Download - Carbon sequestration potential of soils in southeast Germany derived from stable soil organic carbon saturation

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 GEUß ‡ ,

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

Ern€ahrung, Landnutzung und Umwelt, Technische Universit€at Munchen, Freising-Weihenstephan 85350, Germany, †Lehrstuhl

fur Wirtschaftslehre des Landbaues, Wissenschaftszentrum Weihenstephan fur Ern€ahrung, Landnutzung und Umwelt, Technische

Universit€at Munchen, Freising-Weihenstephan 85350, Germany, ‡Bavarian Environment Agency, Hof 95030, Germany,

§Institute for Advanced Study, Technische Universit€at Munchen, Garching 85748, Germany

Abstract

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 of

greenhouse 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

Introduction

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: [email protected]

© 2013 John Wiley & Sons Ltd 653

Global Change Biology (2014) 20, 653–665, 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 <20 lm). He found a strong

correlation of SOC stored in the fraction containing silt

and clay particles in a wide range of uncultivated and

grassland topsoils of temperate and tropical regions

and proposed that this correlation could be used to esti-

mate the stable SOC saturation. The difference between

the potential C saturation of the fine fraction and the

actual measured C content of this fraction corresponds

to the C saturation deficit or, in other words, the C

sequestration potential. Hassink’s approach was used

in several experimental studies to estimate the degree

of C saturation in cultivated soils (Chan, 2001; Six et al.,

2002; Carter et al., 2003; Conant et al., 2003; Sparrow

et al., 2006; Zhao et al., 2006). However, these studies

included only a limited number of investigated loca-

tions, thus the results are not suitable to derive compre-

hensive conclusions for larger (landscape) scales.

Recently, Angers et al. (2011) estimated the C saturation

deficit of French agricultural topsoils using Hassink’s

approach on the basis of about 1.5 million SOC and 0.3

million particle-size determinations. However, the

results must be regarded as a very rough estimation of

the C saturation deficit as the actual C content of the

fine fraction was not measured; instead, an estimated

constant proportion of 85% of total SOC was assumed.

Moreover, no absolute amount of the C sequestration

potential could be derived due to missing data for bulk

density (BD). The need for quantitative studies estimat-

ing the C sequestration potential for various soils and

land uses on larger scales was highlighted in a recent

review of SOC sequestration (Stockmann et al., 2013).

In this study, we used a comprehensive soil data set

within the state of Bavaria in southeast Germany to

estimate the C sequestration potential of soils under the

main land uses of cropland, grassland and forest and to

gain insight into the controlling factors. For 516 soil

profiles, data on soil texture, SOC content, BD and

stone content (SC) enabled a quantification of the C

saturation potential according to Hassink (1997). The

determination of the current C concentration of all

major soil units and land uses in Bavaria allowed a

calculation of the C sequestration potential. The

objectives of the study were to do the following

1. Determine the C saturation deficit in agricultural

and forest soils.

2. Elucidate relationships between land use-specific C

saturation deficits and environmental factors.

3. Quantify the total C sequestration potential of soils

within Bavaria.

Materials and methods

Study area

The state of Bavaria, with an area of 70 550 km2, is located in

southeast Germany and comprises various landscapes. The

north-western part of Bavaria is dominated by the southern

German escarpment landscape that adjoins low mountain

ranges of the Bohemian Massif in the east. Southwards the

Molasse basin ascends to the mountain range of the Alps at

the southern border of Bavaria. Elevation ranges between 107

and 2962 m above sea level. Due to its location in central

Europe, Bavaria exhibits a suboceanic climate that is charac-

terized by a transitional situation between a maritime climate

in the north-west and subcontinental influences in the east.

Mean annual temperature and precipitation from the escarp-

ment landscape in the north-west to the Alps in the south

range between 9 and 4 °C and 550 and 2500 mm respectively.

Dominant soil classes are soils with well-developed B hori-

zons (Cambisols) at 45% of the total area, soils with initial soil

formation (Leptosols, Regosols) at 14% and soils with water

stagnation (Stagnosols, Albeluvisols, Planosols) at 13% accord-

ing to the German soil systematic (AD-HOC AG Boden, 2005)

and the equivalent Reference Soil Groups of the WRB system

(IUSS Working Group WRB, 2006).

Compilation of soil data

Available data from different soil surveys and permanent soil

observation sites in Bavaria overseen by the Bavarian Environ-

ment Agency and the Bavarian State Institute for Forestry

were screened to compile a representative data set. Sampling

locations were incorporated only where topsoil material had

been analysed for SOC, BD, SC and soil texture. The minimum

requirement for SOC analysis was a determination by dry

combustion using a CN elemental analyser. Generally, only

soil data collected after 1990 were considered to reduce the

impact of temporal SOC changes. The selected sampling loca-

tions that fulfilled all requirements comprised 516 sites. The

© 2013 John Wiley & Sons Ltd, Global Change Biology, 20, 653–665

654 M. WIESMEIER et al.

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 <20 lm was

obtained by physical fractionation of available topsoil material

from 95 locations of the compiled soil data set which are rep-

resentative of the main land uses within Bavaria. For each

land use type, three sampling locations were selected for all

major soil units within Bavaria to cover the range of environ-

mental conditions in terms of soil type, parent material and

climate. For the fractionation, 30 g of soil material <2 mm was

suspended in 150 ml of deionized water and dispersed using

a calibrated ultrasonic probe-type (Bandelin, Berlin, Germany)

with an output energy of 22 J ml�1. This relatively low energy

was applied to disrupt only weakly stabilized soil macroag-

gregates and to prevent the disruption of mineral-associated

SOM (Amelung & Zech, 1999). The stable SOC fraction was

separated by sieving the suspension at 20 lm using pressure

filtration to overcome capillary forces at a 20 lm level

(Amelung & Zech, 1999; Spielvogel et al., 2006; Steffens et al.,

2009) followed by a subsequent extraction of labile dissolved

organic carbon (OC) using a 0.45 lm membrane filter. The

fraction <20 lm was dried at 40 °C, weighed, ground and

analysed in duplicate for OC concentration together with bulk

topsoil samples by dry combustion using a Vario EL elemental

analyser (Elementar, Hanau, Germany).

For the analyses of soil texture, soil samples (<2 mm) were

oxidized with H2O2 to remove organic matter (OM). The

remaining material was dispersed with Na4P2O7 and shaken

for 16:00 hours to 24:00 hours, followed by wet sieving to iso-

late sand fractions >63 lm. To determine silt and clay frac-

tions, approximately 3 g of the <63 lm fraction was

suspended in deionized water using Na4P2O7 and an ultraso-

nication for 3 min with 75 J ml�1 was conducted. Afterwards,

the distribution of silt and clay fractions was obtained by

measuring the X-ray absorption of the soil–water suspension

during sedimentation of the soil particles using a Micromeri-

tics Sedigraph 5100 (Micromeritics, Norcross, GA, USA)

(Sp€orlein et al., 2004). The proportion of the fine frac-

tion <20 lm consisted of medium silt (20–6.3 lm), fine silt

(6.3–2.0 lm) and clay (<2.0 lm). BD was quantified with the

mass of the oven-dry soil (105 °C) divided by the core volume

(Hartge & Horn, 1989). Soil pH values were measured in

0.01 M CaCl2 at a soil to solution ratio of 1 : 2.5 at room tem-

perature.

Calculation of C saturation and C sequestration potential

The potential C saturation of particles <20 lm was calculated

using the equation of Hassink (1997):

Csat�pot ¼ 4:09þ 0:37� particles � 20 lm(%) ð1Þwhere Csat-pot is the potential C saturation (mg g�1), which is

calculated using a linear regression with an intercept of 4.09

and a slope of 0.37 multiplied by the proportion of fine soil

particles <20 lm (%). For the calculation of the C saturation

deficit of a location, the current C concentration of the fine

fraction <20 lm has to be determined. Due to the laborious

soil fractionation, the current C concentration of the fine frac-

tion <20 lm was determined for representative soils under

main land uses of the study area at 95 locations (21 cropland,

32 grassland and 42 forest sites). The relative proportion of the

current C concentration of the fine fraction <20 lm was calcu-

lated for soils under the main land uses cropland, grassland,

forest and other uses and multiplied by the total SOC concen-

tration of topsoils of all 516 sampling locations, which were

allocated to these land uses. To account for uncertainty that

may result from natural variation in the measured current C

concentration of each land use type, a Monte Carlo simulation

was applied (Larocque et al., 2008). To calculate the C satura-

tion deficit of the sampling locations, the estimated current C

concentrations of the fine fraction were subtracted from the

potential C saturation:

Csat�def ¼ Csat�pot � Ccur ð2Þwhere Csat-def is the C saturation deficit (mg g�1) and Ccur is

the current mean C concentration of the fine fraction <20 lm(mg g�1). The total amount of the C sequestration potential

was calculated using the following equation:

Cseq ¼ Csat�def � BD� ð1� RFÞ � T� 10�2 ð3Þwhere Cseq is the C sequestration potential (kg m�2), Csat-def is

the C saturation deficit (mg g�1), BD is the bulk density

(g cm�3), RF is the volumetric fraction of rock fragments

>2 mm (%) and T is the topsoil thickness (10 cm).

Environmental variables

Several environmental parameters that potentially impact the

C saturation deficit were compiled to gain insight into the

processes controlling the C sequestration potential of soils. On

the basis of a digital elevation model with a resolution of 25 m

from the Bavarian Surveying and Mapping Authority, differ-

ent topographical parameters were calculated (Wilson &

Gallant, 2000). As primary terrain attributes, elevation, slope


SOC SATURATION AND SEQUESTRATION POTENTIAL 655

and curvature were determined. As secondary parameters,

the contributing area (CA) and the topographic wetness index

(TWI) were calculated using the following equation:

TWI ¼ lnCA

tan a

� �ð4Þ

where CA is the specific upslope contributing area derived by

a geographical information system and a is the slope. The TWI

is 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 (B€UK1000N) 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.

Statistical analysis

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 <20 lm from relatively few 95 representative locations to

all 516 sampling locations within the study area, natural vari-

ability had to be accounted for (Landres et al., 1999). Larocque

et al. (2008) highlight the importance of input data amplitudes

for modelling, due to their effect on the model outcomes sensi-

tive to variations in the inputs. For our simple steady-state

model (output independent of time) thus some degree of sto-

chasticity was included by using a Monte Carlo approach. In

the first step, input data distributions were determined for

each land use type subset. In a second step, random generated

values were applied to the model and results of 1.000 itera-

tions were recorded. Model results were used further for the

prediction of the C saturation deficit and the respective stor-

age potential on the landscape level of the study region.

For the analysis of the importance of a variable to the C

saturation deficit, Pearson’s correlation coefficients among all

parameters were calculated. To include the nominally scaled

parameters soil class and parent material, the different classes

of soil and parent material were ranked according to their

potential to store SOM. For the factor soil class, the ranking

was done by considering the properties soil profile thickness,

soil texture and wetness of the respective soil class which

largely impact total SOC storage. Median values of the soil

profile thickness were calculated for all soil classes and

grouped into four classes (1 = >100 cm; 2 = 95–100 cm;

3 = 90–95 cm; 4 = <90 cm). For soil texture and wetness,

which control the stabilization and degradation of SOM,

empirical values for all soil classes based upon expert knowl-

edge and the literature (AD-HOC AG Boden, 2005) were used

to derive a texture/wetness gradient composed of four classes

ranging from very moist/high clay contents (class 1) to dry/

high sand contents (class 4). The soil classes were grouped into

five classes according to the combined assignment to profile

thickness and texture/wetness classes (the lower the cumula-

tive value of classes, the higher the potential for SOC storage).

To estimate the SOC storage potential of soils based on differ-

ent parent materials, the degree of weatherability and clay con-

tents in the parent material as well as in the weathering

product derived from expert knowledge were considered. For

the ranking, the different parent materials were grouped into

10 units ranging from highly weathered, clay-rich material

with a high potential for soils to store SOC to material with a

low degree of weatherability and low proportions of clay.

Incorporation of nominally scaled parameters using dummy

variables was dismissed due to the high number of soil classes

and parent materials. Due to strong intercorrelations between

environmental parameters, a principal component analyses

(PCA) was carried out to extract the main factors controlling

SOM storage. A stepwise multiple linear regression model was

calculated using the extracted factors. All statistical calcula-

tions were performed using the software IBM SPSS Statistics 19.

Results

Current C concentration and potential C saturation of thefine fraction

The determination of soil texture revealed similar

contributions of particles <20 lm among different land

uses. Topsoils under the main land uses cropland,

grassland and forest had almost identical proportions

of the fine fraction (44–45%; Table 1). For other land

uses, a slightly lower percentage (42%) was deter-

mined. Therefore, the potential C saturation of the fine

Table 1 Proportion of particles <20 lm, SOC concentration, pH value and potential C saturation (Csat-pot) for topsoils (0–10 cm)

under different land uses in Bavaria (median values with 25th and 75th percentile in parentheses)

n Particle<20 lm (%) SOC (mg g�1) pH (CaCl2) Csat-pot (mg g�1)

Cropland (C) 115 45 (34/56) 14.0 (11.2/19.9) 6.3 (5.6/6.8) 20.8 (16.8/24.7)

Grassland (G) 110 45 (32/63) 26.4 (17.7/34.6) 5.5 (4.8/6.1) 20.6 (15.9/27.3)

Forest (F) 249 44 (30/58) 49.0 (31.6/73.3) 3.5 (3.2/3.9) 20.2 (15.3/25.4)

Other use (O) 42 42 (30/63) 14.5 (12.1/20.7) 5.8 (5.3/7.0) 19.7 (15.1/27.3)



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 g�1.

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 <20 lm related to the total OC

content of the bulk soil was in a relatively narrow

range, with a median value of 77%. Soils under grass-

land and forest showed generally lower proportions as

well as a higher variability in fine fraction OC. For

grassland soils, OC contributions of the fine fraction

ranged between 49 and 68%, with a median of 60%.

Under forest, soils showed even lower proportions of

OC stored in the fine fraction, with values of 26–46%with a median of 38%. The estimation of the current C

concentrations of all 516 sampling locations using a

Monte Carlo simulation from the results of 95 fraction-

ated locations revealed considerable differences

between the land uses (Table 2). Cropland soils

showed a relatively low median current C concentra-

tion of 9.6 mg g�1. For grassland and forest soils,

significantly (P < 0.05) higher values of 14.9 and

18.5 mg g�1 were determined. Soils under other uses

showed a current C concentration of 10.6 mg g�1.

Compared with the potential C saturation of the fine

fraction, cropland soils had a mean C saturation of

47%, grassland soils of 73%, forest soils of 93% and

soils under other land uses of 62%.

The variability in the current C concentration was

determined as a function of the proportion of the fine

fraction (Fig. 2). No significant relationship between

the current C concentration of the fine fraction and the

proportion of the fine fraction was determined for soils

under agricultural use soils and soils under other land

uses. In contrast, forest sites showed increasing

current C contents of the fine fraction with increasing

proportions of the fine fraction. However, a generally

high variability in under- and oversaturated sites was

found. For sites which were apparently oversaturated

(current C concentration exceeded the potential C satu-

ration), no clear relationship with the contribution of

the fine fraction was found except for cropland soils,

where oversaturated sites were restricted to fine frac-

tion contents below 40%.

The C sequestration potential as affected byenvironmental factors

The C sequestration potential, calculated as the differ-

ence between the current C concentration and the

potential C saturation, was considerably different

between the land uses (Table 2; Fig. 3). In cropland

soils, a low current C amount in the fine fraction of

1.3 kg m�2 compared with a potential C saturation of

3.0 kg m�2 resulted in a relatively high C sequestration

potential of 1.5 kg m�2. In grassland and forest soils,

current C amounts of 1.8 and 1.6 kg m�2 were close to

potential C saturation values of 2.4 and 1.7 kg m�2

respectively. Thus, grassland and forest soils had lower

C sequestration potentials of 0.6 and 0.1 kg m�2 respec-

tively. Soils under other uses showed a current C

amount of the fine fraction of 1.5 kg m�2 and a poten-

tial C saturation of 2.6 kg m�2, resulting in a C seques-

tration potential of 0.9 kg m�2. The C saturation deficit

of soils under agricultural use was positively correlated

Table 2 Monte Carlo simulation of the current C concentra-

tion of the fine fraction <20 lm (Ccur) and the C saturation

deficit (Csat-def) in topsoils (0–10 cm) under cropland (C),

grassland (G), forest (F) and other uses (O) derived from 95

locations with measured values for the current C concentra-

tion of the fraction >20 lm

C

n = 115

G

n = 110

F

n = 249

O

n = 42

Ccur

(mg g�1)

Minimum 4.4 4.5 2.0 3.1

Maximum 24.4 57.4 156.6 101.7

Mean 10.8 16.3 22.5 23.2

Median 9.6 14.9 18.5 10.6

Standard

deviation

4.0 8.7 17.9 25.9

Skewness 0.9 1.7 3.5 1.9

Kurtosis 0.4 4.4 19.8 2.7

Csat-def

(mg g�1)

Minimum �7.6 �35.6 �124.4 �75.4

Maximum 22.5 27.3 25.4 24.8

Mean 10.3 5.0 �2.1 �1.7

Median 11.1 5.0 1.1 4.5

Standard

deviation

6.4 9.6 16.6 23.3

Skewness 0.6 0.7 �3.3 �1.7

Kurtosis 0.1 2.3 19.6 2.9

Fig. 1 Relative proportion of the OC content of the fraction

<20 lm to the total OC content of the bulk soil for cropland (C),

grassland (G) and forest (F) soils of Bavaria.



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 <20 lm and the current C concentration of this soil fraction (Ccur) for cropland (C),

grassland (G), forest (F) and other land uses (O). The black line indicates the potential C saturation (Csat-pot) according to Hassink (1997).

Fig. 3 Current C concentration of the fine fraction <20 lm (Ccur) derived by a Monte Carlo simulation of 95 locations as well as poten-

tial C saturation (Csat-pot) and the C sequestration potential (Cseq) of topsoils (0–10 cm) under cropland (C), grassland (G), forest (F) and

other uses (O). Lines within the boxes give the median, boxes represent the 25th and 75th percentile, whiskers show the lowest and

highest values excluding outliers, circles represent outliers (1.5–3.0 interquartile range), triangles represent extremes (more than 3.0

interquartile ranges).



parameters and the C saturation deficit were examined

(Table 3). Strong positive correlations (P < 0.01) were

found 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 <20 lm and the C saturation deficit (Csat-def) for cropland (C), grassland (G), forest

(F) and other land uses (O).

Table 3 Correlation matrix of the C saturation deficit (Csat-def) and different pedogenetic, topographical and environmental

parameters

Csat-def Temp. Prec. Elev. Slope Curv. CA TWI PM Soil C. pH

Temp. 0.418**

Prec. �0.354** �0.840**

Elev. �0.420** �0.940** 0.907**

Slope �0.130** �0.484** 0.534** 0.472**

Curv. 0.065 0.126** �0.144** �0.151** �0.106*

CA �0.056 0.032 �0.029 �0.032 �0.044 �0.011

TWI 0.066 0.372** �0.290** �0.316** �0.702** 0.000 0.312**

PM �0.117** �0.333** 0.531** 0.474** 0.151** �0.117** 0.034 0.074

Soil C. �0.254** �0.208** 0.144** 0.176** 0.116** �0.009 �0.023 �0.095* �0.023

pH 0.367** 0.295** �0.149** �0.241** �0.119** �0.045 0.063 0.213** 0.051 �0.310**

SC �0.014 �0.189** 0.052 0.088* 0.193** 0.025 �0.025 �0.189** �0.059 0.111* �0.049

Level of significance: *P ≤ 0.05; **P ≤ 0.01.

Csat-def, C saturation deficit (mg g�1); Temp., mean annual temperature (°C); Prec., annual precipitation (mm); Elev., elevation (m

a.s.l.); Curv., curvature; CA, contributing area; TWI, topographic wetness index; PM, parent material; Soil C., soil class according to

AD-HOC AG Boden (2005); SC, stone content.



Discussion

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 <20 lmaccording to Hassink (1997) was almost identical

among the main land uses due to their having similar

proportions of this fraction (Table 1). The current C

concentration was distinctly different between agricul-

tural and forest soils, pointing towards a clear impact

of cultivation (Table 2; Fig. 3). Remarkably, in cropland

soils the current C content of the fine fraction was on a

constant median level of around 10 mg g�1 indepen-

dent from the fine fraction content (Fig. 2). Fine-

textured cropland soils with a high potential to stabilize

SOM showed almost the same C contents as sand-

dominated soils that were close to the potential C satu-

ration or even exceeded it. This probably indicates that

the investigated cropland soils, most of which were

under long-term cultivation, had reached a constant

equilibrium level of mineral-associated SOM that is not

(or no longer) related to the proportion of the fine frac-

tion. The fact that the relatively low amount of SOC

associated with the fine fraction in coarse-textured

cropland soils was only marginally depleted probably

reflects a more careful management of these soils in

terms of SOM supply. A study that estimated the C sat-

uration deficit of agricultural topsoils in France also

determined a high degree of C saturation in very sandy

soils (Angers et al., 2011). In contrast, fine-textured

cropland soils showed a clear depletion of silt- and

clay-associated SOC. This loss can be ascribed to the

effect of tillage that causes destruction of soil macroag-

gregates and leads to a subsequent mineralization of the

released OM (Mann, 1986; Balesdent et al., 2000; Post &

Kwon, 2000; Six et al., 2000). On the other hand, the

physical stabilization of OM is reduced due to a deterio-

ration of microaggregate formation (Six et al., 1999). Fur-

thermore, the input of OM is reduced due to removal of

crop residues with the harvest, resulting in relatively

low SOC concentrations in the topsoils of cultivated

soils (Table 1). However, analyses of a comprehensive

data set of cropland soils in Bavaria revealed that the

depletion of SOC in the uppermost 10 cm, as it was

determined in this study, was compensated by a translo-

cation of SOC with depth due to deepening of the

plough layer to 30 cm (Wiesmeier et al., 2012, 2013a).

Compared with the potential C saturation, cropland

soils of Bavaria lost more than 50% of silt- and clay-

associated SOC in the uppermost 10 cm, resulting in a

median C saturation deficit of 11.1 mg g�1. This is in

line with several studies that investigated C saturation

of cropland soils. A comprehensive study of agricul-

tural topsoils in France revealed a slightly lower med-

ian C saturation deficit of 8.1 mg g�1 (Angers et al.,

2011). However, only a very rough estimation of the

current C amount of the fine fraction (85% of bulk SOC)

was applied. Long-term cultivated temperate cropland

soils in China showed a silt- and clay-associated C

saturation of around 50% (Zhao et al., 2006). For agri-

cultural experimental sites in eastern Canada, coarse-

textured topsoils with silt and clay contents <40% were

nearly saturated, whereas fine-textured soils (>60% silt

and clay) had a C saturation of 65–70% (Carter et al.,

2003). In cropland soils of Australia, a lower C

saturation of around 35% was determined, which was

related to low precipitation and high temperatures

(Hassink, 1997; Chan, 2001). In contrast, agricultural

soils in Tasmania under a cool, temperate climate were

close to or even exceeded the potential C saturation

(Sparrow et al., 2006).

Grassland soils showed a level of potential C satura-

tion similar to that of cropland soils but a considerably

higher current C concentration of the fine fraction

(Tables 1 and 2; Fig. 3). Although the current C concen-

tration was also not significantly related to the propor-

tion of silt and clay particles, a higher median value of

14.9 mg g�1 and generally a higher variability was

detected. This is presumably attributed to a much

lower intensity of the land use and a higher above- and

belowground input of OM into grassland soils. More-

over, the higher variability in the current C concentra-

tion of grassland soils is related to a wider range of

environmental conditions compared with cropland

sites and also includes areas with a cool, humid climate

or water-affected soils, such as fens, where a high pro-

portion of SOC is not associated with the fine fraction

(Fig. 1). Due to the higher current C contents of the fine

fraction, the C saturation of 73% is distinctly higher

Table 4 Rotated component matrix derived from a principal

component analysis of variables controlling the C saturation

deficit in soils of Bavaria

Factor 1 Factor 2

Temperature �0.915 �0.088

Precipitation 0.905 0.281

Elevation 0.928 0.229

Slope 0.702 �0.310

Curvature �0.178 �0.252

Contributing area �0.108 0.417

Topographic wetness index �0.548 0.628

Parent material 0.458 0.620

Soil class 0.264 �0.285

pH value �0.322 0.367



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 Hassink’sequation

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

<20 lm. However, there was a high variability in the

current C concentration in forest soils, with a high

proportion of sites having current C contents above the

potential C saturation and apparently being oversatu-

rated (Fig. 2). As the median current C concentration of

the fine fraction was close to the potential C saturation,

forest soils showed a high C saturation of 93% and a

resulting small C saturation deficit of only 1.1 mg g�1.

However, more than 60% of the total SOC of

investigated forest soils was not associated with the silt

and clay fraction (Fig. 1) and the C saturation deficit

showed no significant relationship with the proportion

of silt and clay (Fig. 4). Moreover, a high number of

forest sites showed a marked degree of apparent over-

saturation. Presumably, C saturation in forest soils is

not strongly related to the proportion of silt and clay as

was found for agricultural soils and thus, Hassink’s

equation provides no reliable estimation of the C satu-

ration in most forest soils. In acidic forest topsoils with

a median pH value of 3.5 (Table 1), other C stabiliza-

tion mechanisms are probably more relevant than an

association with clay minerals.

In several studies, the importance of amorphous iron

(Fe) and aluminium (Al) oxides for the stabilization of

OM in acidic soils was emphasized (Kaiser & Zech,

2000; Kaiser et al., 2002; Kleber et al., 2005; Sch€oning

et al., 2005; Wiseman & Puttmann, 2005; K€ogel-Knabner

et al., 2008; Spielvogel et al., 2008; Dumig et al., 2011).

Reactive surfaces of Fe and Al oxides may have a stron-

ger impact on stabilization of SOM in these soils than

clay minerals. Therefore, Kleber et al. (2005) and

Wiseman & Puttmann (2005) suggested that the C

sequestration potential of acidic soils could be related

to the content of poorly crystalline mineral phases. In a

study that investigated the relationship between the C

content of silt and clay particles and the proportion of

silt and clay particles in forest topsoils, Six et al. (2002)

assumed that Fe and Al oxides could play an important

role in explaining differences between cropland and

forest soils. However, the association of SOM with oxi-

des was predominantly related to subsoils (Kaiser et al.,

2002; Kleber et al., 2005; Spielvogel et al., 2008; Rumpel

& K€ogel-Knabner, 2011). In topsoils, preservation due

to recalcitrance of aliphatic forest OM might further be

a more relevant C stabilization mechanism (Eusterhues

et al., 2005; von Lutzow et al., 2006). Moreover, Baldock

& Skjemstad (2000) pointed out that Hassink’s equation

excluded the potential OM that may be associated with

the <20 lm fraction due to physical protection within

soil aggregates. These C stabilization mechanisms could

be the basis for an alternative method to estimate the C

sequestration potential in acidic soils using soil proper-

ties which are associated with these processes (e.g. reac-

tive surface of oxides or content of recalcitrant

compounds). An alternative method to Hassink’s equa-

tion could not only be used for acidic forest soils but

also for coarse-textured agricultural soils, where satu-

rated or apparently oversaturated conditions also indi-

cated insufficient detection of the C stabilization

processes (Fig. 2). A promising approach to estimate

the capacity of soils to preserve C by its association with

silt and clay more precisely was proposed by Six et al.

(2002), who developed specific regression equations for

cultivated grassland and forest soils as well as for dif-

ferent types of clay minerals. Moreover, Feng et al.

(2013) suggested an organic C loading method based on

the measurement of soil mineral-specific surface area as

well as a boundary line analysis using only data from

soils that have reached the maximal organic C stabiliza-

tion (upper tenth percentile) as alternative methods to

Hassink’s linear regression. The suitability of these

methods to estimate the C sequestration potential of

different soils should be tested in further studies.

Estimation of the soil C sequestration potential in Bavaria

A PCA and a multiple linear regression of the extracted

factors revealed that the C saturation deficit was mainly

controlled by climate (Tables 3 and 4). Therefore, the C

saturation deficit of soils under different land uses was

correlated with temperature and precipitation. Positive,

significant (P < 0.01) correlations with temperature and

negative correlations with precipitation were found for

all land uses (Figs 5 and 6). These relationships were

used for a rough estimation of the total C sequestration

potential of Bavarian soils. A similar approach was

used to estimate the C sequestration potential of



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.



(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. ha�1 yr�1 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).

Outlook

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 ha�1) Cseq-0-10 (Mt) Cseq-A (Mt)

Cropland (C) >6.4 <1450 105 22817 13.9 32 96

Grassland (G) >7.0 <1150 458 9267 6.7 6 12

Forest (F) >8.1 <850 11561 11699 3.1 4 4

Other use (O) >8.0 <1000 3104 11312 7.9 9 22

Total 15228 55095 50 134



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.

Acknowledgements

We thank Alfred Schubert from the Bavarian State Institute forForestry for providing forest soil data. Ulrike Maul, NadineEheim, Wiebke Wehrmann and Sigrid Hiesch are acknowledgedfor laboratory work. We are grateful to the Bavarian State Minis-try of the Environment and Public Health for funding theproject ‘Der Humusk€orper bayerischer B€oden im Klimawandel– Auswirkungen und Potentiale’.

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