1. Introduction
Soil organic carbon (SOC) represents the largest part of the terrestrial carbon reservoir (Doetterl et al., 2016).
SOC determines qualities and functions of soils and is partly released into the atmosphere possibly contributing to climate change (e.g. Batjes, 1996; Lal, 2004; Meersmans et al., 2016; Wiesmeier et al., 2019). Quantification of soil organic carbon storage at various spatial and temporal scales is important in the context of climate and environmental changes as well as land management. Current regional studies have used a multiple regression approach to predict SOC (e.g. Grimm et al., 2008; Meersmans et al., 2008; Suuster et al., 2012).
Wiesmeier et al., (2019) proposed a quantification of SOC storage using many different indicators based on mainly factors controlling SOC content in soils.
Soil types and its features (i.e. clay content, soil texture, Ca and Mg cations, metal oxides, micro and macro soil fauna), soil moisture (drainage class), climate (air temperature, precipitation, potential evapotranspiration), topography (slope gradient, elevation and other terrain attributes) and land use are factors most often used for SOC assessment, as well as for mapping and predicting SOC distribution (e.g. Tan et al., 2004; Vos et al., 2019; Wiesmeier et al., 2014; Wiesmeier et al., 2019). However, it should be emphasized that SOC content also changes with soil depth depending on different controlling factors (e.g. Hobley & Wilson, 2016; Jobbágy & Jackson, 2000; Vos et al., 2019). The SOC content is most frequently studied in the topsoil (i.e. the upper 0.15-0.3 m), and rarely in the entire soil profiles (e.g. Arrouays et al., 2001, Bellamy et al., 2005; Pignard et al., 2000; Rumpel & Kögel–Knabner, 2011; van Wesemael et al., 2005).
Rumpel & Kögel–Knabner, (2011) suggest that ca. 50% of the total SOC stocks is found in subsoil. Especially in eroded landscapes SOC buried at depositional sites may be underestimated when one assesses SOC stocks in topsoil only. Therefore, SOC content in the entire soil profile should be assessed (Doetterl et al., 2016).
In landscapes where significant soil erosion occurs, the importance of topographic conditions for the spatial distribution of SOC and its storage is emphasized (Doetterl et al., 2016).
Some studies revealed that topographical features are of major importance for understanding SOC distribution only at local scales where there are small variations in soil properties. At regional scale topography is usually considered to be less important for SOC storage (e.g. Thompson et al., 2006; Vos et al., 2019). Soil moisture (low C mineralization and microbial activity under high soil moisture content) is considered to be the most important factor for SOC storage and its spatial distribution at regional scale (e.g. Neufeldt, 2005; Tan et al., 2004; Wiesmeier et al., 2019).
It should be noted, however, that the spatial variation of soil moisture content is affected by the topography of an area. The presence of concave forms, small elevation differences and gentle slopes favour the retention of soil moisture, while steep slopes and large elevation differences are conducive to the drainage of surface and subsurface water rather than the retention of moisture in the soil. Therefore, topography is a significant factor and may have not only a direct (SOC erosion control) but also an indirect (moisture control) effect on SOC distribution in landscapes.
The models predicting SOC dynamics are often used at regional and national scales. Martin et al. (2014) compared and evaluated available models used for SOC stocks prediction and mapping.
In previous studies it was observed that some landscapes in loess regions may have up to 60 CDs per km² closed depressions (CDs) in which colluvial deposits containing SOC have been reported (Kołodyńska-Gawrysiak & Poesen, 2017; Kołodyńska-Gawrysiak, Poesen & Gawrysiak, 2018). The aim of this study is therefore to quantitatively assess the influence of such CDs on the spatial SOC distribution in eroded loess landscapes at the regional scale. This study addresses SOC across the whole soil profiles.