EARTH SURFACE PROCESSES AND LANDFORMS
Earth Surf. Process. Landforms (2009)
Copyright © 2009 John Wiley & Sons, Ltd.
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/esp.1796
Agricultural soil erosion and global carbon cycle:
controversy over?
Chichester,
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Research Group
Agricultural soil erosion and global carbon cycle
Nikolaus J. Kuhn,1 Thomas Hoffmann,2 Wolfgang Schwanghart1 and Markus Dotterweich3
University of Basel, Department of Environmental Sciences, Basel, Switzerland
2
Department of Geography, University Bonn, Bonn, Germany
3
Institute for Environmental Sciences, University of Koblenz-Landau, Landau, Germany
1
Received 31 July 2008; Revised 29 December 2008; Accepted 5 January 2009
* Correspondence to: Nikolaus J. Kuhn, University of Basel, Department of Environmental Sciences, Basel, Switzerland. E-mail: [email protected]
ABSTRACT: Recent research on the contribution of soil erosion on agricultural land to atmospheric carbon dioxide (CO2)
emphasizes either the contribution of soil organic matter (SOM) mineralization during transport as source for atmospheric CO2, or
the deep burial of SOM-rich sediment in agricultural landscapes as a sink. The contribution of either process is subject to a
controversial debate. In this letter, we present preliminary results on our research on interrill carbon (C) erosion, SOM transport by
rill erosion and the stationarity of C erosion during the Holocene. None of those issues has been incorporated comprehensively
and with global coverage in the debate on the role of C erosion in the global C cycle. Therefore, we argue that only an ecogeomorphologic perspective on organic C movement through landscapes can reconcile the two positions. Copyright © 2009 John
Wiley & Sons, Ltd.
KEYWORDS:
soil erosion; agricultural land; global carbon cycle; eco-geomorphology
Introduction
The exchange of greenhouse gases between terrestrial ecosystems
and the atmosphere represents one of the greatest uncertainties
in our understanding of the global carbon (C) cycle (e.g.
Stallard, 1998; Harden, 1999; Liu et al., 2003; Berhe et al.,
2007; Denman et al., 2007). In particular, the lateral movement
and fate of eroded organic C are subject to a controversial
debate (see review by Berhe et al., 2007). Conventionally,
selective erosion of organic C and subsequent mineralization
during transport and landscape deposition are considered to
represent a major source of atmospheric C (e.g. Lal, 2004).
This position has been challenged (e.g. Quine and van Oost,
2007; van Oost et al., 2007) and discussed controversially
(Lal and Pimentel, 2008; van Oost et al., 2008). Based on
erosion and deposition rates estimated from cesium (Cs) 137
data from 10 watersheds in Europe and the US, van Oost
et al. (2007) concluded that agricultural erosion currently
represents neither a major source or sink of atmospheric C,
effectively leading only to a removal of 0·06 to 0·27 Pg of C
from the atmosphere per year. A key question in this debate is
the fate of C while moving through a landscape system (e.g.
Battin et al., 2008). Stallard (1998) already concluded that
only a combined eco-geomorphologic and geomorphologic
approach to erosion research, encompassing erosion and
fate of organic C on its route through a landscape system, will
deliver a comprehensive answer to the role of erosion on
agricultural land in the global C cycle. Eco-geomorphology
studies the interaction between the biosphere, landforms and
geomorphic processes at or near the land surface (see Viles,
1998). Our research in the UK, Switzerland and Germany
highlights some significant gaps in the understanding of C
movement through landscapes. Three issues in the context
of organic C erosion and eco-geomorphology are presented
in preliminary form in this letter: (i) sediment differentiation
by interrill erosion, (ii) landscape deposition of organic C
from rills, and (iii) the stationarity of eroding system behavior
on soil-atmosphere C fluxes. Our intention is to renew Stallard’s
(1998) call for a comprehensive research effort into the ecogeomorphology of C movement through terrestrial ecosystems
and thereby contribute to an end of the controversy over the
role of C erosion in the climate system.
Interrill Erosion
In the context of organic C erosion, mineralization and deposition, estimates of C erosion are based on multiplying soil
erosion estimated on a global scale by soil organic matter
(SOM) content of the eroding soil (e.g. Lal, 2004; van Oost
et al., 2007). This approach is generally problematic because
C content of sediment often differs significantly from that of
the eroding soil (e.g. Wan and El-Swaify, 1997; Chaplot and
Le Bissonnais, 2000; Quinton et al., 2006; Kuhn, 2007). Also,
on both field and watershed scale, this approach considers
only the export of sediment by rill erosion into the fluvial system.
The potential role of interrill erosion on organic C dynamics
on a field scale is ignored. Our research on interrill erosion
2
EARTH SURFACE PROCESSES AND LANDFORMS
Figure 1. C content of interrill sediment generated by experimental storms on soils from Germany (Eifel) the UK (Devon). For a description of the
experiments see Kuhn (2007).
identifies two issues which indicate that it may have a
significant role in the global C cycle. First, while net field and
watershed sediment yield by interrill processes is small
compared to rill erosion and mass wasting by tillage (Govers,
1987), literally all arable soil surfaces are affected by interrill
processes. Interrill erosion processes such as rainsplash and
rainwash mobilize organic C preferentially (e.g. Mermut et al.,
1997), which leads to an enrichment of C in interrill sediment
(e.g. Rumpel et al., 2006; Kuhn, 2007). Our experiments
on soils from the UK and Germany (Kuhn, 2007) show C
enrichment rates of sediment of up to 50% (Figure 1). The
second, potentially more important issue in the context of
soil-atmosphere C fluxes, is the fate of this organic C in interrill
sediment. Interrill sediment, unless suspended during a runoff
event, travels only short distances and accumulates in
depositional crusts (Le Bissonnais et al., 2005). The organic C
in such depositional crusts is largely broken out of a coherent
soil structure (Wan and El-Swaify, 1997; Rodríguez Rodríguez
et al., 2006; Rumpel et al., 2006) and exposed to the
atmosphere. In such crusts, eroded SOM therefore cannot
be considered to be deeply buried. Assuming that interrill
processes act on all arable land (globally: 14·2 million km2)
to a depth of 2 mm, the affected layer would contain between
0·57 and 1·33 Pg of organic C (for calculations see Table I).
These values correspond to between 15% and 30% of the
annual increase of 4 Gt of C in the atmosphere, and 25% to
50% of the annual uptake in the residual land C sink
(Denman et al., 2007). They are also in the same order of
magnitude than the estimate of C removal from agricultural
land in the UK made by Quinton et al. (2006). They found
that 76 to 312 kg of C are eroded annually while our numbers
correspond to 400 to 936 kg ha–1 a–1 interrill C mobilization
during one cropping period. The similarity of the range of values
highlights the potentially significant size of the interrill C
pool. Clearly, not all organic C in the soil affected by interrill
processes is mineralized. However, due to its location at the
soil–atmosphere interface and the size of the C pool mobilized
Table I.
Rill Erosion
A second issue requiring consideration is the differentiation
of sediment during transport through a geomorphic system.
Conventionally, sediment generated by rill erosion is considered
to have identical properties to the soil it is derived from because
of the non-selective nature of rill erosion. However, this
assumption is not valid once the rill sediment is transported
through an agricultural landscape. Here a differentiation of
sediment may occur due to cyclic rilling or changes in topography
and flow obstruction, e.g. at field borders. In undisturbed rill
systems, a regular pattern of erosion and deposition evolves
due to the cyclic nature of energy expenditure on erosion and
transport (Bryan and Brun, 1999). Deposition of sediment is
not uniform, but affects large particles first (Starr et al., 2000).
Similar differentiation of sediment also occurs when rills are
obstructed at the edges of fields or when slope angles are
declining (Figure 2). Organic C content of sediment varies
with grain size. Therefore, deposition of rill sediment leads to
a differentiation of C content of both the deposited sediment
and the sediment that is transported further downslope. In March
2008, a short sampling campaign on the quality of deposited
rill sediment was conducted near the town of Oberkail (50°02′ N,
Estimates of the amount of organic C affected by interrill processes globally per year
Conservative estimate
High estimate
a
by interrill processes, the sediment generated by those processes
would have a high potential for affecting the C exchange between
soil and atmosphere. Interrill processes are also highly sensitive
to climate change due to the dependence of crusting on initial
soil moisture (e.g. Moore and Singer, 1990; Kuhn and Bryan,
2004) and rainfall amount and kinetic energy (e.g. Torri et al.,
1999). Therefore, a definite assessment of the effects of soil
erosion on the global C cycle should include interrill processes
and C mineralization at the soil surface. Research needs to
involve an improved capacity of modeling organic C erosion
by interrill processes and an analysis of the fate of C in shallow
and temporary sinks at the soil surface.
Bulk density
(t m–3)
Roughness
effect on area
Organic C
content (%)
Soil volume affected
by interrill processes
million m3)
Organic C affected
by interrill processes
per year globally (Pg)
Organic C affected
by interrill processes
(kg ha–1)
1
1·3
1
1·2a
2
3
28 400
34 080
0·57
1·33
400
936
Based on laser scanning DEMs by Anderson and Kuhn (2007) and unpublished data collected at the University of Basel.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms (2009)
DOI: 10.1002/esp
AGRICULTURAL SOIL EROSION AND GLOBAL CARBON CYCLE
Figure 2. Rill and rill deposit at a field edge (foreground) near
Oberkail, Germany, after storm Emma in March 2008. Water and
sediment delivered from the rill (middle of image), unless deposited
on the edge of the field (lower section of the image), moved away
from the rill mouth to either side, leaving a body of continuously
fining sediment (lighter colored, smooth soil surface across the lower
section of the image). Sampling of soil and sediment was conducted
in regular intervals (1 m) along the sediment body and in 5 m
intervals on each side of the rill. Texture was determined by
sedigraph and organic C through Loss on Ignition at 500 °C for four
hours.
6°43′ E) in the Eifel region of Germany. Before the sampling,
the cyclonic depression ‘Emma’ had delivered up to 50 mm
of rainfall to the region and caused the first significant rill
erosion of the winter season. The predominant soil type in the
region is a Luvisol with a silty texture, which has developed on
hills formed by Triasic Muschelkalk (Werle, 1978). Soil and
sediment samples were collected on fields which have been
under continuous crop farming for more than 25 years, most
recently several years of maize–wheat rotation. To ensure that
spatially averaged values of organic C content were achieved,
sampling of soil and rill sediment was conducted along bodies
of sediment deposition and the rills which had delivered the
deposited sediment. A pairwise comparison of source area
soil and corresponding rill sediment (Figure 3) confirmed that
the soil had a significantly higher (t = 2·9, tcrit = 1·7, p > 0·005
in one way analysis of mean) organic C content than the
sediment. Consequently, the sediment that remained in
suspension had a higher organic C content than the sediment
deposited at rill mouths and field edges. Such differentiation
of sediment along its pathway through a geomorphic system
highlights a critical issue for estimating the organic C balance
along slopes (e.g. Quine and van Oost, 2007). ‘Balance’ is
based on the assumption that sediment moved from a source
to a sink does not change in quality. Our data indicate that
the differentiation of rill sediment causes an enrichment of
Copyright © 2009 John Wiley & Sons, Ltd.
3
Figure 3. Scatterplot of total organic C (TOC) of sampled soils and
corresponding rill sediments. Error bars indicate the standard
deviation of the mean. Uniform distribution of TOC in soil and
sediment would imply a 1:1 relationship as indicated by the black
line. The distribution of values below the line shows that the rill
sediment is depleted in organic C. There also appears to be a trend of
declining depletion with increasing soil organic C content. Inorganic
C in the soil and sediment was below 0·1% and therefore neglected
in this figure.
organic C in the sediment with distance from the source
area. The degree of C enrichment depends on the nature of
the landscape system the sediment is moving through, i.e.
whether it is hydraulically ‘rough’ or ‘smooth’ surface. Assuming
constant C contents of sediment moved by rill erosion throughout
a watershed may therefore lead to an underestimation of
organic C exports from slopes to rivers or an overestimation
of slope C erosion, depending on whether erosion rates are
based on the C content of soil or alluvial deposits.
Geomorphologic research should focus on developing a
good understanding of hillslope-channel coupling not only
for sediment, but also organic C. In addition, like interrill
erosion, the fate of organic C in shallow landscape sinks along
rill sediment pathways has to be investigated.
Erosion System Stationarity
Global estimates of C-fluxes coupled to soil erosion assume
stationarity of soil erosion rates (Lal and Pimentel, 2008; Van
Oost et al., 2007). In the case of the study of van Oost et al.
(2007), soil erosion rates are estimated based on Cs-137
measurements, which give mean values for the past 50 years.
However, the evaluation of the impact of soil erosion on the
global C budget requires the consideration of long-term changes
of soil erosion and soil C contents. Most erosion of agricultural
land is currently associated with land-cover changes and soil
degradation (UNEP, 2007). Land-cover change generally results
in an initial increase of erosion followed by the establishment
of a new equilibrium between climate, land cover and erosion
(e.g. Bryan, 1994). During the transience phase, the soil organic
C stocks are depleted and consequently rates of C erosion
and deposition decline. Many soil and colluvial profiles in
central Europe illustrate the dynamic nature of soil and C
erosion, which started already 7500 years BP (Gerlach et al.,
Earth Surf. Process. Landforms (2009)
DOI: 10.1002/esp
4
EARTH SURFACE PROCESSES AND LANDFORMS
2006) when agriculture was introduced. Due to the high
agricultural productivity of loess, the first farmers mainly
settled in loess covered areas where fully developed Luvisols
dominated already during the Neolithic. To evaluate the longterm impact of soil erosion on terrestrial C fluxes, the Cerosion of a Luvisol consisting of a typical sequence of Ah,
Al, Bv, Bt and Cv horizons was estimated (Hoffmann et al.,
2009). We assumed typical thicknesses and organic C
contents of the horizons of 30 cm (Ah), 30 cm (Al), 20 cm
(Bv) and 70 cm (Bt) and 1·4%, 0·3%, 0·5% and 0·2%,
respectively. The C erosion since the Neolithic was calculated
based on soil erosion rates for the Holocene, which were
derived from the frequency analysis of optically stimulated
luminescence ages (OSL-ages) found on colluvial deposits
in south Germany (Lang, 2003). The frequency distribution of
the OSL-ages f(t) was converted to erosion rates ER(t) = a × f(t)
by assuming cumulative erosion
t
E c = a × ∫ f (t )dt
0
of a 1 m, 2 m, and 3 m layer during the last 7500 years
(Figure 4). The C-erosion was estimated by
CR(t) = ER(t) × Corg(t)
(1)
with ER(t) equal to erosion rates at time t and Corg(t) equal to
organic C-content of soil horizon eroded at time t.
Soil formation effects on C content is neglected in this
calculation, which therefore represents minimum rates and
thus a conservative estimate of C erosion. Figure 4 shows soil
and C erosion rates on the Luvisol for the cumulative erosion
of a 1 m, 2 m or 3 m topsoil layer. In general, C erosion rates
do not show the same pattern as erosion rates. Initially,
organic C erosion is high when the upper, organic matter rich
horizons of the profile were eroded. During later stages of
increased soil erosion, C erosion increased only slightly due
to the reduced topsoil C content of the truncated profile.
For the erosion of the 2 m and 3 m layer, higher erosion
rates result in maximum C-erosion rates already 2100 and
2500 years BP, much earlier than maximum soil erosion at
approximatley 900 years BP. While this simulation has to be
interpreted cautiously, it clearly illustrates that organic C
erosion is not stationary in a changing environment and does
not necessarily correlate with soil erosion rates.
The conceptual model of organic C erosion during the
Holocene presented earlier is supported by colluvial fan
deposits in the Wolfsschlucht, about 50 km east of Berlin. The
fan (Figure 5) shows an approximately 1200 year old sequence
of buried soils and colluvial sediments. The short duration of
Figure 4. (a) Holocene soil erosion rate in south Germany calculated based on cumulative frequency distribution of OSL-ages in colluvial
deposits (15). (b) Corresponding organic C erosion rate of a Central European Luvisol. Organic C contents of the soil horizons are chosen to
represent a typical Central European Luvisol (Hoffmann, 2007).
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms (2009)
DOI: 10.1002/esp
AGRICULTURAL SOIL EROSION AND GLOBAL CARBON CYCLE
5
Figure 5. 1200 year old sequence of colluvial sediments and in situ developed buried soils of the Wolfsschlucht about 50 km east of Berlin.
PSD, grain size distribution; TOC, total organic C, PoSE, period of sedimentation, PoSF, period of soil formation. Dates are estimated by a
combination of radiocarbon, archaeological and historical data (Dotterweich, 2008). This figure is available in colour online at
www.interscience.wiley.com/journal/espl
sediment accumulation (PoSE = Period of sedimentation in
Figure 5) suggests that at least 50% of the colluvial sediments
are a result of few extreme events, including gully erosion
(see also Dotterweich, 2008). The stable periods between the
extreme events are characterized by the formation of soils.
The fossil soils show increased organic C contents compared
to the sediment that was accumulated during extreme events
and that functioned as their parent material. The pattern of
sediment deposition and soil formation illustrates the lack of
stationarity associated with C erosion in agricultural landscapes
during the Holocene. The organic C content of soil and sediment
profile in the fan does not decrease or increase with depth,
indicating no general trend of C erosion through time. By
itself, this is not in accordance with the simulation conducted
on the Luvisol described earlier. However, the buried humic
horizons contain only 5% of the amount of SOM of the
recent soils. For example, the dark brownish A-horizon (layer
2d-1 in Figure 5) of a well developed medieval buried
Cambisol contains only 0·08% organic C, while the A horizon
of an adjacent Cambisol contains 1·5% to 2%. The large
difference between the recent and the fossil soil suggests only
a limited stability of organic C in buried sediments, but it is
still unclear which long-term processes led to the C release.
Taking the release of buried organic C into account, C
erosion rates could therefore have been much higher when
medieval soil erosion began, which in turn is in good
agreement with the modeled C flux based on the long-term
soil erosion (Figure 4).
Conclusions
The data presented in this study are intended to illustrate that
the role of C erosion on agricultural land for the global C
cycle cannot be addressed comprehensively without taking
an eco-geomorphologic perspective on the movement and
fate of C through a landscape system. Interrill erosion mobilizes
a significant amount of C right at the soil–atmosphere interface.
The significance of this ‘eroded’ C in the global C cycle has
so far not been included in studies on C erosion and climate
Copyright © 2009 John Wiley & Sons, Ltd.
change. The differentiation of sediment C content by landscape
deposition of rill sediment shows that sediment C content
changes while eroded soil moves through a landscape system.
Therefore, a comparison of C stocks between eroding areas
and landscape sinks has to include the effects of transport on
sediment C concentration. Finally, the dynamic nature of C
erosion in the past shows that an extrapolation of C erosion
results obtained under relatively stable erosion and land-use
conditions may not apply to a global scenario on time scales
with significant land-use change and land degradation. The
controversy on the role of C erosion in the global C cycle
therefore appears not to be over yet, but requiring a
significant research effort by geomorphologists. Three key
issues for geomorphologic research emerge. First, erosion,
transport and deposition of C have to be considered and
understood from an eco-geomorphologic perspective focused
on C cycling, not just simply the bulk movement of sediment
through watersheds. Similar sediment quality-focused
research has been carried out already, for example when
dealing with phosphorous (e.g. Quinton et al., 2001), but
has to be expanded to rill erosion and hillslope-channel
coupling. Second, the changes in C erosion over time have
to be understood. Conventionally, erosion of C over time is
based on the C content of sediment sinks and current soil
organic matter content. Without considering the C content
of soils at the time of erosion and understanding the
differentiation of sediment between source and sink area, this
approach is limited to systems that are stationary in terms of
properties, i.e. soil C content, and spatial patterns of erosion
and transport processes. Finally, the fate of C in ‘shallow’
landscape sinks, from crusts to rill and colluvial deposits has
to be explored. Particular attention should be paid to
understanding the ecological conditions and the likelihood
of C mineralization in those shallow sinks
Acknowledgements—The funding of various projects by the University
of Basel, Bonn, Exeter and Koblenz-Landau which delivered the
data used in this paper is gratefully acknowledged. The paper also
commemorates the dedication of Tim Heinen (1980–2008) to Geography,
who tragically lost his life in the line of duty far too young.
Earth Surf. Process. Landforms (2009)
DOI: 10.1002/esp
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EARTH SURFACE PROCESSES AND LANDFORMS
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DOI: 10.1002/esp