Carbon sequestration potential of soils in southeast Germany derived from stable soil organic carbon saturation
Post on 27-Mar-2017
Carbon sequestration potential of soils in southeastGermany derived from stable soil organic carbonsaturationMART IN WIESME IER * , R ICO HUBNER , P ETER SP ORLE IN , UWE GEU ,
EDZARD HANGEN , ARTHUR RE I SCHL , B ERND SCH ILL ING , MARG IT VON L UTZOW*
and INGRID K OGEL-KNABNER*
*Lehrstuhl fur Bodenkunde, Department fur Okologie und Okosystemmanagement, Wissenschaftszentrum Weihenstephan fur
Ernahrung, Landnutzung und Umwelt, Technische Universitat Munchen, Freising-Weihenstephan 85350, Germany, Lehrstuhl
fur Wirtschaftslehre des Landbaues, Wissenschaftszentrum Weihenstephan fur Ernahrung, Landnutzung und Umwelt, Technische
Universitat Munchen, Freising-Weihenstephan 85350, Germany, Bavarian Environment Agency, Hof 95030, Germany,
Institute for Advanced Study, Technische Universitat Munchen, Garching 85748, Germany
Sequestration of atmospheric carbon (C) in soils through improved management of forest and agricultural land is
considered to have high potential for global CO2 mitigation. However, the potential of soils to sequester soil organic
carbon (SOC) in a stable form, which is limited by the stabilization of SOC against microbial mineralization, is largely
unknown. In this study, we estimated the C sequestration potential of soils in southeast Germany by calculating the
potential SOC saturation of silt and clay particles according to Hassink [Plant and Soil 191 (1997) 77] on the basis of
516 soil profiles. The determination of the current SOC content of silt and clay fractions for major soil units and land
uses allowed an estimation of the C saturation deficit corresponding to the long-term C sequestration potential. The
results showed that cropland soils have a low level of C saturation of around 50% and could store considerable
amounts of additional SOC. A relatively high C sequestration potential was also determined for grassland soils. In
contrast, forest soils had a low C sequestration potential as they were almost C saturated. A high proportion of sites
with a high degree of apparent oversaturation revealed that in acidic, coarse-textured soils the relation to silt and clay
is not suitable to estimate the stable C saturation. A strong correlation of the C saturation deficit with temperature
and precipitation allowed a spatial estimation of the C sequestration potential for Bavaria. In total, about 395 Mt
CO2-equivalents could theoretically be stored in A horizons of cultivated soils four times the annual emission ofgreenhouse gases in Bavaria. Although achieving the entire estimated C storage capacity is unrealistic, improved
management of cultivated land could contribute significantly to CO2 mitigation. Moreover, increasing SOC stocks
have additional benefits with respect to enhanced soil fertility and agricultural productivity.
Keywords: agricultural management, climate change, CO2 mitigation, soil organic carbon stocks, soil fractionation, stabilization
of soil organic matter
Received 18 April 2013 and accepted 30 August 2013
Sequestration of atmospheric carbon (C) in soils is
considered to contribute significantly to CO2 mitiga-
tion, and several management options for increasing
SOC stocks have been discussed. For forest ecosystems,
practices such as a change in tree species composition,
afforestation, thinning, drainage, fertilization, liming,
site preparation and harvest management are associ-
ated with an increase in SOC stocks and are conse-
quently viewed as having a high potential for soil C
sequestration (Goodale et al., 2002; Liski et al., 2002;
Karjalainen et al., 2003; Lal, 2005; Jandl et al., 2007; Ciais
et al., 2008; Lorenz & Lal, 2010; Luyssaert et al., 2010;
Carroll et al., 2012; Vesterdal et al., 2012; Wiesmeier
et al., 2013b). An even higher C sequestration potential
is assumed for agricultural soils because a distinct
depletion of SOC stocks has been observed in most
cultivated soils (Paustian et al., 1997; Lal, 2004; Smith,
2004). Among several agricultural practices that may
increase C sequestration in cultivated soils, promising
management options are promotion of organic inputs,
conservation/zero tillage, converting cropland to grass-
land, introduction of perennials, improved manage-
ment of farmed peatland and organic farming (Cole
et al., 1997; Paustian et al., 2000; Sauerbeck, 2001; Vlees-
houwers & Verhagen, 2002; Freibauer et al., 2004; Hol-
land, 2004; Lal, 2004; Johnson et al., 2007; Smith, 2012).Correspondence: Martin Wiesmeier, tel. +49 (0)8161 71 3679,
fax +49 (0)8161 71 4466, e-mail: firstname.lastname@example.org
2013 John Wiley & Sons Ltd 653
Global Change Biology (2014) 20, 653665, doi: 10.1111/gcb.12384
However, C sequestration by improved management
of forest and agricultural soils reaches a new equilib-
rium at a higher SOC level after a certain period of
time. Several studies have shown that there is an upper
limit of SOC storage, confirming the hypothesis of soil
C saturation (Six et al., 2002; Goh, 2004; Stewart et al.,
2007, 2008; Chung et al., 2008). This is related to the lim-
ited potential of soils to stabilize soil organic matter
(SOM) against microbial mineralization (Baldock &
Skjemstad, 2000). There are three major SOM stabiliza-
tion mechanisms: selective preservation due to recalci-
trance of SOM, spatial inaccessibility of SOM due to
hydrophobicity or occlusion in soil aggregates, and
interaction with mineral surfaces (Sollins et al., 1996;
von Lutzow et al., 2006). The last is regarded as quanti-
tatively the most important in a wide range of soils, as
indicated by a strong correlation of SOC stocks with
clay contents (e.g. Oades, 1988; Arrouays et al., 2006).
Hassink (1997) assumed that the capacity of soils to
preserve SOC is limited by the proportion of silt and
clay particles (fine fraction
main land uses were adequately represented, with 115 loca-
tions (22% of the data) as cropland (34% of the total area), 110
locations (21%) as grassland (16%), 249 locations (48%) as
forest (35%) and 42 locations (8%) under other land uses
(15%). The main part of the data constituted a grid sampling
within Bavaria (Joneck et al., 2006). Between 2000 and 2004,
soil profiles were sampled using grids of 8 9 8 km within
Bavaria. For each soil profile, a representative location was
selected within a radius of 500 m around the grid node to
achieve a homogeneous sampling area in terms of vegetation,
relief, soil type and parent material as well as a central posi-
tion in the particular land use type. Anthropogenic distur-
bances in the subsoil were excluded in a pre-exploratory
survey using a soil auger. Topsoil material was collected as a
composite sample from eight sub-locations around one main
soil profile to cover the small-scale heterogeneity of the soils.
At the main soil profile, steel core samples with a diameter of
10 cm were extracted for topsoil horizons. A small number of
soil profiles originated from permanent soil monitoring sites
(Schubert, 2002) and other regional soil surveys.
Determination of soil properties
The proportion of SOC stored in the fraction
and curvature were determined. As secondary parameters,
the contributing area (CA) and the topographic wetness index
(TWI) were calculated using the following equation:
TWI ln CAtan a
where CA is the specific upslope contributing area derived by
a geographical information system and a is the slope. The TWIis a topographical variable that indicates soil moisture condi-
tions (Beven & Kirkby, 1978; Sorensen et al., 2006). To include
geology as a potential parameter influencing the C saturation,
parent material data were assigned from a map with 35 parent
material classes (BAG500) with a resolution of 2 km from the
Bavarian Environment Agency. Information about the soil type
was included using a generalized soil map (BUK1000N) with
28 superior soil classes (Leitbodenassoziationen) with a resolu-
tion of 2 km from the Federal Institute for Geosciences and
Natural Resources. The factor land use was incorporated by
using 2006 satellite data from the CORINE Land Cover project
(CLC2006) from the German Remote Sensing Data Center. For
climatic variables, annual precipitation and mean annual tem-
perature determined between 1981 and 2010 by the German
Weather Service with a resolution of 1 km were allocated. All
environmental parameters were assigned to 25 9 25 m cells.
Descriptive statistics were applied to describe the soil data sets
including mean, minimum and maximum values, median,
interquartile range, extremes and outliers, skewness and kur-
tosis. In order to scale current C concentrations of the fine frac-
tion 100 cm; 2 = 95100 cm;3 = 9095 cm; 4 =
fraction, which was calculated according to the equa-
tion of Hassink (1997), was also similar between land
uses and ranged between 19.7 and 20.8 mg g1.The current C concentration of the fine fraction was
measured for soils under major land uses in Bavaria
(Fig. 1). For cropland soils, the proportion of OC
stored in the fraction
with the proportion of the fine fraction, particularly
in cropland soils (Fig. 4). In contrast, forest soils and
soils under other land uses showed no significant
relationship with the fine fraction content. To gain
insight into the factors that control the C sequestration
potential, correlations between several environmental
Fig. 2 Correlation between the proportion of particles
parameters and the C saturation deficit were examined
(Table 3). Strong positive correlations (P < 0.01) werefound with mean annual temperature and pH and
strong negative correlations (P < 0.01) with annual pre-cipitation, elevation, slope and soil class. However, a
multiple linear regression analysis that included two
factors derived from a PCA (Table 4) revealed that the
C saturation deficit was strongly controlled by one fac-
tor that showed high loadings of temperature, precipi-
tation and elevation.
Fig. 4 Correlation between proportion of particles
The C saturation deficit of agricultural soils
The estimation of the C saturation deficit in soils of
Bavaria revealed that it was dependent on the current
C content of the fine fraction. The potential C saturation
as a function of the proportion of particles
compared with cropland soils. This is in the range
reported for pastures in the south-eastern United
States, where a silt- and clay-associated C saturation of
60% was determined (Conant et al., 2003). A higher C
saturation in pastures compared with cropland was
also detected in Australia, though on a lower C satura-
tion level (Chan, 2001).
In summary, both cropland and grassland soils in
Bavaria have a substantial potential to sequester addi-
tional amounts of C in a stable form. A positive rela-
tionship of the C saturation deficit with the silt and clay
content (Fig. 4) revealed that soils that are particularly
fine textured have a large potential for C sequestration.
C saturation in forest soils and limitations of Hassinksequation
In forest soils, a distinctly different level of C saturation
in the fine fraction was observed as compared with
agricultural soils. The current C concentration
increased slightly with the proportion of particles
degraded grasslands worldwide (Conant & Paustian,
2002). For each land use, thresholds of temperature and
precipitation were derived (point of intersection of the
regression line at a C saturation deficit of 0) that divide
the areas of the respective land uses into regions with
saturated and unsaturated conditions (Table 5). The C
sequestration potential for each land use was estimated
by multiplying the area of unsaturated conditions by
the median value of the C saturation deficit of this area.
A regionalization of the C sequestration potential using
geostatistical methods was not conducted as the C satu-
ration deficit of the investigated locations was calcu-
lated using only an estimation of the current C
saturation of the fine fraction, which was based on a
smaller data set.
The results revealed that cropland and grassland
soils of Bavaria could potentially sequester 32 and 6 Mt
C in the uppermost 10 cm respectively. The high poten-
tial of cropland soils is related to the high C saturation
deficit of intensively cultivated soils and a large area
with unsaturated conditions. Less than 1% of the total
cropland area of Bavaria was assigned to saturated
conditions as cultivation is not feasible in cool, humid
areas. For forest soils, a C sequestration potential of
only 4 Mt was estimated with a high uncertainty as
previously explained. Other land uses have a potential
to sequester 9 Mt C. The low potential of forest soils to
sequester C can be ascribed to almost saturated condi-
tions in forest soils and the fact that only half of the
total forest area was associated with unsaturated condi-
tions. About 50% of Bavarian forests are located in
regions where cool, humid conditions result in a
complete C saturation of silt and clay particles.
The C sequestration potential estimated for the first
10 cm of the soil was extrapolated to the median depth
of A horizons of each land use, assuming that soil
texture and SOC contents are comparable within the A
horizon. The C sequestration potential of A horizons
under cropland, grassland, forest and other uses was
estimated to be 96, 12, 4 and 22 Mt respectively. In total,
soils of Bavaria could additionally sequester 134 Mt C,
18% of total SOC stocks of 764 Mt. This amount corre-
sponds to 490 Mt CO2-equivalents (CO2-eq.), which is
more than five times higher than the annual greenhouse
gas emission (in 2009) in Bavaria of 94 Mt CO2-eq.
(UGRdL, 2012). The majority, 395 Mt CO2-eq.
Fig. 5 Correlation between mean annual temperature and the C saturation deficit (Csat-def) for cropland (C), grassland (G), forest (F)
and other land uses (O). Values above the dashed line refer to a deficit of C saturation, values below the dashed line refer to an oversat-
uration of C.
2013 John Wiley & Sons Ltd, Global Change Biology, 20, 653665
662 M. WIESMEIER et al.
(approximately 80%), could be sequestered in agricul-
tural soils. An increase in C saturation in forest soils
and soils under other land uses is associated with high
uncertainty. Assuming that due to an improved man-
agement of cultivated soils the theoretical stable C stor-
age potential is reached after a mean period of 30 years
(West & Six, 2007), a mean annual amount of 13 Mt
CO2-eq. could be sequestered in Bavarian soils over this
period of time, which is 14% of the annual emission of
greenhouse gases in Bavaria (in 2009). On an area basis,
4.1 t CO2-eq. ha1 yr1 could potentially be seques-
tered in agricultural soils, which is considerably higher
than observed and modelled C accumulation rates
through various management options aimed at increas-
ing SOC stocks in cultivated soils (Vleeshouwers &
Verhagen, 2002; West & Post, 2002; Freibauer et al.,
2004; Smith et al., 2008).
A comparison of the current C amount with the poten-
tial C saturation of silt and clay particles according to
Hassink (1997) revealed high C sequestration potential
of agricultural topsoils in Bavaria. Although there are
some large uncertainties regarding the efficiency and
practicability of proposed management options to
Fig. 6 Correlation between annual precipitation and the C saturation deficit (Csat-def) for cropland (C), grassland (G), forest (F) and
other land uses (O). Values above the dashed line refer to a deficit of C saturation, values below the dashed line refer to an oversatura-
tion of C.
Table 5 Threshold of mean annual temperature (MATunsat) and annual precipitation (MAPunsat) for unsaturated soils, area of satu-
rated (Areasat) and unsaturated (Areaunsat) soils, C sequestration potential to a depth of 10 cm (Cseq-0-10) and extrapolated for the A
horizon (Cseq-A) for different land uses within Bavaria
MATunsat (C) MAPunsat (mm) Areasat (km2) Areaunsat (km
2) Cseq (t ha1) Cseq-0-10 (Mt) Cseq-A (Mt)
Cropland (C) >6.4 7.0 8.1 8.0
increase SOC stocks, the estimated high potential of
agricultural soils for C sequestration justifies optimized
SOM management of cultivated soils. One has to bear
in mind that besides the stable C sequestration in the
fine fraction, a significant additional amount of labile
SOC will also be sequestered as a result of improved
agricultural management. Furthermore, it is important
to note that there are benefits associated with C seques-
tration beyond CO2 mitigation because increased SOM
is associated with improved soil fertility, soil structure,
water holding capacity and thus a higher productivity.
Further important aspects are reduced risk of soil
erosion, decreased eutrophication and water contami-
nation as well as reduced costs for fossil fuel and fertil-
izer inputs (Paustian et al., 1998; Lal, 2007). Further
studies are needed to connect the estimated C seques-
tration potential of Bavarian soils with the economical
and political feasibility of agricultural practices aimed
at increasing SOC stocks. Such considerations should
include not only the possible range of CO2 mitigation,
but also additional benefits of SOM increases such as
improved soil fertility and productivity.
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