"Regardless of the challenges involved, desertification control and restoration of degraded soils is a win-win strategy. It needs to be done."

• Introduction
• Soils of drylands
• Areal extent and land use of drylands in the U.S.
• Desertification and its impact on the soil carbon pool
• Carbon sequestration through desertification control
  • Sequestration of soil organic carbon
  • Sequestration of soil inorganic carbon
• Conclusion
• Endnotes
• References
• Author information

Introduction

(Back to top)
Desertification implies "land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors including climatic variations and human activities" (UNESCO 1979; UNEP 1992; UNCED 1992). The term "land" includes whole ecosystems comprising soil and water resources, land surface, vegetation, animals and crops. The term "degradation" implies reduction of resource potential by one or a combination of processes (e.g., water and wind erosion and sedimentation, long-term reduction in amount and diversity of natural vegetation and animals, and salinization [Lal et al. 1999]). Therefore, desertification involves degradation of both vegetation and soil. Degradation of soil encompasses decline in soil physical, chemical and biological qualities leading to reduction in biomass productivity. Degradation of vegetation involves reduction in species diversity and decline in the quality and quantity of biomass produced.

Thumbnail link to table of drylands characteristics

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.

Soils of drylands

(Back to top)
Over large areas of deserts, soils are poorly developed and mostly comprise unweathered rocks or regolith because of the predominance of physical weathering (freeze/thaw, wet/dry cycles) over chemical weathering. Dominant pedogenic or soil-formation processes in drylands are calcification in well-drained soils and salinization in poorly drained sites. Thus, predominant soils are Aridisols and Entisols in the subtropical steppes and in both subtropical and temperate deserts. These soils are characterized by low soil organic carbon (SOC) content and high concentrations of carbonates. In particular, Entisols (which are young soils) are characterized by coarse texture, low water and nutrient retention capacities, low inherent soil fertility, and a low SOC pool. In contrast, Mollisols, which are rich in SOC content, occur in temperate steppes. Some Mollisols occur in temperate desert regions with grass vegetation (Bailey 1996). Thus, predominant soils are Aridisols and Entisols in the sub-tropical steppes, and Mollisols and Aridisols in temperate steppes. Well-drained soils contain large concentrations of precipitated calcium carbonate (CaCO3). Dryland soils also have large pools of soil inorganic carbon (SIC) because of calcification and formation of caliche.

Areal extent and land use of drylands in the U.S.

Thumbnail link to table of drylands of U.S.

Thumbnail link to table of uses of U.S. drylands

(Back to top)
Estimates of the area of drylands of the U.S. vary depending on ecoregions included. Bailey (1978) showed that semi-arid steppes and arid desert regions cover 273 million hectares (Mha). However, when combined with dry sub-humid regions, the total area may be 372 Mha. Desert regions are generally too dry to support any crop cultivation without irrigation. Extensive grazing, one animal over several hundreds of hectares, is the most common form of agriculture. Sub-tropical steppes also support limited grazing, and crop cultivation is possible only with irrigation. Crop cultivation with irrigation or rainfed farming is practiced in semi-arid steppes and adjacent dry sub-humid regions. Predominant land uses in the drylands of the U.S. include 15 Mha of irrigated cropland, 30 Mha of rainfed cropland, and 325 Mha of rangeland.

Desertification and its impact on the soil carbon pool

(Back to top)
The risks of desertification are exacerbated by inappropriate land use and by soil/vegetation mismanagement that leads to drastic disturbance of fragile ecosystems in harsh environments. Soil degradative processes (e.g., erosion, salinization, depletion of soil fertility) leading to desertification diminish vegetative cover, decrease net primary productivity (NPP), reduce the amount of biomass returned to the soil and deplete the soil carbon pool. The exposure of carbonaceous sub-soil, by water and wind erosion, can also lead to emissions of carbon dioxide (CO₂) by dissolution of carbonates.

Thumbnail link to table estimating extent of desertification

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.

Carbon sequestration through desertification control

(Back to top)
Soil C sequestration can be achieved through restoration of degraded soils and adoption of recommended agricultural practices. Desertification control may also lead to C sequestration through increases in biomass produced and by means of SIC returned to the soil through formation of secondary carbonates.

Sequestration of soil organic carbon

(Back to top)
The basic strategies of SOC sequestration are to decrease losses of C from the soil and to increase NPP of the soil.

Thumbnail link to diagram of strategies

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.

Sequestration of soil inorganic carbon

(Back to top)
Formation of secondary carbonates or pedogenic carbonates (PIC) and leaching of carbonates into the ground water are among two possible mechanisms of SIC sequestration. Although the chemical reaction underlying the formation of PIC (Birkeland 1984; Nordt et al. 2000) is simple, the importance of SIC in C dynamics in dryland ecosystem is not very well understood. Some argue that rate of formation of secondary carbonates is merely 30 to 50 Kg/ha/yr (Schlesinger 1997) while others believe that the rate may be 3 to 4 times higher (Monger and Gallagos 2000; Nordt et al. 2000). An important mechanism of SIC sequestration is through leaching of carbonates in irrigated soils, if the irrigation water is not already saturated (Nordt et al. 2000). Assuming the low rate of formation of secondary carbonates of 30 to 50 Kg/ha/yr, the potential of SIC sequestration in 372 Mha of drylands of the U.S. is 11 to 18 TgC/yr.

Conclusion

Thumbnail link to table of potential for C sequestration

(Back to top)
While depletion of the SOC pool can be rapid, through conversion from natural to agricultural ecosystems and increase in the extent and severity of soil degradation, increasing the SOC pool is a difficult task even in soils of humid climates. It is an even greater challenge in dryland soils where water is the most limiting factor. Yet, restoration and desertification control of drylands of the U.S. has a potential to sequester carbon at the rate of 31 to 54 TgC/yr. This potential compares with 75 to 208 Tg for that of the cropland (Lal et al. 1998) and 22 to 98 TgC/yr for grazing lands (Follett et al. 2001). Realization of this potential depends on identification of appropriate policies for erosion control, establishment of vegetative buffers along riparian zones, afforestation of highly erodible land, elimination of summer fallow and adoption of improved agricultural practices on cropland. Regardless of the challenges involved, desertification control and restoration of degraded soils is a win-win strategy. It needs to be done.

Endnotes

(Back to top)
1. One megagram (Mg) = 1 metric ton = 1,000 kilograms
back to text

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.)
back to text

References

(Back to top)
Bailey, R.G. 1978. Description of the ecoregions of the United States. Ogden, Utah: U.S. Dept. of Agriculture, Forest Service.

Bailey, R.G. 1996. Ecosystem geography. New York: Springer-Verlag.

Bailey, R.G. 1998. Ecoregions: The ecosystem geography of the oceans and continents. New York: Springer-Verlag.

Barrow, C. 1999. Alternative irrigation: The promise of runoff agriculture. London: Earthscan.

Birkeland, P.W. 1984. Soils and geomorphology. New York: Oxford Univ. Press.

Bruins, H.J. and P.R. Berliner. 1998. Bioclimatic aridity, climatic variability, drought and desertification: definitions and management options. In The arid frontier: Interactive management of environment and development, eds. H.J. Bruins and H. Lithwick, 97-116. Dordrecht: Kluwer.

Burke, I.C., C.M. Yonker, W.J. Parton, C.V. Cole, K. Flach and D.S. Schimel. 1989. Texture, climate and cultivation effects on soil organic matter content in U.S. grassland soils. Soil Science Society of America Journal 53: 800-805.

Burke, I.C., W.K. Lauerenroth and D.P. Coffin. 1995. Soil organic matter recovery and N mineralization in semi-arid grasslands: implications for the conservation reserve program. Ecological Applications 5: 793-801.

Ciais, P., P.P. Tans, M. Troiler, J.W.C. White and R.J. Francey. 1995. A large northern hemispheric terrestrial CO₂ sink indicated by the 13C/12C ratio of atmospheric CO₂. Science 269: 1098-1102.

Debano, L.F. 2000. The role of fire and soil heating on water repellency in wildland environments: a review. Journal of Hydrology 231-232: 195-206.

Dregne, H.E. and N.T. Chou. 1992. Global desertification dimensions and costs. In: Degradation and restoration of arid lands, ed. H.E. Dregne, 249-281. Lubbock, Tex.: Texas Tech Univ.

Emmerich, W.E. and J.R. Cox. 1992. Hydrologic characteristics immediately after seasonal burning on introduced and native grasslands. Journal of Range Management 45: 476-479.

Follett, R.F., J.M. Kimble and R. Lal (eds) 2001. The potential of U.S. grazing lands to sequester carbon and mitigate the greenhouse effect. Boca Raton, Fla.: CRC/Lewis Publishers.

Gebhart, D.L., H.B. Johnson, H.S. Mayeux and H.W. Polley. 1994. The CRP increases soil organic carbon. Journal of Soil and Water Conservation 49: 488-492.

Haigh, M.J., ed. 2000. Reclaimed land: Erosion control, soils and ecology. Rotterdam: A.A. Balkema.

Hester, J.W., T.L. Thurow and C.A. Taylor, Jr. 1997. Hydrologic characteristics of vegetation types as affected by prescribed burning. Journal of Range Management 50: 199-204.

IPCC (Intergovernmental Panel on Climate Change) 2000. Land use, land use change and forestry. Oxford: Oxford Univ. Press.

Keeling, C.D., J.F.S. Chin and T.P. Whorf. 1996. Increased activity of northern vegetation inferred from atmospheric CO₂ measurements. Nature 382:146-149.

Kern, J.S. 1994. Spatial patterns of soil organic carbon in the contiguous United States. Soil Science Society of America Journal 58: 439-455.

Lal, R. 1995. Global soil erosion by water and carbon dynamics. In Soils and global change, eds. R. Lal, J. Kimble, E. Levine and B.A. Stewart, 131-142. Boca Raton, Fla.: CRC/Lewis Publishers.

Lal, R. 2001. Soil erosion and carbon dynamics on grazing lands. In The potential of U.S. grazing lands to sequester carbon and mitigate the greenhouse effect, eds. R.F. Follett, J.M. Kimble and R. Lal, 231-247. Boca Raton, Fla.: CRC/Lewis Publishers.

Lal, R., J.M. Kimble, R.F. Follett and C.V. Cole. 1998. The potential of U.S. cropland to sequester carbon and mitigate the greenhouse effect. Chelsea, Mich.: Ann Arbor Press.

Lal, R., H.M. Hassan and J. Dumanski. 1999. Desertification control to sequester carbon and mitigate the greenhouse effect. In: Carbon sequestration in soils: Science, monitoring and beyond, eds. N. Rosenberg, R.C. Izaurralde and E.L. Columbus, Ohio: Malone Battelle Press.

Manley, J.T., G.E. Schuman, J.D. Reeder and R.H. Hart. 1995. Rangeland soil C and N responses to grazing. Journal of Soil and Water Conservation 50: 294-298.

Michalson, E.L., R.I. Papendick and J.E. Carlson, eds. 1999. Conservation farming in the United States. Boca Raton, Fla.: CRC Press.

Mohamed, A.M.O. and K.I. Al Hosani, eds. 2000. Geoengineering in arid lands. Rotterdam: A.A. Balkema.

Monger, H.C. and R.A. Gallegos. 2000. Biotic and abiotic processes and rates of pedogenic carbonate accumulation in the southwestern United States - relationship to atmospheric CO₂ sequestration. In: Global climate change and pedogenic carbonates, eds. R. Lal, J.M. Kimble and B.A. Stewart, 273-289. Boca Raton, Fla.: CRC/Lewis Publishers.

Nordt, L.C., L.P. Wilding and L.R. Drees. 2000. Pedogenic carbonate transformations in leaching soil systems: implications for the global carbon cycle. In: Global climate change and pedogenic carbonates, eds. R. Lal, J.M. Kimble and B.A. Stewart, 43-64. Boca Raton, Fla.: CRC/Lewis Publishers.

NRC 1996. A new era for irrigation. Washington, D.C.: National Research Council.

Reeder, J.D., G.E. Schuman and R.A. Bowman. 1998. Soil C and N changes on conservation reserve program land in the Great Central Plains. Soil and Tillage Research 47: 339-349.

Robichaud, P.R. and R.D. Hungerford. 2000. Water repellency by laboratory burning of four northern Rocky Mountain forest soils. Journal of Hydrology 231-232: 207-219.

Schlesinger, W.H. 1997. Biogeochemistry: An analysis of global change. 2nd ed. New York: Academic Press.

Tanji, K.K. and C.A. Enos. 1994. Global water resources and agricultural use. In: Management of water use in agriculture, eds. K.K. Tanji and C.A. Enos, 4-24. Berlin: Springer-Verlag.

UNCED (U.N. Conference on Environment and Development). 1992. Earth Summit Agenda 21: Programme for action for sustainable development. Nairobi: U.N. Environmental Programme (UNEP).

UNEP. 1992. World atlas of desertification. London: Edward Arnold.

UNESCO (U.N. Educational, Scientific and Cultural Organization) 1979. Map of the world distribution of arid regions. UNESCO, Programme on Man and the Biosphere (MAB) Technical Notes 7. Paris: UNESCO.

Uri, N.D. 1999. Conservation tillage in U.S. agriculture: Environment, economic and policy issues. New York: Haworth Press.

USDA-NRI 1992. National Resources Inventory. Washington, D.C.: USDA.

Wilhite, D.A. and M.J. Hayes. 1998. Drought planning in the United States: Status and future directions. In The arid frontier: Interactive management of environment and development, eds. H.J. Bruins and H. Lithwick, 33-53. Dordrecht: Kluwer.