One of the tree-mediated benefits of considerable advantage in the tropics is that trees and other vegetation improve the productivity of the soil beneath them. Research results during the past two decades show that three main tree-mediated processes increased nitrogen (N) input by N2-fixing trees (NFTs), enhanced availability of nutrients resulting from production and decomposition of tree biomass, and greater uptake and utilization of nutrients from deeper layers of soils by deep-rooted trees determine the extent and rate of soil improvement in agroforestry systems. Furthermore, the presence of deep-rooted trees in the system can contribute to improved soil physical conditions and higher soil microbiological activities under agroforestry.
NFTs that are common in the tropics are a particularly valuable resource for soil improvement. Farmlands in many parts of the developing world generally suffer from the continuous depletion of nutrients as farmers harvest without fertilizing adequately or fallowing the land. One promising way for overcoming the acute problem of low nutrient status of African soils is to enable smallholders to use fertilizer tree systems that increase on-farm food production. After years of experimentation with a wide range of soil-fertility replenishment practices, three major types of simple, practical, fertilizer-tree systems have been developed:
1. improved fallows using trees and shrubs such as sesba-nia (Sesbania sesban) or tephrosia (Tephrosia vogelii);
2. mixed intercropping with species such as gliricidia (Gliricida sepium); and
3. biomass transfer with species such as wild sunflower (Tithonia diversifolia) or gliricidia.
These practices can provide 50-200 kgN ha-1 yr-1 to the associated cereal crops.
The other major avenue of soil improvement with agroforestry is through soil conservation. About 1.9 billion hectares of land, a third of total farmland, in developing nations are estimated to be degraded through erosion, salinity, and fertility depletion. The potential of agrofor-estry to reduce the hazards of erosion and desertification as well as to rehabilitate such degraded land and to conserve soil and water has been well recognized. Although this approach is more recent in the temperate zone, research has demonstrated the ameliorative potential of agroforestry. For example, research results show that the agroforestry designs of grass-shrub-tree buffers (riparian buffer) are superior to grass buffers in reducing sediment losses, and trees used as windbreaks around agricultural fields reduce soil erosion caused by wind and water. Agroforestry practices have also been recognized for their role in reducing the hazard of forest fires.
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