Chernozems of Eastern European forest-steppe lowland landscapes: Morphology, classification and spatial distribution

Abstract

The chernozems of the flat interfluves in the Oka-Don Lowland exhibit unique characteristics within the chernozem zone. These soils show distinct signs of groundwater-induced waterlogging in the lower part of soil profile and, unlike the chernozems of elevated plains, do not typically experience summer desiccation. This study is based on an analysis of morphological properties and extensive soil surveys encompassing over 2500 soil profiles across an area exceeding 500,000 ha of the Oka-Don Lowland. The research integrates topographic and lithological factors of soil formation alongside the results of large-scale soil mapping conducted on a 400-ha key site.

Using machine learning and digital soil mapping techniques, we identified the key environmental drivers governing the spatial distribution of waterlogged chernozems within the Oka-Don Lowland. The key site revealed distinct spatial patterns of chernozem distribution and their associations in catenas with Solonetz, Gleysols, and Stagnosols. Despite the minimal elevation differences across the flat interfluves, topography emerged as the primary factor influencing soil spatial differentiation.

A comparison of forest-steppe soil classifications between the USSR system and WRB reveals significant discrepancies in their Chernozem categories. For instance, according to the WRB, Chernozems cover 80–90 % of the southern Oka-Don Lowland, whereas the USSR SC classification assigns only 65 % of this area to chernozems. This difference arises because the WRB includes additional soil types in its Chernozem category that the USSR SC classifies differently. Notably, not all soils identified as chernozems in the USSR SC qualify as Chernozems under the WRB; some of them belong to the Phaeozems. Correlating soil taxa at the species level between the USSR SC and WRB results in a loss of critical information regarding soil depth and organic carbon content. This discrepancy can lead to differences of up to 3.5 times in estimates of organic carbon stocks within the 50 cm soil layer.

These inconsistencies underscore the urgent need to harmonize soil classification data and systematically address these differences when assessing soil organic carbon stocks. Such efforts are essential for ensuring accurate carbon accounting and enhancing our understanding of the role of these soils in the global carbon cycle.