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 18.104.22.168 discusses definitions and databases; Section 22.214.171.124 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.
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.
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.
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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