Fact Sheet 4.19. Restoration of Severely Degraded Land
Severely degraded land is understood in this context to mean land that will
not revert to its former state through good agricultural, rangeland management,
or forestry practice alone. Practices include restoration of severely eroded
land and land polluted with heavy metals or mine spoils (Bennett, 1977; Akala
and Lal, 1999), as well as reclamation of deserts (Lal et al., 1999b),
saline soils (Glenn et al., 1993), and alkaline soils (Garg, 1998; Sumner
and Naidu, 1998). Section 3.2.5.7 discusses definitions and databases; Section
4.6.1.4 includes a small discussion in relation to degraded forest soils. Other
relevant categories include acidified soils; soils polluted with hydrocarbons;
compacted, sealed, or crusted soils; and waterlogged and slumped soils.
Use and Potential
The specific techniques needed depend on the nature of the degradation. The
potential is large, but there is always a reason why land has become degraded;
unless this cause is removed, restoration and carbon sequestration will not
occur. Globally, very large areas are affected, although there is much variability
in estimates of the extent of the problem. Most authors (Dregne et al.,
1991; Glenn et al., 1993; Gupta and Rao, 1994; Sumner and Raidu, 1998;
Lal and Bruce, 1999; Lal et al 1999b) estimate, however, that no more
than 20-40 percent of badly degraded land is restorable at a reasonable cost.
Current Knowledge and Scientific Uncertainties
Eroded soil may have become unstable and susceptible to further erosion. Remaining
soil may be highly weathered and lacking basic fertility to permit restoration
by growing crops, in particular hyperaccumulators or soil microorganisms. Where
land has become degraded-for example, by overgrazing-the underlying cause must
be addressed, or the overgrazing is likely to happen again once vegetation is
restored. Certain soils are always going to be susceptible simply because of
their constitution; stabilizing these soils or achieving anything other than
low amounts of carbon stocks may be impossible. Remaining fertility may be extremely
variable across a site, making restoration patchy. Resistance to further erosion
depends partly on establishing full vegetative cover to provide protection from
the action of wind or water.
Time Scales
Highly degraded soils often start from a low carbon base in relation to undisturbed
conditions. Given this difficulty, full restoration of the soil may take many
years. The combination of large potential and low rates, however, means that
some degraded lands could function as a sink for very many years-perhaps more
than half a century.
Monitoring, Verifiability, and Transparency
Monitoring depends on the sequestration/revegetation method chosen. The extent
or effectiveness of vegetation cover can be monitored by using satellites, and
carbon can be estimated in the usual way. Soil carbon can be estimated using
models and verified by taking samples on-site. If a soil is still susceptible
to erosion, there is a danger that erosion may continue during the monitoring
period. The height and location of soil may need to be referenced. Removal of
organic pollutants requires special treatment. This practice is as verifiable
as any other means of changing soil carbon stocks. Organic pollutants may cause
problems that require special attention. Transparency depends on the method
of analysis chosen. Uncertainty may be greatest if erosion continues to remove
soil or if the original pollutant is organic.
Permanence
If degraded soils start from a low carbon stock, the permanence of additional
carbon is probably high, but adequate measures must be taken to prevent repeat
degradation.
Associated Impacts
This practice may lead to an increase in aboveground biomass on soil that previously
could not support cropping; increased fertility and profitability; reduced erosion
and environmental problems such as landslip and silting of water courses; and
"recycling" of land where land is scarce.
Relationship to IPCC Guidelines
The Guidelines do not explicitly deal with restoration of severely degraded
lands. Changes in soil carbon stocks could be incorporated similarly to what
is done with conversions to set-asides, provided appropriate time-dependent
"Base Factors" were determined. Changes in non-woody biomass stocks for restoration
of degraded lands are not included in the Guidelines.
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