|Sustaining the Future: Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 pages)|
|Part 2: Environmental issues and futures|
|Tropical deforestation and its impact on soil, environment, and agricultural productivity|
Deforestation and conversion to arable land use have drastic impacts on soil properties, water and energy balance, and soil erosion hazard (Lal 1987). The worst-case-scenario local effects are outlined in table 9.8. The principal soil degradation effects include adverse effects on soil structure leading to crusting, compaction, and hardsetting. Reduction in infiltration, increase in surface runoff, and soil exposure to raindrop impact and to the shearing effect of overland flow accentuate soil erosion risks. Alterations in pore size distribution and reduction in the colloid content of the surface soil, owing to eluviation, and preferential removal of clay and organic carbon by erosion drastically reduce plant-available water reserves. High soil temperatures, often reaching 40-45°C at 0-5 cm depth for 4 to 6 hours a day, further aggravate the frequency and intensity of drought stress experienced by shallow-rooted crops.
The principal impact of deforestation on chemical and nutritional properties is related to a decrease in the organic matter content of the soil and to disruption in nutrient-recycling mechanisms owing to the removal of deep-rooted trees. The decrease in soil organic matter content is mostly due to the high rate of mineralization caused by high temperatures. The absence of actively growing roots in the subsoil horizon leads to leaching of bases (e.g. Ca, Mg, K, Na) and increase in soil acidity. In addition to leaching, loss of N and S also occurs owing to volatilization.
Table 9.8 Worst-case scenario regarding the adverse effects of deforestation on soil productivity
|Physical effects||Chemical and nutritional effects|
|· Compaction, crusting, increased strength||· Loss of soil organic matter, nitrogen, and sulphur|
|· Accelerated erosion||· Leaching of bases|
|· Loss of clay and soil colloids||· Acidification|
|· Drought stress||· Reduction in soil biological activity|
|· High soil temperatures||· Disruption of nutrient recycling|
Table 9.9 Technological options to minimize the adverse effects of deforestation
|Time of land clearing||· During dry season when soil moisture is low|
|Method of land-clearing||· Manual felling with chainsaw|
|Mechanized clearing||· Preferably with shearblade|
|Management of fell biomass||· In situ burning, no windrows|
|Stumping and root removal||· Remove manually from the top 30 cm, or leave intact|
|Protective cover||· Plant an aggressive cover crop, e.g. Mucuna, Desmodium spp. Puereria, etc.|
|Seedbed preparation||· No-till or conservation tillage|
|Erosion management||· Vegetative hedges, e.g. Vetiver, Leucaena, etc.|
The magnitude of these adverse effects depends on the method of deforestation and on the soil and crop management practices. In addition, there exists a strong interaction with soil type, rainfall regime, nature of the existing vegetation, ambient soil moisture content, and microclimate. Technological options for minimizing the adverse effects of deforestation are outlined in table 9.9. It is well known that the adverse effects of deforestation are more severe for mechanical than for manual land-clearing. The adverse effects of mechanical clearing (soil compaction and accelerated erosion) are generally less severe for shearblade than for treepusher or bulldozer blade methods of tree felling. Compaction and structural degradation are more severe when soil wetness is high at the time of landclearing (Ghuman and Lal 1992). Stumping and removal of roots, which is necessary only to facilitate mechanized farm operations, should preferably be done manually. Root ploughing is disruptive and causes considerable soil disturbance. Windrowing also scrapes topsoil and concentrates nutrient-rich ash in narrow strips. Management of the soil structure and erosion control can be achieved by sowing a quick-growing cover crop. The cover crop should preferably be a legume, e.g. Mucuna utilis, Pueraria phaseoloides, Centrosema spp., or Desmodium spp. Erosion control on sloping lands can be achieved by establishing vegetative hedges (e.g. Vetiver, Leucaena, Gliricidia) and other multi-purpose trees and woody shrubs. Adoption of agro-forestry practices also enhances nutrient recycling and minimizes leaching losses of bases.
Enhancing the nutrient capital of the soil is critical to increasing the agricultural productivity of these soils of low inherent fertility. Soil fertility is further depleted by deforestation and biomass removal and/or burning. Therefore, judicious application of fertilizer needs careful consideration. To some extent, nitrogen can be supplied through biological fixation. However, other nutrients, including Ca, Mg, and P, must be made available from off-farm sources. Admittedly, resource-poor farmers cannot afford capital-intensive inputs. None the less, essential nutrients must be supplied, through application of either organic manure or mineral fertilizer, if high yields are expected on a sustained basis.