4.4.7. Restoration of Severely Degraded Lands

Restoration of degraded land generally involves revegetation that increases carbon stocks in biomass and soil. It can occur on croplands, grazing lands, forests, or "other" lands (mine spoils, deserts, etc.) As such, many of the practices relating to restoration of partially degraded land are discussed elsewhere in this Special Report under the appropriate land use. This section deals with soils that are so badly degraded that normal practices no longer have an effect, soils that are no longer capable of supporting crops, and soils that must be taken out of agricultural practice before progress can be made (Fact Sheet 4.19). For example, where land is polluted with heavy metals, these pollutants may have to be removed before revegetation can proceed. Other important categories of degraded land are salinized, sodic, desertified, and eroded soils; Section 2.2.5.7 discusses the nature of degradation and lists definitions. Areas of most categories of severely degraded land are increasing. Restoration brings multiple benefits: Not only can carbon be sequestered, the loss of carbon can be arrested and nonproductive land brought back into use. Restored lands may be put in to crops (Section 4.4.2, Fact Sheets 4-1, 4-2, 4-3 and 4-4), pasture (Section 4.4.3, Fact Sheets 4-6, 4-7, 4-8 and 4-9), or forest (Chapter 3, Section 4.4.4, Fact Sheets 4-12, 4-13, 4-14, 4-15, 4-16 and 4-17) to sequester still more carbon.

Degraded land has a large potential for sequestration in relation to undisturbed land because it often contains little carbon, but there are almost always factors that limit this potential. Nabuurs et al. (1999) cite rates of 0.2-2 t C ha^-1 yr^-1. Rates as large as 7-9 t C ha^-1 yr^-1 appear in the literature (Table 4-11), but these rates often involve other measures such as the application of animal manures or deal with special problems. With severely degraded land, it is probably unreasonable to expect the largest rates to apply everywhere. Lal and Bruce (1999) use a conservative rate of 0.25 t C ha^-1 yr^-1, which is used in Table 4-4. The very low carbon levels in most of these soils means that a 1 percent increase in the carbon content of soil mass is feasible. Such an increase would take about 60 years at a rate of 0.25 t C ha^-1 yr^-1; in practice, however, the land is likely to be used for some other purpose, such as agriculture, before that rate and other rates of carbon sequestration will apply (Table 4-4).

Because severely degraded soils require intervention above and beyond revegetation, extra costs inevitably are involved. Remote sites may impose additional costs for transporting the means of intervention to the site. Although remediation is expensive, additional income from carbon credits may make restoring some land that would otherwise remain abandoned worthwhile.

Estimates of the net global carbon sequestration potential of polluted land are difficult to make. On one hand, these lands might be the last to be restored because of the expense and difficulty. On the other hand, their polluted or damaged nature means that in many cases-especially in developed countries-legislation may compel owners to restore the land. Where land remediation is a legal requirement, a decision might be needed on whether to allow the claiming of carbon credits.

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