Agroforestry on Soil Organic Carbon

The three major land use categories in Zambia are cropland, forest reserves, and national parks. Forest reserves and national parks cover about 10% and 8% of the country, respectively, and are managed by the Forest Department and the National Parks and Wildlife Services (German Technical Cooperation [GTZ], 1995).

Forests in Zambia are used for agriculture, wood fuel, and timber harvesting. Out of 753,000 km2 total land area, about 420,174 km2 or 56% of the land area is potentially available for agriculture. However, the land area suitable for crop production is between 110,000 to 150,000 km2, that is, between 15% to 20% of the total land area (GTZ, 1995). Currently, average woodland clearing for chitemene is estimated at 4.15 ha per household, culminating in an estimated annual deforestation of 600,000 ha (Centre for Energy Environment & Engineering Zambia [CEEEZ], 1999).

Wood fuel and charcoal are the main sources of energy in rural and urban areas, respectively. Charcoal was introduced in the Copperbelt in 1947, and has since become a major urban household fuel in Zambia. In 1990, at least 40,000 ha were cleared for urban wood fuel, causing deforestation in forest reserves and unprotected areas (CEEEZ, 1999; Chidu-mayo and Chidumayo, 1984). This type of deforestation does not affect forests in the national parks because these are preserved for wildlife conservation.

Timber harvesting is another major use of forests in Zambia. Although only a few valuable indigenous timber species exist, indigenous hardwoods are used for various subsistence and commercial purposes (CEEEZ, 1999).

Data on various land uses in Zambia from 1990 to 1994 indicate that there has been no increase in any category or total land area usage (CEEEZ, 1999). Thus, C loss or sequestration should not vary significantly. However, two- to threefold increases in deforestation activities for various enterprises are projected by 2010 and 2030 (CEEEZ, 1999). Therefore, SOC losses in the future are anticipated.

Decline in soil fertility in Zambia is making traditional farming unsustainable. Poor land husbandry is practiced by small-scale farmers. Data show that:

1. A total of 35 metric tons ha-1 of topsoil or 160 million metric tons of soil are lost annually.

2. An estimated 339,090 metric tons of nitrogen and 10,915 metric tons of phosphate, valued at $300 million are lost annually.

3. Average maize yields are declining by 4% to 11% annually.

4. Sixty percent of the population is under the poverty level defined by the UN Development Programme (Zambia Department of Agriculture, 1999).

When the forest is cleared or fallow land brought into continuous production, SOM declines and reaches new steady-state levels depending on the quantity and quality of annual organic inputs and tillage methods. In the conversion of forest to cropland, SOC is reduced from 43 to 25 metric tons ha-1 in 15 years. It is replenished at 2 metric tons ha-1 instead of 11 metric tons ha-1 through organic dry matter per year (Greenland et al., 1992). For drier savannah conditions, a decrease of SOC from 18 to 14 metric tons ha-1 was predicted as a consequence of converting natural vegetation to cropland.

When natural vegetation is converted into cropland, soils rapidly become more acidic and lose nutrients through crop harvests, residue removal, and/or burning and leaching. Physically, the soil seems to collapse on itself. It becomes more dense, and erosive forces often cause the finer particles to disappear, due to poor tillage practices that leave a sandy or gravely material. The soil loses its capacity to form stable aggregates because the binding material, the SOM, has been lost. These degradation processes result in reduced biomass production and reduced amounts of organic matter returned to the soil, depletion of the SOC pool, decline in soil quality, and greater emissions of CO2, CO4, and N2O to the atmosphere, where soil biota play an important role in all of these processes (Metting, 1993; Mendes et al., 1999). Emissions of these greenhouse gases caused by traditional farming practices lead to mining of soil C and N reserves. Since the SOC pool is very labile and highly dynamic, and its amounts depend on the input-output balance of the system, the result of the traditional farming system is a rapid decline of soil productivity, food production, food shortages, and malnutrition (Stewart et al., 1991). Intervention with improved farming methods, such as integrated nutrient management to rebuild SOM becomes very important. This is especially true since SOM affects the dynamics of various nutrients, such as N and P; influences soil physical and chemical properties and microbial biomass; and directly impacts the atmospheric concentration of CO2.

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