4. Discussion
The results show that the SOC content in the whole soil cover of the
agriculturally used loess slopes and plateau tops (including
microtopography i.e. CDs) of the Nałęczów Plateau is 100-144
Mg·ha-1. If we disregard the specific for loess area
microtopography (CDs), this value in the area under study will be
slightly lower, i.e. 102.38 Mg·ha-1 of SOC. In eroded
sites (loess slopes and plateaus) 51.07 Mg·ha-1 SOC
are stored in the topsoil (i.e. the A horizon, mean depth 0.24 m). In
depositional sites (i.e. CDs) about ¼ (38.26 Mg
SOC·ha-1) of SOC are stored in the topsoil. This
confirms the findings of Rumpel and Kögel-Knabner (2011) that high SOC
storages may be present below the topsoil and not only within the
topsoil. Therefore, more attention should be given to subsoils when
assessing SOC stocks. Otherwise, calculated SOC stocks will be strongly
underestimated.
For Wallonia, Chartin et al. (2016) determined the mean predicted SOC
stock in the topsoil at 59.9 Mg C·ha-1 in cropland
(53.5-107 Mg C·ha-1 for different regions). For the
same region, Lettens et al. (2005b) determined the mean SOC stock value
at 48 Mg C·ha-1 in cropland. These differences results
from the different research methods adopted by the authors. In France,
mean SOC stock for cropland is 55.7 Mg C·ha-1(Meersmans et al., 2012). At the scale of the whole territory of
Belgium, Meersmans et al. (2011) forecasted a SOC stock value of 49.5 Mg
C·ha-1 for cropland. In Poland, the present soil
organic carbon stock (SOC) in topsoils for the main soil types range
from 36 to 42 Mg C·ha-1 (Faber et al., 2012). The
results of SOC content in topsoil obtained in this study are comparable
with the results for other European regions.
In the agriculturally used loess areas subject to erosion, topography
has both a direct and an indirect impact on SOC distribution in the
landscape. The direct impact consists of the accumulation of SOC-rich
colluvial sediments in depressions; these sediments are products of soil
erosion on hillslopes (Sorensen et al., 2006). Erosion causes a loss of
SOC in topsoils of sloping land and a reduction of their thickness. In
the eroded loess landscapes of the Nałęczów Plateau, CDs are SOC pools
where soil erosion-derived colluvial sediments of considerable thickness
are accumulated. SOC storage in the CDs originates not only from soil
erosion in their catchment but also from fossil soils that have been
buried by SOC-rich colluvial sediments. Together with the subsurface
fossil soils, closed depressions store more (almost twice) SOC (mean
192.85 Mg·ha-1) than eroded soils on sloping cropland
(mean 102.38 Mg·ha-1). These results demonstrate that
if one neglects microtopographic features like CDs the true SOC storage
will be underestimated.
The study of Grimm et al. (2008) indicated that topographic features
explained most of the spatial variability of soil organic carbon content
in the topsoil (0–0.1 m) at the regional scale. In the subsoil
(0.10–0.50 m), SOC distribution was related to soil texture.
The investigations conducted in the Nałęczów Plateau indicate that the
microtopography has also a significant impact on the vertical
distribution of SOC in soil profiles. About 1/4 (38.26
Mg·ha-1) of the mean SOC storage in CDs is found in
the topsoil (0-0.24 m) of these forms (Table 5). Most SOC stored in the
CDs occurs in the subsoil and fossil soil (at a depth ca. 1.0 m). The
topsoil at the loess plateau tops and slopes (without CDs) contains
about half (51.07 Mg·ha-1) of the mean amount (102.38
Mg·ha-1) of SOC within the soil cover of these areas
(Table 7).
The indirect impact of topography on the spatial SOC distribution in the
landscape is linked to its influence on the larger soil moisture
contents (and even water logging) within the CDs and hence on SOC
storage. This has been confirmed in studies by Meersmans et al. (2011)
who demonstrated that, at the catchment scale, a higher SOC content is
correlated with soils of waterlogged valley bottoms. The bottoms of CDs
are areas with a larger moisture content than the surrounding slopes,
which can be deduced from the redoximorphic features in colluvial
sediments deposited in the CD bottoms. The SOC mass stored in the
colluvial sediments of the CDs is similar to that of the underlying
fossil soils. On average, 10.23 Mg SOC per CD is stored in the colluvial
sediments of the studied CDs, while 10.39 Mg SOC per CD is stored in the
underlying fossil soils (Table 2). Furthermore, SOC distribution in
fossil soil profiles indicates a vertical leaching of SOC deeper into
the soil profiles as a result of surface water infiltration in the CD
bottom. This is deduced from the morphology and micromorphology of the
soils in the CDs and their geochemical properties (Kołodyńska-Gawrysiak
et al., 2017). The indirect impact of topography on SOC storage,
expressed by the topographic wetness index (TWI), was emphasized by
Wiesmeier et al. (2013). Depressed areas display on average a larger
soil moisture content which delays the decomposition of organic matter
and is conducive to the accumulation of SOC.
According to Wiesmeier et al. (2019), the influence of topography
(topographic position, slope gradient, TWI) on the variation of SOC
storage is significant only at a local scale. It is less significant at
regional scale where SOC storage is determined by climate, vegetation,
texture, parent material, land use and management. Models predicting SOC
content at the national or large regional scale usually do not take
topography into account either. The model of SOC prediction at the
national scale, proposed by Martin et al. (2010) showed that SOC
distribution is strongly correlated with land use (forest soils,
cropland soils). This study reveals the significant impact of
(micro)topography (microrelief) on the SOC storage for the regional
scale. For the Nałęczów Plateau area, the inclusion of microtopographic
features (i.e. closed depressions), typical for loess areas, enabled a
more accurate estimation of SOC storage. 95.86 % of the studied eroded
landscape actually holds to 10% more SOC than what models excluding SOC
storage in CDs would suggest (Fig. 9). For about 4.13% of these areas,
the exclusion of CDs results in an underestimation of SOC storage in the
landscape from 10% to 30 % (Fig. 9).
The presence of thick colluvial sediment layers of different ages in the
CDs suggests the influence of these microtopographic landforms on SOC
storage in the past (Kołodyńska-Gawrysiak, Poesen & Gawrysiak, 2018).
Today this carbon storage is often not quantified when conducting
organic carbon inventories at local or regional scale. A similar remark
has been made for buried peatlands that have accumulated significant C
stocks over millennia (Treat et al., 2019).