Agriculture Reference
In-Depth Information
productivity reduces the amount of plant
residuesreturnedtothesoil(Lal,2003).
The major drivers of SOC losses include:
1. Demandforfuel,forexampleinformof
charcoal, leading to deforestation and re-
moval of other natural vegetation; large-scale
deforestationandconsequenterosionmay,
for example, take place in locations in re-
ceipt of migrants (Stringer, 2012).
2. Overgrazing, leading to vegetation deg-
radation, soil compaction and wind and
water erosion.
3. Arable agriculture for market and sub-
sistence crops.
4. Overexploitation of vegetation for do-
mestic use; for example, for fuel, fencing
etc.
of ~0.12 Pg C year 1 (range 0.06-0.27).
This figure is, however, admittedly over-
estimated as it does not account for decom-
position losses from the exported SOC,
of  which the long-term stability is highly
uncertain.
Since 1000 ad , an estimated 2 billion
ha of once productive land has been irrevers-
ibly degraded; that is, more land than is cur-
rently under agricultural production (Lal,
2003).ThedepletionoftheglobalSOCpool
as a result of conversion of natural to agroeco-
systems is estimated at 40-100 Pg (Lal et al .,
2004;Smith,2008;Lal,2012).
Global Soil Carbon Fluxes
Change in land cover and land use generally
leads to a decrease in SOC. The largest losses
of SOC per area occur where the C stocks are
largest; for example, in highly organic soils
such as peatlands. SOC tends to be lost
when converting grasslands, forest or other
native ecosystems to croplands, or by drain-
ing, cultivating or liming highly organic
soils. SOC tends to increase when restoring
grasslands, forests or native vegetation on
former croplands, by changing native forest
to pasture and by rewetting organic soils
(GuoandGifford,2002;Smith,2008).
The decrease in SOC continues in min-
eral soils until a new, stable level is reached
atabout 30- 50%oftheoriginallevel(Lal,
2002, 2004), depending on tillage practice
and the level of C inputs into the soil as
crop residue or animal manure (McLauch-
lan, 2006). The period to stabilize SOC
levels is around 100 years for soils in the
temperate region, whereas tropical soils
maystabilizemorequicklyandborealsoils
more slowly.
While erosion may have severe conse-
quencesforlocalsoilfertility,itseffecton
carbon emissions to the atmosphere de-
pends on the fate of the SOC exported from
the eroded areas. van Oost et al . (2007) esti-
mate that as a result of agricultural erosion
over the past 50 years globally, ~ 16-21 Pg C
have been moved laterally over Earth's sur-
face and have been buried within agricultural
landscapes, constituting a global carbon sink
The stock of SOC results from the balance
between the input and output of organic
matter in the soil. Inputs are primarily from
leaf and root detritus, and growth of subsur-
face plant and associated microbial biomass
fed directly by photosynthesis. Outputs
consist predominantly of emissions of carbon
dioxide (CO 2 ) to the atmosphere, whereas
emissions of volatile organic compounds
and methane (CH 4 ),andtransportofDOC
andPOCmayalsobeimportant(Lal,2003;
HeimannandReichstein,2008).
The rate of input is related to the net
primary production (NPP) of the vegetation,
which varies with climate, land cover, spe-
cies composition and soil type. Part of NPP
enters the soil as organic matter root growth,
exudates and plant litter, where it is partly
converted back to CO 2 and CH 4 via (hetero-
trophic) soil respiration or leaches away as
DOC and POC. Also, harvest of below-
ground biomass and fire (peat fires) may re-
move belowground carbon. The balance of
all these processes determines whether the
soilisasourceorasinkofC(Smith,2008).
For the 1990s, the worldwide litter in-
put in the soil was estimated to be 4.0 Pg C,
ofwhich3.9Pgeventuallyoxidizedtoat-
mospheric CO 2 directly or after leaching
and translocation. An estimated 0.1 Pg was
added to the long-term SOC pool ( Fig. 19.2 ;
Houghton, 2007; cf. van Oost et  al ., 2007).
This assumed small net soil carbon gain,
 
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