World’s Soil Resources

of the distribution of organic C, a separate class of ‘Agriculture and Managed Areas’ was created by merging
the GLC 2000 classes 16 and 23. For each of the broad vegetation classes the organic C stock was extracted by
pool. The results are presented in Table 6.2.
The single largest stock for terrestrial C is attributed to areas with broadleaf forest (509.4 Pg C). This forest
type contains approximately one quarter of all terrestrial organic C in either the soil or the biomass.
The proportions of the C stored in the soil and biomass stocks by broad vegetation class is graphically
presented in Figure 6.7.
For the biomass C stock alone, broad forests account for over 50 percent of the C in that pool, but only 16.8
percent of the organic C is stored in the soils under this vegetation type. With the exception of the ‘Forest/
Cropland Mosaic’, in all other vegetation classes the soil stores more C than the biomass.
6.2.6 | Historic trends in soil carbon stocks
The SOC stocks are more susceptible to anthropogenic activities and natural factors than are SIC stocks.
Conversion of natural to agro-ecosystems in the past has led to decline in the SOC stock of the surface layers
and also in SOC in the total profile for most soils. The magnitude of the historic loss, however, differs among
soils and climates. The magnitude and rate of loss are higher for soils within the tropics than for those of
temperate climates. Losses are also higher for coarse-textured than for heavy-textured soils, higher for
soils containing higher SOC stocks, and higher for soils under subsistence or ‘extractive’ farming than for
those farmed with more science-based agricultural practices. Depletion is also exacerbated by drainage of
wetlands, by ploughing, and by biomass burning or removal. Some soils in the tropics can lose 50 percent of
their previous pool within five years following deforestation and conversion to agricultural land use. The rate
and magnitude of SOC loss are exacerbated in soils vulnerable to accelerated erosion, salinization, nutrient
depletion or imbalance, structural decline and compaction, acidification, elemental toxicity, pollution and
contamination.
Estimates of the magnitude of historic SOC loss vary widely. The historic loss has been estimated at 40 Pg by
Houghton (1995), 55 Pg by IPCC (1995) and Schimel (1995), 150 Pg by Bohr (1978), 500 Pg by Wallace (1994) and
537 Pg by Buringh (1984). The average of these estimates is 223 Pg C year -1 . Lal (1999) estimated the magnitude
of SOC loss since 1850 at 47 to 104 Pg for different biomes (Table 6.3); 66 to 90 Pg for major soils (Table 6.4);
Biome
Change in Area
10 6 ha
Historic SOC Loss
Pg C
Forests 1300 23 - 53
Woodlands 180 3 - 7
Shrublands 140 1 - 4
Grasslands 660 20 - 40
Total 47 - 104
Table 6.3 Estimate of the historic SOC depletion from principal biomes. Source: Lal, 1999.
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