Colluvial soils as a soil organic carbon pool in different soil regions

Abstract Subsoil has been recognized as large reservoir of soil organic carbon in recent years. In our study, we investigated deep colluvial soils as a potentially important source of SOC due to high mass redistribution driven by soil erosion. Three agriculture plots from the Chernozem, Luvisol, and Cambisol regions were studied to assess the SOC storage in topsoil (0–25 cm), at 2 m depth (0–200 cm), and over the total soil depth (0–450 cm) as a function of relief. The study is based on 558 borings, and soil profile description and classification to facilitate the colluvial soil delineation. Among these locations, SOC content was measured at 230 sampling points. Prediction of the SOC stock for the plots was based on support vector machine algorithms using digital elevation model derivatives as predictors. Total SOC stock varied among the study plots. The highest relative SOC stock was measured in the Chernozem (CH) plot (144.7 t·ha− 1), while at the Luvisol (LU) plot, it reached 68.4 t·ha− 1 and was 73.4 t·ha− 1 at the Cambisol (CM) plot. The role of colluvial soils regarding their spatial extent and SOC stock differs among the studied plots. Colluvial soils at the CH plot represent an important soil cover both spatially (13%) and by the volume of SOC stock (37%). A moderate importance of colluvial soils is determined for the LU plot (12% of SOC stock), and a low importance for the CM plot (5% of SOC stock). SOC stock contained in topsoil and subsoil differs in each plot. In the Luvisol and Cambisol plots, more than one half of SOC is retained in topsoil (53.4%, 60.3%). In contrast, more than two thirds of the SOC stock (73.1%) occurs in subsoil in the Chernozem plot. Moreover, 19.0% of the total SOC stock occurs at depths below 2 m. This finding indicates the importance of the incorporation of deep colluvial soil horizons in SOC stock estimations.