Impact of Climate Change on the Distribution of Carbonates, Acidity, and Morphological Properties of Soils in a Forest-Steppe Depression (Oka–Don Lowland) over a Half-Century Period

Abstract— The article presents the results of a repeated field survey of the morphological features, carbonate content, and acidity of four soils along a catena within a shallow depression in the forest-steppe zone of the Oka–Don Lowland. These data are analyzed in conjunction with soil moisture records for 1960–2020, model estimates of vertical (gravitational) water in a layer of 140–150 cm, and information on the long-term dynamics of the groundwater level. The vertical water flux was calculated using an extended version of the Richards equation for the movement of liquid water and water vapor and the heat transfer equation, including freeze–thaw dynamics. The soils demonstrate a differentiated response to the observed climatic changes. The mucky-humus gley solod (Stagnosol) in the accumulative position at the bottom of the depression is characterized by a significant increase in the thickness of humus horizon, acidification, leaching of carbonates beyond the soil profile, and a downward displacement of the zone with iron–manganese nodules. The quasigley chernozem (Chernozem (Bathygleyic)) and dark agrosolonetz (Solonetz (Aric, Bathygleyic)) of the transaccumulative position (lower part of slope) are characterized by a lowering of the carbonate horizon, a pronounced decrease in the carbonate content, and an upward displacement of the zone with iron–manganese nodules. Taking into account the catena topography, the concretions in these three soils occur at a comparable hypsometric position presumably marking a new level of seasonal waterlogging. Despite the leaching of carbonates, the pH of the Solonetz (Aric, Bathygleyic) has remained virtually unchanged, unlike that in other soils. This is probably due to the high proportion of exchangeable sodium in the soil exchange complex, which acts as an alkaline buffer maintaining pH stability. A quasigley agrochernozem (Chernozem (Aric, Bathygleyic)) of the autonomous position is characterized by minor changes in the content of carbonates and the depth of effervescence. The observed soil changes are possibly related to a lowering of the groundwater table and an increase in liquid precipitation, including heavy rainfall, which leads to the removal of carbonates beyond the profile even under conditions of lower spring moisture recharge. Analysis of climate data and modeling of vertical soil moisture flux suggest that the observed changes occurred in 1980–2000 and 2010–2020.