World’s Soil Resources

Figure 7.6 Some soil-related feedbacks to global climate change to illustrate the complexity and potential number of response
pathways. Source: Heimann and Reichstein, 2008.
CO 2
concentration in the shorter term, buying time to develop longer term emission reduction solutions
across all sectors of the global economy (Smith, 2012).
Just as reductions in soil C stocks are associated with negative consequences for soil function, increased
soil carbon stocks are associated with increased soil fertility, workability, water holding capacity, reductions in
greenhouse gas emissions and reduced erosion risk (Lal, 2004). Increasing soil carbon stocks can thus reduce
the vulnerability of managed soils to future global warming (Smith and Olesen, 2010). Management practices
effective in increasing SOC stocks include: (i) improved plant productivity through nutrient management,
rotations and improved farming practices; (ii) reduced or conservation tillage and residue management; (iii)
more effective use of organic amendments; (iv) land use change, for example from crops to grass or trees; (v)
set-aside; (vi) agroforestry; (vii) optimizing livestock densities; and (viii) planting legumes or improving the
crop mix (Smith et al., 2008). While these measures have the technical potential to increase SOC stocks by
about 1 – 1.3 Pg C yr -1 (Smith et al., 2007a, 2008), they are dependent on economics: the economic potential for
SOC sequestration was estimated to be 0.4, 0.6 and 0.7 Pg C yr -1 at carbon prices of up to US$20, $50 and $100
per tonnes CO 2
-eq. yr–1, respectively (Smith et al., 2008). In addition, the size of the potential sequestration
is relatively small in comparison to the threats: only a small loss of C from permafrost or peatlands could
offset this potential sequestration (Joosten et al., 2014). However, an increase in SOC through improved
management is expected to also reduce vulnerability of the soils to future SOC loss under global warming.
As such, soil carbon sequestration can, in many respects, be regarded as a ‘win-win’ and a ‘no regrets’ option
(Smith et al., 2007b).
7.3.2 | Nitrous oxide emissions
Soils emit nitrous oxide (N 2
O), a greenhouse gas that is around 300 times more potent for radiative forcing
(climate warming) over 100 years than CO 2
. Of the approximately 16 Tg N 2
O-N yr–1 emitted globally in the
1990s, between 40 and 50 percent was a result of human activities (Reay et al., 2012). The main sources were
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