No. 49, May/June 2001
Linkages between Climate Change and Desertification
by Rattan Lal
"Regardless of the challenges involved, desertification control and restoration of degraded soils is a win-win strategy. It needs to be done."
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Drylands comprise a wide range of ecoregions, from hyper-arid to sub-humid, characterized by low and erratic rainfall/precipitation and short growing season. Drylands typically have less than 75 growing days, semi-arid lands have 75 to 120 days, and dry sub-humid regions have 120 to 150 days. A principal feature of drylands is a negative water balance, as expressed by annual rainfall/precipitation and potential evapotranspiration and by high inter-annual rainfall variability. The ratio of annual precipitation to potential evapotranspiration is less than 0.05 for hyper-arid, 0.05-0.20 for arid, 0.20-0.50 for semi-arid and 0.50-0.65 for sub-humid regions (UNEP 1992).
There are two principal categories of drylands: the arid desert and the semi-arid steppe, both of which may occur in regions with sub-tropical and temperate climates (Bailey 1996, 1998). The steppe is a transitional zone surrounding the desert and separating it from dry sub-humid regions. The steppes predominantly comprise short-grass grassland with local concentrations of shrubs and woodland. In contrast, desert vegetation primarily comprises xerophytic plants such as spiny shrubs and cacti.
Globally, change in land use is one of three anthropogenic activities that have contributed to atmospheric enrichment of CO2, the other two being fossil fuel combustion and cement manufacturing (IPCC 2000). Therefore, conversion of drylands to an appropriate land use and adoption of recommended soil/vegetation management practices can lead to C sequestration in soil and biomass, and reduce the net gaseous emissions by partly off-setting those emissions due to fossil fuel combustion (Ciais et al. 1995; Keeling et al. 1996; Lal et al. 1998). The objective of this report is to assess the potential of dryland ecosystems of the U.S. to sequester carbon and reduce the country's net gaseous C emissions.
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Desertification-caused degradation of vegetation has occurred in the southwestern U.S. For example, overgrazing and trampling by livestock has caused encroachment of woody species (Prosopis spp.) in grasslands in New Mexico. Further, inter-shrub areas are prone to erosion by water and wind. Dregne and Chou (1992) show that estimates of the extent of desertification include 4.0 Mha (26.0%) of irrigated cropland, 3.6 Mha (12.0%) of rainfed cropland and 276 Mha (85.0%) of rangeland. The reliability of these estimates needs to be improved through ground truthing based on surveys of soil and vegetation resources.
Lal et al. (1999) assumed that desertification may lead to an average reduction in the SOC pool by 8 to 12 megagrams of carbon per hectare (MgC/ha).(1) However, the reduction may be in the order irrigated cropland > rainfed cropland > rangeland. It is thus possible that reduction in the SOC pool may be 15 to 20 MgC/ha in irrigated cropland, 10 to 15 MgC/ha in rainfed cropland, and 1 to 2 MgC/ha in rangeland. If so, the historic loss of soil C due to desertification in the U.S. may be 60 to 80 teragrams of carbon (TgC) from irrigated cropland, 36 to 54 TgC from rainfed cropland and 0.27 to 0.55 petagrams of carbon (PgC) from rangeland.(2) Thus, the total historic loss of SOC due to desertification may be 0.28 to 0.65 PgC. In contrast, losses form cropland soils of the U.S. are estimated at 1 to 3 Pg (Kern 1994) to 5 PgC (Lal et al. 1998). These estimates of historic C loss are tentative, crude, do not include the losses of SIC, and need to be improved.
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There are about 100 Mha of highly erodible rangelands in the U.S. that lose 1750 +/- 117 Tg/yr of sediment by wind erosion and 482 +/- 15 Tg/yr of sediment by water erosion (USDA-NRI 1992). Assuming that SOC content is 2.0% in wind-blown sediments and 3.0% in water-transported sediments, the total amount of SOC translocated by erosion from U.S. grazing land is about 49 TgC/yr. If 20% of the C displaced is mineralized (Lal 1995), the SOC emitted into the atmosphere by erosion from U.S. grazing land is about 10 TgC/yr with a range of 8 to 12 TgC/yr. Erosion management can reduce these emissions. Fire, both natural and prescribed, can increase runoff and soil erosion (Emmerich and Cox 1992; Hester et al. 1997; Debano 2000) by altering soil structure and wettability (Robichaud and Hungerford 2000); it also emits gases into the atmosphere. Therefore, identifying strategies of fire management is also important to enhancing SOC sequestration.
Restoration of severely degraded rangelands, through afforestation with quick growing species, is another option of increasing SOC content (Manley et al. 1995). The objective is to improve vegetative cover and enhance net primary productivity (NPP) through establishing appropriate species, and through managing riparian zones. Restoration of 100 Mha of highly erodible rangeland may enhance the SOC pool at the rate of 100 to 200 Kg/ha/yr (Lal 2001) with a total potential of 10 to 20 TgC/yr.
There is also some potential of improving the SOC content in cropland through enhancing water use efficiency (WUE) and nutrient use efficiency (NUE). For a total US cropland area of 45 Mha, adoption of improved agricultural practices (e.g., conservation tillage and residue management, integrated nutrient management, elimination of summer fallow) may lead to SOC sequestration at the rate of 50 to 100 Kg C/ha/yr (IPCC 2000) with a total potential of 2 to 4 TgC/yr.
Most water reserves in drylands are saline, and some halophytes can be grown by irrigation with brackish water. Some useful halophytes with potential to produce large biomass include pickle weed (Salicornia spp), salt grass (Distichlis spp), salt brushes (Atriplex spp) and a common algae Spirulina sp. (Lal et al., 1999). The biomass, useful as forage or as raw material for industry, can also improve SOC sequestration.
Overall, there have been several important advances in arid lands management (Bruins and Berliner 1998; Wilhite and Hayes 1998), including geoengineering techniques (Mohamed and Hosani 2000) and reclamation of drastically disturbed lands (Haigh 2000), that can aid in achieving the above goals for carbon sequestration. Potentially appropriate strategies include alternative irrigation systems (NRC 1996; Barrow 1999); management of water use in agriculture (Tanji and Enos 1994); and conservation tillage (Uri 1999; Michalson et al. 1999). Adoption of conservation reserve programs (Gebhart et al., 1994; Reeder et al., 1998) and restoration of grasslands in semi-arid regions can also lead to SOC sequestration (Burk, Yonker et al. 1989; Burk, Lauerenroth et al. 1995).
Finally, identification of landscape with potential to sequester carbon is an important strategy. Riparian zones, surrounding seasonal water courses, would be appropriate for afforestation. Halophytes can be grown on saline soils and/or irrigated with saline ground water. Irrigated lands can be used for adopting recommended agricultural practices to enhance WUE and NUE.
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2. One teragram (Tg) = 1 million [1,000,000] metric
tons; one petagram (Pg) = 1 thousand million metric tons (this quantity
is often referred to as 1 billion in the U.S. or 1 milliard in the U.K.)
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Dr. Rattan Lal is a professor of Soil Science in the School of Natural Resources at Ohio State University. You can reach him for comment as follows:
Dr. Rattan Lal
School of Natural Resources
The Ohio State University
2021 Coffey Road
Columbus, OH 43210
Tel: +1 (614) 292-9069
Fax: +1 (614) 292-7432
Web site: http://www.ag.ohio-state.edu/~natres/faculty/lal.html
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