Current Soil Carbon Loss and Land Degradation Globally: Where are the Hotspots and Why There? - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 227

iron and aluminium oxides are less prone to
C storage.
The C content of soils under different
land cover types varies substantially ( Table
19.1 ). The soils of savannahs contain rela-
tively little SOC, but the carbon stocks are
globally significant, as this biome covers
large areas of land. In contrast, peatlands
coveronly3%ofthegloballandareabut
contain a disproportionately large amount
of soil carbon, making them the most spa-
tially intensive carbon store among all ter-
restrial ecosystems.
Within the biomes, natural soil carbon
densities may be modified strongly by land
use, which may substantially change the car-
bon fluxes to and from the soil. Forests gener-
ally have the largest (all year-round) input of
recalcitrant material, grasslands a large input
of often less recalcitrant material, whereas
croplands have a small input of rather labile
material only when a crop is growing (Smith,
2008).Withintheland-usetypes,land-use
intensity, drainage conditions and soil type
(cf. organic versus mineral soils) play a fur-
ther important role in controlling soil carbon
content, losses or gains.

Mechanisms and Drivers
of Soil Carbon Loss

Three properties make SOM/SOC highly la-
bileandvulnerable(Lal,2003).

• Thesupericialsetting:whereasocean
and bedrock carbon is largely physic-
ally inaccessible, SOC lies close to the
surface and is subject to transport and
transformation processes.
• Thelightweight:incomparisontosilica-
based soil components (gravel, sand,
clay), SOM is light. With a bulk density
only somewhat larger than that of water,
SOM - when not protected by vegeta-
tion - is transported easily with water
and wind flows. Erosion preferentially
removes the light organic fraction.
• Thechemicalconstitution:SOMis-
indeed - organic, implying that it oxi-
dizes in the presence of O 2 , in some
cases completely to CO 2 and H 2 O.

DegradationoftheSOCpoolcantakefour
main forms (Oldeman, 1992; Plate 10;
Table 19.2) .

Table 19.1. Indicative soil organic carbon pool in various biomes and land cover types. Soil carbon
stocks refer to the upper metre of soil, except in the case of peatlands. (From Amundson, 2001; Lal,
2012; Victoria et al ., 2012.)

Soil carbon content

Biome Area ( 10 6 ha) Mg ha - 1 (mean) Mg total

Tundra 880 218 191,840

Boreal desert 200 102 20,400

Cool desert 420 99 41,580

Warm desert 1,400 14 19,600

Tropical desert bush 120 20 2,400

Cool temperate steppe 900 133 119,700

Temperate thorn steppe 390 76 29,640

Tropical woodland and savannah 2,400 54 129,600

Boreal forest, moist 420 116 48,720

Boreal forest, wet 690 193 133,170

Temperate forest, cool 340 127 43,180

Temperate forest, warm 860 71 61,060

Temperate forest, very dry 360 61 21,960

Tropical forest, dry 240 99 23,760

Tropical forest, moist 530 114 60,420

Tropical forest, wet 410 191 78,310

Boreal and subarctic peatland 340 1,340 455,600

Tropical peatland 44 2,000 88,000