Earth’s upper mantle related to large-scale con-
vective processes.
References and Notes
1. P. M. Shearer, J. Geophys. Res. 96, 18147 (1991).
2. Y. Fei et al., J. Geophys. Res. 109, 10.1029/
2003JB002562 (2004).
3. M. Akaogi, E. Ito, A. Navrotsky, J. Geophys. Res. 94,
15671 (1989).
4. E. Ito, E. Takahashi, J. Geophys. Res. 94, 10637
(1989).
5. M. P. Flanagan, P. M. Shearer, J. Geophys. Res. 103,
2673 (1998).
6. J. Gossler, R. Kind, Earth Planet. Sci. Lett. 138, 1 (1996).
7. Y. J. Gu, A. M. Dziewonski, J. Geophys. Res. 107, 2135
(2002).
8. S. Lebedev, S. Chevrot, R. D. van der Hilst, Science 296,
1300 (2002).
9. K. H. Liu, S. S. Gao, P. G. Silver, Y. K. Zhang, J. Geophys.
Res. 108, 10.1029/2002JB002208 (2003).
10. J. D. Collier, G. R. Helffrich, Geophys. J. Int. 147, 319
(2001).
11. M. P. Flanagan, P. M. Shearer, J. Geophys. Res. 103,
21165 (1998).
12. N. Schmerr, E. Garnero, J. Geophys. Res. 111, 10.1029/
2005JB004197 (2006).
13. Materials and methods are available as supporting
material on Science online.
14. B. Efron, R. Tibshirani, Stat. Sci. 1, 54 (1986).
15. A. Deuss, S. A. T. Redfern, K. Chambers, J. H. Woodhouse,
Science 311, 198 (2006).
16. Y. C. Zheng, T. Lay, M. P. Flanagan, Q. Williams, Science
316, 855 (2007).
17. N. A. Simmons, H. Gurrola, Nature 405, 559
(2000).
18. C. R. Bina, B. J. Wood, J. Geophys. Res. 92, 4853
(1987).
19. L. Stixrude, J. Geophys. Res. 102 , 14835 (1997).
20. J. R. Smyth, S. D. Jacobsen, in Earth's Deep Water Cycle,
S. D. Jacobsen, S. Van der Lee, Eds. (American
Geophysical Union, Washington, DC, 2006), vol. 168,
pp. 1–11.
21. M. M. Hirschmann, Annu. Rev. Earth Planet. Sci. 34, 629
(2006).
22. S. Karato, in Water in Nominally Anhydrous Minerals,
H. Keppler, J. R. Smyth, Eds. (Geochemical Society,
St. Louis, MO, 2006), vol. 62, pp. 343–375.
23. G. M. Leahy, D. Bercovici, J. Geophys. Res. 112, 10.1029/
2006JB004631 (2007).
24. Y. Fei, C. Bertka, in Mantle Petrology: Field Observations
and High Pressure Experimentation, Y. Fei, C. Bertka,
B. Mysen, Eds. (Geochemical Society, Houston, TX, 1999),
vol. 6, pp. 189–207.
25. X. Li, S. V. Sobolev, R. Kind, X. Yuan, C. Estabrook, Earth
Planet. Sci. Lett. 183, 527 (2000).
26. P. M. Shearer, T. G. Masters, Nature 355, 791
(1992).
27. D. Bercovici, S. Karato, Nature 425, 39 (2003).
28. R. Dasgupta, M. M. Hirschmann, Nature 440, 659
(2006).
29. M. Akaogi, A. Tanaka, E. Ito, Phys. Earth Planet. Inter.
132, 303 (2002).
30. P. Bird, Geochem. Geophys. Geosys. 4, 1027 (2003),
10.1029/2001GC000252.
31. B. Steinberger, J. Geophys. Res. 105, 11127 (2000).
32. We thank A. McNamara and J. Tyburczy for numerous
discussions and helpful suggestions, and two anonymous
reviewers for their comments. This work was supported by
NSF grants EAR-0711401 (E.J.G.) and EAR-0453944
(N.S.) and by an Achievement Rewards for College
Scientists Fellowship (N.S.).
Supporting Online Material
www.sciencemag.org/cgi/content/full/318/5850/623/DC1
Materials and Methods
Figs. S1 to S18
References
4 June 2007; accepted 25 September 2007
10.1126/science.1145962
The Impact of Agricultural Soil
Erosion on the Global Carbon Cycle
K. Van Oost,
1
*†‡ T. A. Quine,
2
* G. Govers,
1
S. De Gryze,
3
J. Six,
3
J. W. Harden,
4
J. C. Ritchie,
5
G. W. McCarty,
5
G. Heckrath,
6
C. Kosmas,
7
J. V. Giraldez,
8
J. R. Marques da Silva,
9
R. Merckx
10
Agricultural soil erosion is thought to perturb the global carbon cycle, but estimates of its effect
range from a source of 1 petagram per year
−1
to a sink of the same magnitude. By using
caesium-137 and carbon inventory measurements from a large-scale survey, we found consistent
evidence for an erosion-induced sink of atmospheric carbon equivalent to approximately 26% of
the carbon transported by erosion. Based on this relationship, we estimated a global carbon
sink of 0.12 (range 0.06 to 0.27) petagrams of carbon per year
−1
resulting from erosion in the
world’s agricultural landscapes. Our analysis directly challenges the view that agricultural
erosion represents an important source or sink for atmospheric CO
2
.
H
umans have drastically altered the global
carbon cycle, mostly through increased
use of fossil fuels and land use change
(1). Global earth system models (2, 3) represent
well the changes in carbon flux between soil and
atmosphere resulting from the reduced carbon
inputs to soil and the accelerated decomposition
of soil organic carbon (SOC) that accompany
conversion of land from an undisturbed state to
agricultural use (4, 5). In contrast, the carbon dy-
namics of the well-documented acceleration of
soil erosion and deposition (and resultant lateral
fluxes of SOC) associated with conversion of
land to agricultural use are poorly understood (6).
Soil erosion removes SOC from the site of
formation and results in its burial in depositional
environments. Recent analyses have identified
three key mechanisms whereby these geomor-
phic processes, together or separately, may result
in a change in the net flux of carbon between the
soil and atmosphere (fig. S1). Mechanism M1
involves replacement of SOC at eroding sites as a
result of continued inputs from plants and de-
crease in SOC available for decomposition (6, 7);
mechanism M2 is the deep burial of allochtho-
nous and autochthonous carbon (8) and inhibited
decomposition upon burial (6, 9, 10); and mech-
anism M3 is the enhanced decomposition of SOC
as a result of the chemical or physical breakdown
of soil during detachment and transport (11). The
fundamental controls on the magnitude of the
erosion-induced sink or source are then the rate at
which SOC is replaced at sites of erosion, changes
in the reactivity of SOC as a result of transport
and burial, and the rates of soil erosion and
deposition. Previous global assessments of the
influence of erosion and deposition on carbon
dynamics have made markedly different assump-
tions about these controls, resulting in the diamet-
rically opposed assertions of a global net release
or source of 0.37 to 1 Pg C year
−1
(12, 13) ver-
sus a net uptake or sink of 0.56 to 1 Pg C year
−1
(6, 9, 10) as a consequence of erosion on agri-
cultural lands.
The controversy about the role of erosion in
the global carbon cycle reflects the inherent dif-
ficulty of quantifying a net flux controlled by
interacting processes that are most often studied
in isolation. We examined the integrated effect of
the interacting processes using evidence for (i)
the rate of SOC replacement at sites of erosion,
(ii) the fate of the eroded and buried SOC within
agricultural watersheds, and (iii) global soil ero-
sion and soil carbon erosion rates (14). The first
two lines of evidence were derived from a com-
prehensive large-scale survey of the SOC and
caesium-137 (
137
Cs) inventories (mass per unit
area to given depth) of agricultural soils in Europe
and the United States (table S1) that allows us to
assess quantitatively the relationships between
lateral and vertical SOC fluxes. We examined
1400 soil profiles from 10 watersheds (1 to 14 ha),
including noneroded soils and eroding hill slopes
as well as colluvial soils where sediment and
SOC are buried. The artificial fallout radioisotope
1
Physical and Regional Geography Research Group,
Katholieke Universiteit Leuven, 3001 Heverlee, Belgium.
2
Department of Geography, University of Exeter, EX4 4RJ
Exeter, UK.
3
Department of Plant Sciences, University of
California, Davis, CA 95616, USA.
4
U.S. Geological Survey,
Menlo Park, CA 94025, USA.
5
U.S. Department of
Agriculture, Agricultural Research Service, Hydrology and
Remote Sensing Laboratory, Beltsville, MD 20705–2350,
USA.
6
Department of Agroecology and Environment,
Research Centre Foulum, University of Aarhus, 8830
Tjele, Denmark.
7
Laboratory of Soils and Agricultural
Chemistry, Agricultural University of Athens, 11855
Athens, Greece.
8
Department of Agronomy, University of
Cordoba, 14080 Cordoba, Spain.
9
Instituto de Cie˜ncias
Agrárias Mediterrânicas, Department of Rural Engineer-
ing, University of Évora, Évora, Portugal.
10
Division of Soil
and Water Management, Katholieke Universiteit Leuven,
3001 Heverlee, Belgium.
*These authors contributed equally to this work.
†Present address: Département de Géographie, Université
Catholique de Louvain, 1048 Louvain-la-Neuve, Belgium.
‡To whom correspondence should be addressed. E-mail:
kristof.vanoost@uclouvain.be
26 OCTOBER 2007 VOL 318 SCIENCE www.sciencemag.org
626
REPORTS
on November 2, 2007 www.sciencemag.orgDownloaded from