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Global soils store in their first metre three times more carbon than all forest biomass of the world combined, and double the CO 2 content of the atmosphere. The natural soil carbon density is controlled by climate, soil properties and vegetation. Land-use intensity, drainage conditions and soil type (organic versus mineral soils) play an important role in controlling soil carbon losses or gains.

Because of its superficial setting, small bulk density and organic constitution, soil organic carbon (SOC) is highly susceptible to water and wind erosion and chemical and physical degradation. The major drivers of SOC loss include demand for fuel, overgrazing, arable agriculture and other overexploitation of vegetation. The resulting depletion of the global SOC pool is estimated at 40-100 Pg.

Three global hotspots can be distinguished where environmental and socio-economic conditions currently lead to large soil carbon losses:

Peatlands, especially those in the tropics. Drained peatlands on 0.3% of the global land area lose around 0.5 Pg C year(-1).

Drylands (arid, semi-arid and dry subhumid areas). Degraded drylands on 4-8% of the global land area (and home to >2 billion people) lose around 0.3 Pg C year(-1).

Tropical forests, which experience large-scale clearing, with the vast majority of new cropland coming from intact and disturbed rainforests.

The future of SOC will ultimately depend on whether land management will continue to mine soil carbon for short-term gains, but with long-term detriment, or whether it will manage to conserve and enhance soil carbon. Soil carbon management will thus decide whether a legacy of land resources will remain to sustain future generations.