 | Sustaining the Future. Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 p.) |
 |  | Part 2: Environmental issues and futures |
| | Tropical deforestation and its impact on soil, environment, and agricultural productivity |
|
|
Deforestation and the emission of radiatively active gases
Deforestation strongly affects the dynamics of soil organic matter. Experiments conducted in Africa (Greenland and Nye 1959; Nye and Greenland 1960; Lal 1976; Juo and Lal 1977; Aina 1979; Lal et al. 1980; Ghuman and Lal 1991,1992) show rapid decline in soil organic matter content following deforestation and cultivation. The magnitude of carbon decline in the top 5 cm depth can be as much as 50 per cent in 12 months and 60 per cent in 18 months. The organic carbon (C) content of the top 30 cm depth declines by about 50 per cent within 10 years of deforestation and intensive cultivation. Examples of the carbon loss from soils of the humid tropics within 10 years of deforestation and intensive cultivation are shown in tables 9.10-9.12. The rate of C loss may be as much as 1.13 mg/ha/yr from soil managed by conservation tillage and agro-forestry to 5.60 mg/ha/yr for soils managed with a plough-based conventional tillage system. That being the case, newly cleared land in the humid tropics may release between 98.7 billion kg C/yr and to 218.8 billion kg C/yr, with a mean emission rate of about 154.3 billion kg C/yr.
Table 9.10 Loss of organic curbon with continuous and intensive cultivation with no-till and agro-forestry in 10 years following deforestation
| Depth (cm) | Organic C (%) | Bulk density (mg/m3) | Total soil carbon (mg/ha) | Carbon emission in 10 years (mg/ha) | |||
| Initial | Final | Initial | Final | Initial | Final | ||
| 0-10 | 2.50 | 1.50 | 1.10 | 1.40 | 27.5 | 21.0 | 6.5 |
| 10-25 | 1.40 | 1.00 | 1.25 | 1.45 | 26.3 | 21.8 | 4.5 |
| 25-50 | 0.90 | 0.80 | 1.30 | 1.45 | 29.3 | 29.0 | 0.3 |
| Total | 83.1 | 71.8 | 11.3 | ||||
Source: Lal (1991).
Table 9.11 Loss of organic carbon with continuous and intensive cultivation using plough-based mechanized systems in 10 years following deforestation
| Depth (cm) | Organic C (%) | Bulk density (mg/m3) | Total soil carbon (mg/ha) | Carbon emission in 10 years (mg/ha) | |||
| Initial | Final | Initial | Final | Initial | Final | ||
| 0-10 | 2.5 | 0.5 | 1.10 | 1.5 | 27.5 | 7.5 | 20.0 |
| 10-25 | 1.40 | 0.4 | 1.25 | 1.45 | 26.3 | 8.7 | 17.6 |
| 25-50 | 0.9 | 0.3 | 1.30 | 1.45 | 29.3 | 10.9 | 18.4 |
| Total | 83.1 | 27.1 | 56.0 | ||||
Source: Lal (1 991).
The loss of organic C from soils under shifting cultivation is less than that from soils under intensive cultivation. Nye and Greenland (1960) observed that the loss of organic carbon in 100 years may be 20 per cent for a soil with 12-year fallow cycle to 45 per cent for a soil with 4-year fallow cycle. The annual loss of C due to shifting cultivation may be as much as 0.27 mg/ha. If shifting cultivation is prac tised on about 25 million ha, the total loss of C due to shifting cultivation is estimated at 6.25 billion kg C/yr. In addition to C, biomass burning also causes release of several other greenhouse gases, e.g. CO2, CO, CH4, and NOx.
Table 9.12 Changes in soil organic carbon (SOC) content of the surface 0-5 an layer of two soils in southern Nigeria
|
Alfisol at Ibadana |
Ultisol at Okomub | ||||||
|
DC |
DC | ||||||
| Year | Organic carbon (%) | %/yr | Average (%/yr)c | Year | Organic carbon (%) | %/yr | Average (%/yr)c |
| 1978 | 2.17 | 1984 | 1.8 | ||||
| 1979 | 1.61 | - 25.8 | - 25.8 | 1985 | 1.4 | - 22.2 | - 22.2 |
| 1982 | 1.54 | -1.5 | -7.3 | 1986 | 1.45 | +3.6 | -9.7 |
| 1984 | 1.14 | -13.0 | -7.9 | 1987 | 1.05 | -27.6 | -13.9 |
| 1985 | 1.24 | +8.8 | -6.1 | 1988 | 1.15 | +9.5 | -9.0 |
| 1986 | 1.30 | +4.8 | -5.0 | ||||
| 1987 | 1.09 | -16.2 | -5.5 | ||||
a. The data from Ibadan are from Watershed 1.
b. The data from Okomu are
from the manually cleared plots; data recalculated from Ghuman and Lal
(1991).
c. The average (%/yr) is calculated for each year on the basis of
the original SOC
content.
|
|