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Atmospheric C pool 790 Pg
(+ 3.3 Pg year
-1
)
CO
2
CO
2
CO
2
Oxidation
Particle
detach-
ment
C added in
above-
ground
residues,
and root
biomass
Tr ans-
location
Humification
SOC
pool
4000 Pg
(+ 0.1 Pg year
-1
)
Redistribution
over the
landscape
Erosion
(
2-
20% of
the C added)
CO
2
CO
2
Deposition
in aquatic
ecosystems
and burial in
depressional sites
Leaching
Aquatic systems
Fig. 19.2.
Processes affecting soil organic carbon (SOC) dynamics (peatlands excluded). Arrows pointed
upward indicate emissions of CO
2
to the atmosphere. Under anaerobic conditions (peatlands, rice fields)
emission of CH
4
also takes place. DOC is dissolved organic carbon. (From Lal, 2004, 2012; Houghton, 2007.)
however, does not include the losses from
drained peatland, which have increased
from0.3PgCyear
−
1
in 1990 to 0.5 Pg C year
−
1
in2008(Joosten,2010),turningtheglobal
carbon flux from soil strongly negative.
ofsoil.LADA(LandDegradationAssess-
mentinDrylands)indsthat24%oftheglo-
bal land area has been degrading over the
past
25
years. The 2011 FAO SOLAW study
(Nachtergaele
et al
., 2011) presents the state
of land and water resources for food pro-
duction and analyses threats to food secur-
ity and sustainable development.
Rather little focus has been on the loss
of SOC because the absolute amount of SOC
cannotbeusedunequivocallyforcompar-
ing degradation among different types of
soil and for different types of ecosystem ser-
vices (e.g. carbon storage versus food provi-
sion; cf. Biancalani
et al
., 2012; Victoria
et al
., 2012). Work-in-progress presented at
the United Nations Convention to Combat
Desertiication(UNCCD)SecondScientiic
Conference 'Economic assessment of desert-
ification, sustainable land management and
resilience of arid, semi-arid and dry sub-
humidareas'inApril2013inBonn,Germany,
showed, not surprisingly, that the decline in
soil organic matter was related strongly to
land use, with India and East Asia as out-
standing examples of very intensive agricul-
tural practices.
Hotspots of Soil Carbon Change
In the last two centuries, humans have
clearedorconverted70%ofthegrassland,
50%ofthesavannah,45%ofthetemperate
deciduousforestand27%ofthetropicalfor-
est biome for agriculture (Foley
et al
., 2011).
Between1985and2005,theworld'scrop-
lands and pastures expanded by
154
million
hectares (Mha) (Foley
et al
., 2011).
Several global studies have tried to as-
sess the extent of land degradation, with
strongly varying results and accuracy (see
Stringer, 2012; Plate 11). Oldeman
et al
.
(1991)identiied15%ofsoilasdegraded.
The Millennium Ecosystem Assessment
(2005)concludesthatmorethan60%ofthe
ecosystem services have been degraded, but
provides limited information on the status
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