Soil Degradation And Climate Change

Projected change in climate may influence several soil processes with a consequent adverse impact on soil quality (Brinkman and Sombroek, 1996). Important among these processes with an attendant adverse impact on soil quality are:

• Hydrolysis: the leaching of silica and basic cations

• Cheluviation: removal of Al and Fe by chelating organic acids

• Ferrolysis: transformation of clay by alternating oxidation and reduction processes, and reduction in cation exchange capacity

• Dissolution: of clay minerals by strong acids producing acid aluminum salts and amorphous silica

• Clay formation: reverse weathering leading to clay formation and transformation

Hydrolysis and cheluviation may accelerate with temperature increases, and ferrolysis may occur in soils subject to reduction and oxidation in high latitudes and monsoonal climates. These processes increase soil erodibility and decrease waterand nutrient-retention capacity. Two schools of thought exist with regard to the effects of projected climate change on soil quality.

The first school argues that climate change is likely to exacerbate global food insecurity, with increased risks of soil degradation. Accelerated soil erosion and other degrading processes already affect soil quality, especially in developing countries of the tropics and subtropics (Table 5.8). The land area affected by water erosion is estimated at 227 million ha in Africa, 441 million ha in Asia, 123 million ha in South America, and 46 million ha in Central America. Soil degradation by other

Table 5.8 Soil Degradation in Developing Countries (millions ha)

Soil Erosion

Soil Erosion

Chemical

Physical

Region

by Water

by Wind

Degradation

Degradation

Africa

227

186

62

19

Asia

441

222

74

12

South America

123

42

70

8

Central America

46

5

7

5

World total

1094

548

240

83

Source: Adapted from Oldeman, L.R. 1994. The global extent of soil degradation. In D.J. Greenland and I. Szabolcs, Eds. Soil Resilience and Sustainable Land Use. CAB International, Wallingford, United Kingdom, pp. 99-118.

Source: Adapted from Oldeman, L.R. 1994. The global extent of soil degradation. In D.J. Greenland and I. Szabolcs, Eds. Soil Resilience and Sustainable Land Use. CAB International, Wallingford, United Kingdom, pp. 99-118.

processes is also a problem in developing areas. In South Asia, 25% of agricultural land is estimated to be affected by water erosion, 18% by wind erosion, 13% by fertility decline, 9% by salinization, 6% by lowering of the water table, and 2% by waterlogging (FAO, 1994). It sounds strange to say that fortunately these are overlapping categories.

Soil degradation is likely to be accelerated by projected climate change, especially in ecologically sensitive regions. Global hot spots of soil erosion include the Himalayan-Tibetan ecosystem, the unterraced slopes of China and Southeast Asia, tropical areas of Southeast Nigeria, the semiarid Sahelian region of West Africa, sloping lands of Central America, and the Andean valleys and cerrado region of South America (Scherr and Yadav, 1996).

Soil erosion rates are likely to change due to the erosive power of rainfall produced by more extreme precipitation events (IPCC, 1995). Since 1910 there has been a steady increase in the area of the United States affected by extreme events (>2" or >50.8 mm of rain in a 24-hour period) (Karl et al., 1996). Areas susceptible to wind erosion usually become drier and become more severely affected (Williams et al., 2002). In the U.S. Corn Belt, Lee et al. (1996) predicted that although a 2°C increase in temperature would decrease water erosion by 3% to 5%, wind erosion would increase by 15% to 18%. Thus, total erosion was predicted to increase as a consequence of increased temperature. Regions prone to salin-ization include the Indus, Nile, Tigris, and Euphrates river valleys; northeast Thailand; northern China; northern Mexico; and the Andean highlands (Scherr and Yadav, 1996; Norse et al., 2001). It is estimated that 20% of total irrigated area is already affected by salinization, and 12 million ha of irrigated land may have already gone out of production for this reason (Nelson and Mareida, 2001).

Higher temperatures due to projected climate change, especially in arid and semi-arid regions, may produce higher evaporative demand for water and exacerbate the drought that often follows the plow (Glantz, 1994). If the soil and water are adequate, as with irrigation, it turns out that an increase in evaporative demand may heighten the risks of salinization (Brinkman and Sombroek, 1996). However, under high atmospheric CO2 conditions, there may be increased salt tolerance of crops (Bowman and Strain, 1987). Revegetation by overgrazing and other factors could exacerbate the problem of desertification (U.N. Environmental Programme, 1997), especially in Sub-Saharan Africa. Risks of overextraction of groundwater for irrigation in South Asia and also in the Near East/North Africa region are already recognized as serious (FAO, 1990).

Soil degradation is thus a major threat to global food security (Oldeman, 1998), and this threat may be increased with anticipated climate change. Soil degradation, especially that caused by accelerated erosion, characteristically involves depletion of soil organic matter. Most degraded soils contain an SOC pool that is below their potential set by ecological factors. Lee et al. (1996) observed that when the SOC pool decreased by 4.8 MT/ha in the U.S. Corn Belt, about 50% of this loss was due to accelerated soil erosion. Further, increased temperature and precipitation accelerate losses of SOC. These can exacerbate nutrient depletion in low-input agricultural systems that are already vulnerable to severe nutrient depletion, as is the case in Ghana (Rhodes, 1995) and elsewhere in Sub-Saharan Africa (Stoorvogel and Smal-ing, 1990). Sustainability of agriculture in the Sahel is already problematic (Reardon, 1995). It becomes even more difficult to attain with increased risks of soil degradation.

Alternatively, some scientists argue that soil quality may be improved, at least in some ecosystems, by the projected changes in climate (Brinkman and Sombroek, 1996). The CO2 fertilization effect would increase biomass productivity with more litter and crop residues returned to the soil, and with higher root mass and greater root exudation. This could result in a gradual increase in soil fertility.

Part of this increase would be attributable to an increase in biological nitrogen fixation. A higher supply of N for plants would enhance growth and nutrient recycling. An increase in mycorrhizal activity due to fungal "infections" of plant roots would also enhance P uptake. Increases in root growth would enhance the SOC pool and the activity of soil biota such as earthworms and termites. Increases in the strength and quantity of stable soil aggregates that these organisms can create would improve soil structure, water infiltration rates, and available water capacity. Improvements in soil structure would decrease rates of soil erosion and reduce leaching losses of plant nutrients. Therefore, it is contended that the CO2 fertilization effect over a long time may increase soil resilience and improve soil quality.

In this scenario, there would also be a positive effect on the weathering of parent material with a resulting increase in the rate of new soil formation, and thus in agroecosystem soil loss tolerance. The importance of increased soil microbial activity due to increases in temperature also should not be ignored. Greater microbial activity would strengthen elemental recycling mechanisms, increase the SOC pool, and enhance soil structure. CO2 fertilization effects at high CO2 concentrations may produce crop residues and litter with a higher C:N ratio, thereby decreasing the rate of mineralization and reducing the turnover rate.

Both schools of thought may be correct under specific ecosystems. Some of the projected climate change would certainly exacerbate soil degradation in some soils and ecosystems. But it could increase soil resilience and enhance soil quality in others. The net effect on soil quality would also depend on the adaptive options or the use of recommended management practices (RMPs) within a particular ecosystem. Adverse effects on soil quality may be more severe in ecologically sensitive regions populated by predominantly resource-poor farmers who cannot use or afford adoption of RMPs.

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