Agriculture Reference
In-Depth Information
Some parts of the world are more at risk
from land and soil degradation and soil car-
bon loss than others. Three global hotspot
land types can be distinguished where dif-
ferent environmental and socio-economic
conditions lead to large soil carbon losses.
Peatlands
, especially those in the trop-
ics, have the highest soil carbon density of all
landcovercategories(
Table
19.1
).Drained
peatlandson
50
Mhaworldwide(=0.3%of
the global land area) are currently respon-
sible for carbon emissions of around 0.5 Pg
per year, with an increasing trend in emis-
sion rate. Half of these emissions stem from
rapidly expanding peatland drainage in South-
east Asia (largely for oil palm and pulpwood
plantations), with associated peatland fires.
Next to these huge emissions, peatland
drainage also leads to subsidence, increased
flooding, saltwater intrusion and eventually
to the loss of large areas of habitable and
productive land (Joosten
et al
., 2012).
Drylands
(arid, semi-arid and dry sub-
humidareas)occupy41%ofEarth'sland
area and are home to more than
2
billion
people.Some
10-
20%ofdrylandsareal-
readydegraded,withanestimated0.3Pgof
soil carbon being lost to the atmosphere
from drylands each year. Desertiication
(land degradation in drylands) has been
ranked among the most urgent global envir-
onmental challenges (Millennium Ecosys-
tem Assessment, 2005).
Tropical forests
are currently experien-
cing a concentration of land-use change
where large-scale forest clearing is taking
place(Houghton,2003;McNeillandWini-
warter, 2008). Between 1980 and 2000,
more than half of new cropland came from
intactrainforestsandanother30%fromdis-
turbed forests (Gibbs
et al
., 2010).
changing primary productivities and chan-
ging rates of decomposition as a result of
changes in temperature and soil moisture.
Rising temperatures could increase biomass
production and inputs of organic materials
into soils. Warming could, however, also ac-
celerate the microbial decomposition and
oxidation of SOM (Houghton, 2007; Lal,
2012), especially in thawing permafrost
soils(HeimannandReichstein,2008).Central
questionswithrespecttofeedbackloopsare:
1.
The temperature sensitivity of soil or-
ganic matter decomposition, especially the
more recalcitrant pools (cf. Melillo
et al
.,
2002; Knorr
et al
., 2005).
2.
The balance between increased carbon
inputs to the soil from increased production
and increased losses due to increased rates
of decomposition, including the effect of
'microbial priming' (Heimann and Reich-
stein,2008).
3.
Interactions between global warming and
other aspects of global change, including
other climatic effects (e.g. changes in pre-
cipitation pattern and resulting water bal-
ance), changes in atmospheric composition
(e.g. increasing atmospheric carbon dioxide
concentration and increased atmospheric
nitrogen deposition; Heimann and Reich-
stein,2008),increaseinintensiicationof
pests and diseases (Lal, 2012) and land‐use
change(cf.McSherryandRitchie,2013).
Scenario analyses show a large spatial het-
erogeneity and regional variation in the re-
sponse of mineral soil SOC to climate
change, which prohibits giving a simple an-
sweronthequestionofwhetherSOCwill
increase or decrease (Gottschalk
et al
., 2012;
see also Chapter
20,
this volume). Clearly,
the simple picture of gradually increasing
CO
2
concentrations and temperatures and
non-interactive effects on assimilation and
respiration needs to be replaced by a strong-
er integrative concept of complex inter-
actionsbetweenecosystemprocesses(Da-
vidson and Janssens, 2006; Heimann and
Reichstein,2008),includinglanduseasa
dominant component (cf. van Wesemael
et al
., 2010; Bell
et al
., 2011).
Current projections indicate that world
population will increase from 6.9 billion
The Future of Soil Organic Carbon
The future of SOC will depend on climate
change, land use and land cover, and feed-
backs within and between these complex
factors.
The response of SOC to climate change
depends on the delicate balance between
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