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 224

19 Current Soil Carbon Loss and
Land Degradation Globally: Where
are the Hotspots and Why There?

Hans Joosten*

Abstract
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 overex-
ploitation 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,especiallythoseinthetropics.Drainedpeatlandson0.3%ofthegloballandarealose
around 0.5 Pg C year - 1 .
• Drylands(arid,semi-aridanddrysubhumidareas).Degradeddrylandson 4- 8%oftheglobal
landarea(andhometo> 2 billionpeople)losearound0.3PgCyear - 1 .
• Tropicalforests,whichexperiencelarge-scaleclearing,withthevastmajorityofnewcropland
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.

Introduction

recognizablehumus(Oades,1989),isavital
element of soil. SOM provides and facilitates
the biological, chemical and physical cap-
acity of soil to sustain plant growth and is fun-
damental for soil fertility. SOM strongly deter-
mines the formation, stabilization and surface
area of soil aggregates, total porosity and pore

Because of its slow formation (~0.1 mm year − 1 ;
Pimentel et al ., 1995; Stockmann et al ., 2014),
soil is essentially a finite resource. Soil or-
ganic matter (SOM), the mixture of recogniz-
able plant and animal remains and no longer