Black soils in the Russian Soil Classification system, the US Soil Taxonomy and the WRB: Quantitative correlation and implications for pedodiversity assessment

Black soils recently proposed by the FAO Global Soil Partnership, include fertile soils, characterized by a thick, dark-colored soil horizon rich in organic matter. This study addressed the proposed concept of "black soils", in terms of taxonomic relationship of their corresponding soil types of existing soil classifications. Similarly studies were based on the calculation of taxonomic distances between selected soil groups with organic matter-enriched horizons of the Russian Soil Classification System, the United States Soil Taxonomy and the World Reference Base for Soil Resources. In the process, we used the so called "centroids", which are the calculated mean values of several soil properties for each of the soil groups using legacy data derived from national and international databases. The results indicated that the Great Groups of the Mollisol Order in the US Soil Taxonomy had small taxonomic distances within the order, except some soils with shallow depth to the hardpan or permafrost. Dark-colored Vertisols and Andisols were found to differ from the Mollisols and similar soils found in similar environments mainly under grasslands. We recommend excluding Vertisols and Andisols from the "black soils" cluster due to the peculiarity in their properties, potential use and management. While the Vertisols and Mollisols of the Soil Taxonomy were completely dissimilar in properties, the WRB Vertisol Reference group and Russian dark-humus compact soils fitted well the Mollisols cluster, presumably due to the different concept of the Vertisols in the studied system. The soil types of the "black soils" of the Russian soil classification had a short distance to the corresponding Reference Groups of the World Reference Base. Also, the similarly was high with the Great Groups of the US Soil Taxonomy for all soil types except of Kastanozems, but the relationship in places was not well interpreted from a pedogeographic point of view. We ascribed the phenomenon to the geographical bias of the databases. Though "black soils" form a distinct cluster that roughly corresponds to the Mollisols Order of the US Soil Taxonomy, the taxonomic distance within the group may be significant, contributing to the pedodiversity of soilscapes of "black soils".