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.