons, most significantly, catalase by 38% (t = 2.2; p = 0.04) and urease by 76% (t = 2.1; p = 0.04). The species composition and structure of earthworm taxocenes reflect the mosaic soil-formation conditions typical for mountainous areas. Close climatic conditions in the biogeocenoses formed in the area of mountainous dark gray forest soils and mountainous brown forest soils are confirmed by the similar composition of dominant earthworm species and the frequent occurrence (recorded in 89% of samples from natural biogeocenoses) of O. lacteum calciphile common in soils with a high content of clay fraction. The earthworm fauna of mountainous brown forest soils is composed predominantly of forest and Caucasian species, whereas that of mountainous dark gray forest soils contains more nemoral-steppe elements. The agrocenotic use of soils changes taxocenes according to the “steppe type,” i.e., elimination of forest species, growing number of individual taxa, and lowering the species richness.