Conservation of and Competition for Water and Nutrients in Semi-arid Agroforestry

The overall objective of this thesis was to increase the ecological knowledge of agroforestry systems in semi-arid regions. More specifically, the work concentrated on two aspects : (1) the water- and nutrient-conserving effects of mulch from agroforestry tree species and other mulch materials and (2) the competition between trees and crops in semi-arid agroforestry, and how this competition is influenced by coppicing the trees. The field work was carried out in the less dry parts of semi-arid Burkina Faso, in an area where shortened fallow periods, declining soil fertility and crop yield, coupled with rapid population growth, are severe problems. In two mulching studies, mulching factors influencing yields (dosage, timing, composition) were evaluated, and growth factors (soil water, temperature, nutrients) that contribute to the mulch effect were identified. The mulch materials used were leaf mulch from Azadirachta indica (neem) and Albizia lebbeck (albizia) and phyllodes from Acacia holocericea (acacia), as well as sorghum straw, wild grass and sorghum straw compost. Decomposition rates and nutrient release from neem and acacia mulch were assessed in a separate study. Competition between trees and crops was investigated in a parkland system with neem and in alley farming systems with neem, albizia and Leucaena leucocephala. The crop in all cases was sorghum. Leaf mulching increased sorghum yields, in a dose-dependent manner. The effects were stronger when no mineral fertilizers had been added, in which case yields were up to 4.2 times that in the control in the highest-yielding treatment. Effects of application timing varied between years, most likely owing to the influence of rainfall variation. Neem leaves generally induced higher yields than other tested materials, owing to their high nutrient content and suitable decomposition rate. These properties favoured a relatively rapid release of nutrients, while at the same time, providing a thick enough soil cover to conserve soil water. It was concluded that nutrient conservation was more important than water conservation or temperature reduction for obtaining high yields. Early tree coppicing reduced competitive effects on crops in the tested agroforestry systems and was associated with increased soil water and light availability in alley farming. Coppicing was therefore concluded to be essential in semi-arid agroforestry. Of the tree species tested, neem was found to be the most appropriate in this climate, as evaluated in terms of crop yield, wood production and coppicing tolerance. In summary, it was shown that intensive agroforestry systems involving coppicing and mulching can be used in less dry parts of semi-arid regions. However, care has to be taken to ensure that such practices are combined with conservation measures, so that nutrient mining from the soil will not occur. The best treatments in the tested systems helped to maintain soil balances with respect to organic matter, N and acidity, but P deficiencies were pronounced. It is therefore recommended that organic fertilizers be combined with strategic mineral inputs.

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