Article
Comparison of Topsoil Organic Carbon Stocks on Slopes under
Soil-Protecting Forests in Relation to the Adjacent
Agricultural Slopes
Paweł Wiśniewski 1, *
and Michael Märker 2
1
2
*
Citation: Wiśniewski, P.; Märker, M.
Comparison of Topsoil Organic
Carbon Stocks on Slopes under
Soil-Protecting Forests in Relation to
the Adjacent Agricultural Slopes.
Forests 2021, 12, 390. https://doi.org/
Department of Landscape Research and Environmental Management, University of Gdansk, Bażyńskiego 4,
80309 Gdańsk, Poland
Department of Earth and Environmental Sciences, Pavia University, via Ferrata 1, 27100 Pavia, Italy;
michael.maerker@unipv.it
Correspondence: pawel.wisniewski@ug.edu.pl
Abstract: Soil erosion is one of the major processes degrading the natural environment but also
agricultural production areas. Soil erosion may lead to soil organic carbon (SOC) loss, especially
from sloping agricultural terrain units. The use of phytomelioration in environmental management,
particularly long-term, permanent forest vegetation, is widely recognized as a possible measure for
soil erosion protection and mitigation of climate change through carbon sequestration. The aim of
this study was to compare of the topsoil organic carbon stocks on the slopes under soil-protecting
forests in relation to the adjacent agricultural slopes. The research was conducted in the young glacial
landscape of North-Central Poland. The study indicated the significant role of forest management on
the increase of soil organic matter content and SOC stock. The results show that land use and slope
gradients are important factors controlling soil organic carbon pools in topsoil in young glacial areas.
This topic is extremely important particularly as the effects of climate change become more and more
visible, and society faces new challenges in preventing these changes.
Keywords: soil organic matter (SOM); soil organic carbon (SOC) stock; global carbon cycle; topsoil;
hillslopes; erosion; soil-protecting forests
10.3390/f12040390
Academic Editors: Cezary Kabała,
Jarosław Lasota and Ewa Blonska
Received: 12 February 2021
Accepted: 23 March 2021
Published: 26 March 2021
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4.0/).
1. Introduction
In the course of the ongoing civilization processes that lead to rapid changes of the
natural environment, the carbon balance is increasingly important not only for a proper
functioning of ecosystems but also for the socio-economic development of many regions of
the world [1]. Soil organic carbon (SOC) in the form of soil organic matter (SOM) plays an
important role in the soil nutrient cycles and soil biodiversity. Benefits of soil organic matter
include improvement of soil quality through increased retention of water and nutrients,
resulting in greater productivity of plants in natural environments and agricultural settings.
SOM improves soil structure and reduces erosion, leading to improved water quality in
groundwater and surface waters, and ultimately to increased food security and decreased
negative impacts to ecosystems [2]. It is a natural resource for the sustainable development
of human society and a key foundation for sustainable forestry development [3]. SOC is
also the most important indicator of soil fertility, and monitoring its space-time changes is
a prerequisite to establish strategies to reduce soil loss and to preserve soil quality [4].
Soil organic matter is an important sink for atmospheric carbon dioxide, thus influencing the global carbon cycle directly. Hence, SOC plays an important role in governing the
dynamics of greenhouse gases (GHG), as its pools and its transformations in the terrestrial
ecosystems may influence the concentrations of carbon dioxide, as well as those of other
greenhouse gases in the atmosphere [5–8]. In the global carbon cycle, soils constitute the
third largest reservoir. It is estimated that carbon contained in soil accumulate 75% of the
Forests 2021, 12, 390. https://doi.org/10.3390/f12040390
https://www.mdpi.com/journal/forests
Forests 2021, 12, 390
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total organic carbon pool, exceeding twice the resources of carbon in the atmosphere [9–11].
Globally, the soil contains a carbon pool estimated at approximately 1500 Gt of organic
carbon in the first 1 m of the soil profile [12]. Current land use changes have a net negative
impact on soil carbon. Desertification and erosion associated with overgrazing and excess
fuelwood harvesting, conversion of natural ecosystems into cropland and pasture land, and
agricultural intensification are causing losses of soil carbon [13]. Therefore, the protection
of soils against erosion and the maintenance of soil organic matter and soil structure are
two pillars helping to protect soils and to enhance soil quality and soil functions [14].
An effective way to protect soils is applying phytomelioration measures. Strategic
landscape-level deployment of plants through agroforestry systems may represent an
efficient way to rebuild total ecosystem carbon, while also stabilizing soils and hydrologic
regimes, and enhancing biodiversity [13]. In particular, the use of long-term, permanent
forest vegetation (especially mixed forest) seems very promising [15–18]. These protective
effects consist primarily in stabilizing the soil through a dense root system. Moreover,
precipitation is intercepted and retained along the stem and in the leaf litter or organic
soil layers. The forests also trigger the melting of snow cover and reduce, distribute, or
facilitate infiltration of surface runoff. Finally, subforest vegetation helps to redeposit
material eroded in the upper parts [19–26].
The aim of this study was to compare of the topsoil organic carbon stocks (SOCP)
and correlations between SOCP and selected soil properties on the slopes under soilprotecting forests (legally recognized forests for soil-protection) in relation to the adjacent
agricultural slopes. Topsoils are more sensitive to environmental conditions and changes
than other parts of soil profiles [27]. Our study is focused on the moraine areas of NorthernCentral Poland. In Poland, soil-protecting forests occupy an area of 323 thousand ha,
which represents 8.9% of the total area of protective forests and 3.5% of all forest resources
managed by the National State Forest Holding [28].
2. Materials and Methods
The research was carried out in the young glacial lake districts in Northern-Central
Poland, on the slopes that are subject to plough tillage. Alongside these ploughed slopes,
we identified slopes that are partially or completely covered with soil-protecting forests
having the same climatic conditions and similar humidity conditions, as well as the same
or similar slope gradient. The landscape morphology of these areas has been mainly
formed during the last glacial (Vistulian) and postglacial period. The study area is located
in the temperate warm transitional climate zone. Its territory is affected by air masses
from both the Atlantic Ocean and the continental Eurasian landmass. Concerning the
transfer of air masses, westerly winds are the most prevailing ones in the study area.
On average, annual totals of precipitation are ca. 500–600 mm. The number of days
with precipitation sum >0.1 mm ranges from 130 to about 170 a year [29]. Apart from
moraines, there are kames, eskers, glacial troughs, river valleys, and glacial lakes. The soil
cover is dominated by Cambisols overlaying boulder clays and loamy sands. The other
important soil units are the Podzols developed on glacial outwash material, as well as
Brunic Arenosols formed from glacio-fluvial and mixed cover sands. Locally, outwash
sands are found [30,31]. Cambisols generally make good agricultural land and are used
intensively. Parent material are mainly medium and fine textured materials derived from a
wide range of rocks. They are characterized by slight or moderate weathering of parent
material and by absence of appreciable quantities of illuviated clay, organic matter, Al,
and Fe compounds. Podzols have an illuvial horizon with accumulation of black organic
matter and reddish Fe oxides. Their parent material are mainly weathering materials of
siliceous rock, including glacial till and alluvial and aeolian deposits of quartz sands. Most
Podzols are under forest or shrubs [32]. Brunic Arenosols (termed rusty soils in Poland) are
developed from glaciofluvial, old-alluvial, and mixed cover sands, and have a relatively
low agricultural suitability. Compared to Podzols, Brunic Arenosols have a somewhat finer
texture, higher pH, and higher base saturation throughout the profile. They also have a
Forests 2021, 12, 390
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thick subsurface Bv horizon [33]. The dominant forest areas are pine forests and mixed
forests, although there are also large areas of deciduous forests. The stands mainly consist
of pine (over 85%). In general, forests are very homogeneous. The majority of forest area is
occupied by single-storied stands [34].
According to Reference [35], we subdivided the slopes in topographic units. Along
the cross sections, we selected different top-slope, mid-slope, and toe-slope positions to
establish soil pits. The soil pits in the top-slope positions cover interfluve and convex
slope units, whereas the mid-slope soil pits characterize straight sloping transport related
land units. The toe-slope positions are described by concave colluvial slope units showing
colluvial processes (Figures 1–5).
The soil profiles were described in terms of morphology in line with the 6th edition
of the Polish Soil Classification [36], which adopted a new classification of soil texture
and mineral materials, compatible with particle sizes and textural classes of the United
States Department of Agriculture (USDA) Soil Taxonomy system [37]. On arable land
and rangeland, pits were dug to a depth of 150 cm, whereas, under the soil-protecting
forests—according to the Forest Management Planning Instruction [38]—we dug to a depth
of 200 cm. From each pit, topsoil samples at a depth of 0–10 cm were taken for further
analysis. The final sample, representative of a homogeneous topsoil unit, had a weight
of 1 kg. Detailed description of the particle size distribution and selected chemical and
physicochemical properties of soils along the analyzed cross sections was presented in
a previous paper of the authors [39] (Table A1 in Appendix A). Based on the land unit
concept, the main slope units and the respective soil forming processes, such as erosion,
transport, and deposition processes related to infiltration, surface runoff, and hypodermic
soil water dynamics, were characterized.
In each soil pit, topsoil samples at a depth of 0–10 cm were taken, and the pools of soil
organic carbon (SOCP) were determined. For the calculation of SOCP accumulated in the
topsoil, we used Equation (1), as follows:
SOCP = Corg × ρ × h/10,
(1)
where: SOCP—pools (stocks) of SOC accumulated in the surface layers (kg m−2 ); Corg—
organic carbon content (%); ρ—soil bulk density (g cm−3 ); h—thickness of soil layer
(cm) [40].
The SOC content was analyzed using the modified Tiurin method [41] to give results
practically identical with those of the dry combustion method. This method is suitable for
almost all soils. Soil bulk density was assessed using the core method (volumetric cylinder
method). The core sampling method is the most common method used to determine ρ in
agricultural soils [42]. Rings of 100 cm3 volume were used as the reference method because
this follows International Organization for Standardization (ISO) [43] and is the common
sample ring size in soil surveys. As recommended by Reference [44], all samples were dried
at 105 ◦ C to constant weight, and the total dry mass was divided by the sampled volume
to obtain the soil bulk density value. Gravel and other particles >2 mm were previously
removed from all soil samples.
The topsoil organic carbon stocks were subjected to statistical and comparative analysis. Subsequently, all relationships between soil variables were evaluated using Pearson’s
correlation tests. In order to calculate the Pearson correlation coefficients, the relationship
between the SOC stocks and the variables, such as of soil texture, concentration of available
forms (P, K, Mg), soil organic carbon content, and soil bulk density, were assessed.
Forests 2021, 12, 390
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Figure 1. The soil cross sections A-B and A’-B’. Slope gradient in %; A: humus horizon; E: eluvial horizon; B: enrichment
horizon; C: parent material (substratum); O: organic horizon; p: plow layer; t (for E horizons): leaching of clay fraction;
t (for B horizons): illuvial accumulation of clay; k: accumulation of pedogenic carbonates; v: occurrence of plinthite; w:
development of color or structure in a horizon but with little or no apparent illuvial accumulation of materials [39].
Figure 2. The soil cross sections C-D and C’-D’. Slope gradient in %; A: humus horizon; E: eluvial horizon; B: enrichment
horizon; C: parent material (substratum); O: organic horizon; p: plow layer; t (for E horizons): leaching of clay fraction; t (for
B horizons): illuvial accumulation of clay; v: occurrence of plinthite [39].
Forests 2021, 12, 390
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Figure 3. The soil cross sections E-F and E’-F’. Slope gradient in %; A: humus horizon; E: eluvial horizon; B: enrichment
horizon; C: parent material (substratum); O: organic horizon; p: plow layer; t (for E horizons): leaching of clay fraction; t (for
B horizons): illuvial accumulation of clay; v: occurrence of plinthite; s: iron and aluminium leaching [39].
Figure 4. The soil cross sections G-H and G’-H’. Slope gradient in %; A: humus horizon; B: enrichment horizon; C: parent
material (substratum); O: organic horizon; p: plow layer; v: occurrence of plinthite [39].
Forests 2021, 12, 390
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Figure 5. The soil cross sections I-J and I’-J’. Slope gradient in%; A: humus horizon; B: enrichment
horizon; C: parent material (substratum); O: organic horizon; p: plow layer; v: occurrence of
plinthite [39].
3. Results and Discussion
Soil cross sections, as well as the analysis of material collected from the soil pits, reveal
the spatial variability of soil morphology, as well as the physical and chemical properties of
the soil within the topsoil layer, consequently, up to a depth of ca. 10 cm. Generally, the soil
properties of the areas under the soil-protecting forests show different characteristics than
the soils under cultivation or not used for agriculture [39]. The soil profiles along the cross
sections illustrate that, on slopes, that are agriculturally used, eroded soils with capped
natural profiles are found. Especially, the Luvisols (cross sections A-B, C-D) show erosion
of the humus rich A-horizons, as well as reduced eluvial and illuvial horizons. Soils on the
slopes covered by the soil-protecting forests are not subject to these visible transformations
by erosion processes. For example, in the case of cross section G’-H’, characterized by
the presence of iron oxides over the entire soil profile, no reduction of horizons in any
soil profile location along the cross section was found. Moreover, on the cross section
Forests 2021, 12, 390
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C’-D’, characterized by a higher variability of soil types, no soil thickness reduction was
observed on the slopes or on the plateau nor in the lower parts of the slope. Differences in
the composition of soil pedons were also noticed comparing soils under soil-protecting
forests and on rangeland. On the cross section I’-J’ in soil pit No. II, located on a concave
side covered with grassland and exposed towards the South, we clearly observe a marked
reduction of a Brunic Arenosol profile. In contrast, no profile reduction is documented
for soil pit No. IV, which is located on the convex slope, exposed towards the North and
covered by soil-protecting forest. The greatest diversity of soils occurs along cross sections
A-B and A’-B’. The diversity may be related to the location of the cross section on the slopes
of an erosional-denudational valley that, in turn, enriches the mosaic landscape due to
different substrates [45]. In the soil pit at the bottom of the slope A-B, we identified a Brunic
Arenosol with fluvioglacial sands, while, in the soil pit, located at the bottom of the valley,
at the foot of slope A’-B’, we found a Haplic Phaeozems (Arenosols).
The cross sections are characterized by loamy sand, sand, and sandy loam texture
(Table A1). On the slopes used for agricultural purposes, the average sand content is 78.7%,
and the silt content is 19.5%, while the clay fraction is 1.8%. The slopes covered with
soil-protecting forests show an average sand fraction of 83.9%, a silt fraction of 15.2%, and
a clay content of 0.9%. In most cases, the soils on the agriculturally used plateau/top slope
position have lower sand content, and a higher percentage of silt and clay fractions in
relation to the top slopes covered with soil-protecting forests. The same relationship exists
for the respective slope sections. At the bottom of the not forested slopes, however, we can
observe higher average contents of sand and clay, with smaller silt fractions, in comparison
the bottom of the forested slopes. The upper soil layers in the studied cross sections are
characterized by a great diversity of pH values. The pH of soils used for agricultural
purposes and those covered with grassland vary from extremely acid to slightly alkaline,
whereas the pH of those soils under soil-protecting forests are mostly lower and range
from ultra-acid to neutral. Significantly higher contents of phosphorus and potassium was
found in soils of arable land than in the protective forest stands. This is mostly due to the
effect of phosphorus and potassium fertilization. The presence of calcium carbonate in the
humus horizons was observed in most soil pits located on the slopes under ploughing and
tillage action, mainly on the slopes and at the foot slope positions (Table A1).
Soil bulk density determines the infiltration, available water capacity, soil porosity,
rooting depth/restrictions, soil microorganism activity, root proliferation, and nutrient
availability [46]. Changes of bulk density are different in various parts of the slopes.
Most often, the highest bulk density occurs along the top-slope [47,48]. Our research also
confirms this characteristic (Tables 1 and 2). In addition, the research of Duan et al. [49]
showed that soil bulk density in the 0–10 cm soil layer is influenced by stand planting density.
Table 1. Soil organic matter content, soil organic carbon content, soil bulk density, and pools (stocks) of soil organic carbon
accumulated in the surface layers (topsoil) along the analyzed cross sections.
Research Cross
Section
A-B
A’-B’
C-D
Soil Pit
Number
Position of
the Soil Pit *
Land Use **
SOM
(%)
Corg
(%)
ρ
(g cm−3 )
SOCP
(kg m− 2 )
I
p
r
0.94
0.55
1.610
2.46
II
s
a
0.72
0.42
1.593
1.66
III
f
a
0.90
0.52
1.272
1.26
I
p
f
0.94
0.55
1.610
2.46
II
s
f
1.85
1.07
1.624
3.14
III
f
r
0.76
0.44
1.606
1.27
I
p
a
0.72
0.42
1.617
1.76
II
s
a
1.35
0.78
1.600
2.88
III
f
a
0.36
0.21
1.610
0.77
Forests 2021, 12, 390
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Table 1. Cont.
Research Cross
Section
Soil Pit
Number
Position of
the Soil Pit *
Land Use **
SOM
(%)
Corg
(%)
ρ
(g cm−3 )
SOCP
(kg m− 2 )
I
p
f
0.79
0.46
1.593
1.68
C’-D’
II
s
f
1.48
0.86
1.477
2.03
III
f
f
1.84
1.07
1.485
2.22
I
p
a
0.53
0.31
1.611
1.54
II
s
a
0.48
0.28
1.560
1.22
III
f
a
0.51
0.30
1.502
1.33
I
p
f
0.49
0.28
1.322
0.68
II
s
f
0.78
0.45
1.299
0.88
III
f
f
0.18
0.10
1.272
0.36
I
p
a
1.16
0.67
1.187
2.24
II
s
a
0.91
0.53
1.388
1.90
III
f
a
1.01
0.59
1.424
2.17
I
p
f
1.60
0.93
1.187
3.08
II
s
f
0.77
0.45
1.176
1.58
III
f
f
0.74
0.43
1.557
1.47
I
f
a
0.49
0.28
1.643
1.26
II
s
a
0.53
0.31
1.544
1.19
E-F
E’-F’
G-H
G’-H’
I-J
III
p
a
0.32
0.19
1.608
0.75
IV
s
a
0.54
0.31
1.490
1.17
V
f
a
0.53
0.31
1.502
1.29
I
f
r
1.74
1.01
1.611
1.95
II
s
r
0.92
0.53
1.598
1.02
III
p
f
1.32
0.77
1.603
3.68
IV
s
f
2.05
1.19
1.432
5.11
V
f
a
0.48
0.28
1.606
1.34
I’-J’
* p—plateau, s—slope, f—foot of the slope; ** a—arable lands, f—soil-protecting forests, r—rangelands; SOM—soil organic matter content;
Corg—soil organic carbon content; ρ—soil bulk density; SOCP—pools (stocks) of soil organic carbon accumulated in the surface layers.
Table 2. Descriptive statistics of the soil organic carbon content, soil bulk density, and pools (stocks) of soil organic carbon
accumulated in the surface layers (topsoil) along the analyzed cross sections.
Position of the
Soil Pit *
ρ (g cm−3 )
Corg (%)
Min
Max
Average
Min
Max
SOCP (kg m−2 )
Average
Min
Max
Average
Research cross sections: A-B, C-D, E-F, G-H, I-J
p
0.19
0.67
0.43
1.19
1.62
1.53
0.75
2.46
1.75
s
0.28
0.78
0.44
1.39
1.60
1.53
1.17
2.88
1.67
f
0.21
0.59
0.37
1.27
1.64
1.49
0.77
2.17
1.35
Research cross sections: A’-B’, C’-D’, E’-F’, G’-H’, I’-J’
p
0.28
0.93
0.60
1.19
1.61
1.46
0.68
3.68
2.32
s
0.45
1.19
0.76
1.18
1.62
1.43
0.88
5.11
2.29
f
0.10
1.07
0.55
1.27
1.61
1.52
0.36
2.22
1.44
* p—plateau, s—slope, f—foot of the slope; Corg—soil organic carbon content; ρ—soil bulk density; SOCP—pools (stocks) of soil organic
carbon accumulated in the surface layers.
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The thickness of the humus horizon along the studied slopes used for agricultural
purposes depends mainly on the plowing depth, that normally ranges from 19 to 31 cm,
on average reaching to a depth of 26 cm. On almost all cross sections, the Ap-horizon
(with the exception of I-J) reaches its maximum thickness in the apical part of the slopes.
On the slopes, it is on average ca. 3 cm shallower, while, at the foot slopes, ca. 2 cm. In
case of sodded and afforested slopes, the thickness of the humus horizon is smaller and
ranges from 12 to 30 cm (average 22 cm). The spatial variability of the A-horizon depth
shows some deviations from the regularities observed on the slopes used for agricultural
purposes. For example, the concave Northern exposed slopes, that are covered with 57year-old soil-protection forest with a fresh mixed forest habitat type (cross section G’-H’)
show the maximum thickness of humus horizons on the slopes, while the minimum depth
was registered on the foot slopes. However, on the convex Northern exposed slope of
cross section I’-J’, covered with a 45-year-old protective forest, the thickness of the humus
horizon does not change throughout the cross section.
The content of organic matter in the upper layers of the agriculturally used soils of the
cross sections amounts to an average of 0.41% ranging from 0.19 to 0.78% (Table 1). Higher
SOC content can be found in soils covered with grassland and under the soil-protecting
forests (average 0.64%), reaching a maximum value of 1.19%. In cross sections A-B, E-F, and
G-H, we observed the depletion of SOC on the slopes according to the erosion dynamics,
with an average of 0.17%. However, in case of most of the afforested slopes (cross sections
A’-B’, C’-D’, E’-F’, I’-J’), the SOC content on the slope is from 0.29 to 0.91% higher than on
the plateau areas. Obviously, the fact that, in some soils, there is an organic horizon, some
SOC values for the tested surface layer are strongly influenced by the thickness of the O
horizon in this pedon and spatial heterogeneity, and not by specific processes in a given
landscape.
Our study shows a high variability of topsoil organic carbon stocks on the slopes, in
relation to the land use (Table 1). In the surface layers of soils used for agricultural purposes,
SOC stock was much lower (varies from 0.75 to 2.88 kg m−2 with an average of 1.58 kg m−2 )
than in forest soils that were not subject to agriculture where it varies from 0.36 to 5.11 kg
m−2 with an average of 2.00 kg m−2 (Table 2). The pool of SOC reaches its maximum value
of 5.11 kg m−2 on the slope transect I′ –J′ , which is covered with 45-year-old protective
forest of fresh mixed forest habitat type. These values are slightly lower, e.g., in relation
to the mean pools of SOC accumulated in surface layers (0–10 cm) of forest soils in the
Karkonosze Mountains in Poland, calculated by Szopka et al. [5]. These authors, referring
to References [50,51], note, however, that the comparison of carbon pools accumulated in
soils, reported by various authors, faces problems due to the different profile depths. In the
cited studies, the depth varies from 20 cm, over 30–50 cm, up to 100 cm, and is including or
excluding the forest litter. Moreover, the determination of soil bulk density, particularly in
organic horizons, also varies since, in some studies, it is estimated in others measured.
Our research has shown that the average SOC stocks on afforested slopes are higher
compared to the adjacent agricultural slopes, regardless of the position of the soil pits
(Table 2). These values are higher by about 33% in the case of the plateau position, by 37%
on the slopes, and by 7% at the foot slope positions under forests in relation to the adjacent
agricultural slopes. These results confirm the conclusions from our earlier work that soilprotecting forests provide an effective protection from the washing out and blowing away
of soil particles, unwanted surface transformations of genetic horizons, and organic matter
decline due to erosion processes. This is very important in the context of climate change
mitigation through carbon sequestration. Indeed, soil management strategies targeting
towards a carbon sequestration will be most effective when accompanied by measures that
reduce soil erosion due to the fact that erosion loss can balance potential carbon uptake,
particularly in sloping areas [52].
The results of the Pearson’s correlation analyses of the topsoil organic carbon stocks
and nine soil properties analyzed are given in Table 3. Pearson’s correlation analysis
revealed that there were no significant correlations between the SOC stocks and most of the
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soil physico-chemical properties, and that the correlation between soil physico-chemical
properties was not consistent across the slope positions. As expected, significant positive,
very high correlation was found between Corg and SOCP both on agricultural slopes and
slopes under soil-protecting forests (r = 0.737 and 0.843, respectively; p = 0.001). In the case
of agricultural slopes, significant positive correlations between SOCP and the content of
clay and potassium were also found. In turn, in the case of slopes under soil-protecting
forests, significant positive correlation between SOCP and the content of potassium and
magnesium were found. The negative correlation between soil organic carbon stocks and
sand content is in agreement with many studies [53,54]. As indicated by Conforti et al. [55],
generally, soils with a high content of sand are well aerated and tend to have low soil
moisture content, which is due to a rapid decomposition and a low stabilization of the
organic carbon. As in some studies [56–58], we also found weak correlations between soil
organic carbon stocks and clay content (mostly on agricultural slopes). As pointed out by
Reference [59], though many studies have established a strong relationship between SOC
and clay contents due to the key role of clays in soil physiochemical processes, there is no
clear-cut evidence on the role of clays stabilizing SOC. Significant differences, depending
on the use of the slopes, were noted for the correlation between SOCP and pH and bulk
density. In the case of agricultural slopes, the correlations were negative, and, in the case of
slopes under forests, they were positive. In contrast, P content positively correlated with
the SOCP on the agricultural slopes, but not on slopes under soil-protecting forests.
Table 3. Pearson’s correlations among the topsoil organic carbon stock (SOCP) and selected soil properties analyzed in the
study area.
Correlation Coefficient
Variable
Agricultural Slopes
Slopes under Soil-Protecting Forests
Sand (2.0–0.05 mm)
−0.350 *
−0.153 *
Silt (0.05–0.002 mm)
0.347 *
0.154 *
Clay (<0.002 mm)
0.365 *
0.140 *
P
0.262 *
−0.149 *
K
0.447 **
0.544 **
Mg
0.415 **
0.470 *
pH
−0.213 *
0.108 *
Corg
0.737 ***
0.843 ***
ρ
−0.201 *
0.336 *
*—significance level p = 0.1; **—significance level p = 0.05; ***—significance level p = 0.001; bold—statistically significant correlations.
4. Conclusions
Soil-protecting forests provide an effective protection from the washing out and
blowing away of soil particles, unwanted surface transformations of genetic horizons,
and organic matter decline due to erosion processes. The surface layers of the soils on
the slopes under the protective forest stands, in relation to the neighboring arable soils,
tend to have a higher content of SOM and higher pool of SOC. The highest pool of SOC
(5.11 kg m−2 ) was observed in the surface layers of Brunic Arenosols on the slope transect
I′ –J′ , which is covered with a 45-year-old protective forest of fresh mixed forest habitat
type. In agricultural soils, the stock of organic carbon was much lower, with an average
SOC stock of 1.58 kg m−2 , than in forest soils, that show on average 2.00 kg m−2 .
Pearson’s correlation analysis revealed that there were no significant correlations between the organic carbon stocks in topsoil and most of the soil physico-chemical properties,
and that the correlation between soil physico-chemical properties was not consistent across
the slope positions. As expected, significant positive, very high correlation was found
between Corg and SOCP and a negative correlation between the soil organic carbon stock
Forests 2021, 12, 390
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and sand content both on agricultural slopes and slopes under soil-protecting forests. In the
case of agricultural slopes, significant positive correlations between SOCP and the content
of clay and potassium were also found. In turn, in the case of slopes under soil-protecting
forests, significant positive correlation between SOCP and the content of potassium and
magnesium were found. Significant differences, depending on the use of the slopes, were
noted for the correlation between SOCP and pH and bulk density. These results show that
land use and slope gradients are important factors controlling soil organic carbon pools in
topsoil in young glacial landscapes.
Our study confirms other investigations that indicated the significant role of forest
management on the increase of soil organic carbon stock. This is very important in the
context of climate change mitigation through carbon sequestration. Soil management
strategies targeting carbon sequestration will be most effective when accompanied by
measures that reduce soil erosion.
Author Contributions: Conceptualization, P.W.; methodology, P.W. and M.M.; formal analysis, P.W.
and M.M.; investigation, P.W. and M.M.; resources, P.W.; data curation, P.W.; writing—original draft
preparation, P.W. and M.M.; writing—review and editing, P.W. and M.M.; visualization, P.W. All
authors have read and agreed to the published version of the manuscript.
Funding: The APC was funded by Department of Landscape Research and Environmental Management, University of Gdansk.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
pH KCl
I
p
r
55.4
40.8
3.8
SL
10.4
12.0
7.5
-
6.9
II
s
a
53.9
42.1
3.9
SL
18.7
12.5
6.0
0.85
7.3
III
f
a
98.1
1.9
0.0
S
11.2
2.0
0.8
0.30
4.7
I
p
f
55.4
40.8
3.8
SL
10.4
12.0
7.5
-
6.9
II
s
f
57.3
39.2
3.5
SL
3.4
8.5
13.1
0.15
7.1
III
f
r
75.2
23.1
1.7
LS
2.8
20.0
5.2
-
5.4
I
p
a
72.1
25.3
2.6
SL
19.9
18.0
6.0
-
5.0
II
s
a
66.4
30.4
3.2
SL
18.3
31.0
6.2
0.35
5.6
III
f
a
81.0
17.4
1.6
LS
11.0
15.5
3.0
traces
5.5
2.0–0.05
0.05–0.002
<0.002
Texture Class ***
CaCO3 (%)
C-D
Land Use **
A’-B’
Concentration of Available
Forms(mg 100g−1 )
Position of the Soil Pit *
A-B
Percentage of Particle Size
Fractions (mm)
Soil Pit Number
Research cross Section
Table A1. Particle size distribution and selected chemical and physico-chemical properties of soils along the analyzed cross
sections, at a depth of 0–10 cm [39].
P
K
Mg
Forests 2021, 12, 390
12 of 14
G’-H’
I-J
I’-J’
I
p
f
74.9
23.2
1.9
LS
4.5
5.5
1.8
-
3.8
II
s
f
81.0
17.9
1.1
LS
1.8
4.5
0.9
-
3.5
III
f
f
79.7
19.5
0.8
LS
2.8
8.5
2.9
-
3.2
I
p
a
78.2
19.6
2.2
LS
53.0
7.5
4.4
2.46
7.4
II
s
a
86.7
12.1
1.2
S
24.6
7.0
3.3
0.70
7.2
III
f
a
84.0
14.6
1.4
LS
36.0
9.0
4.1
0.75
7.4
I
p
f
81.9
17.0
1.1
LS
6.2
2.5
1.3
-
4.1
II
s
f
90.4
9.6
0.0
S
2.7
1.0
0.9
-
3.7
III
f
f
90.2
9.4
0.4
S
5.9
1.5
1.6
-
4.3
I
p
a
92.1
7.5
0.4
S
23.9
5.5
2.2
traces
5.8
II
s
a
87.8
11.5
0.7
S
22.9
6.0
2.0
traces
4.4
III
f
a
81.2
17.3
1.5
LS
20.3
10.0
3.4
-
4.6
I
p
f
93.8
6.2
0.0
S
13.0
1.5
0.5
-
3.6
II
s
f
95.2
4.6
0.2
S
15.9
1.5
0.5
-
4.2
III
f
f
89.4
10.0
0.6
S
9.5
2.0
1.0
-
4.0
2.0–0.05
0.05–0.002
<0.002
Texture Class ***
pH KCl
G-H
CaCO3 (%)
E’-F’
Land Use **
E-F
Concentration of Available
Forms(mg 100g−1 )
Position of the Soil Pit *
C’-D’
Percentage of Particle Size
Fractions (mm)
Soil Pit Number
Research cross Section
Table A1. Cont.
P
K
Mg
I
f
a
68.2
29.2
2.6
SL
18.2
9.8
3.3
0.20
7.2
II
s
a
83.8
14.9
1.3
LS
9.7
5.0
2.0
2.54
7.7
III
p
a
77.1
21.0
1.9
LS
3.8
4.0
1.8
-
4.8
IV
s
a
74.7
23.3
2.0
LS
13.2
9.0
4.2
0.55
7.3
V
f
a
74.6
23.5
1.9
LS
19.0
9.0
3.6
0.93
7.3
I
f
r
62.6
35.0
2.4
SL
31.0
35.0
15.1
traces
6.5
II
s
r
87.1
12.0
0.9
S
7.3
9.0
3.4
2.56
7.4
III
p
f
82.5
16.3
1.2
LS
5.4
10.5
3.4
-
4.1
IV
s
f
90.2
9.6
0.2
S
2.0
6.0
5.1
-
3.6
V
f
a
77.4
21.1
1.5
LS
18.2
13.0
3.2
0.35
7.2
* p—plateau, s—slope, f—foot of the slope; ** a—arable lands, f—soil-protecting forests, r—rangelands; *** Soil texture is defined according
to the USDA textural classification [37]: S—sand, LS—loamy sand, SL—sandy loam.
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