Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Nutrient and water dynamics in rotational woodlots : a case study in Western Tanzania

Auteur : Nyadzi, G.I

Université de soutenance : Wageningen Universiteit

Grade : Doctor Thesis 2004

Résumé partiel
Rotations of trees and crops on farms are considered as a potential technology to overcome the shortage of wood, reverse deforestation of natural forests and improve soil fertility for food security enhancement in western Tanzania, sub-Saharan Africa. However, overexploitation of soil water resources and depletion of soil nutrients have been suggested as possible negative effects of growing trees on farms in the semi-arid tropics. Such possible pitfalls undermine and even threaten a successful implementation of the woodlot technology at larger scale. Evidently, without proper understanding of the interactions and possible competition between trees and crops, the potential benefits of this agroforestry technology will not be realized. Therefore, this study was focused on improving the understanding of tree-soil-crop interactions by examining water and nutrient dynamics of various trees planted in rotational woodlots with a 5-year cycle. Field trials were established with five-year rotational woodlots. Five tree species were compared with natural fallow and continuous maize. Inter-cropping of maize between trees was possible for the first two years of tree establishment without sacrificing maize yield. There was no evidence that trees were over-exploiting the water reserves after three years. Transpiration was greatest in A. crassicarpa and was related to stem diameter, size of the tree canopy and soil water availability. Trees depleted relatively more water than continuous maize and natural fallow, but were able to store more water after rains. Acacia trees had high litter fall but the leaves were low in P and N, which led to N and P immobilization during decomposition. Trees retrieved leached inorganic N and made better use of it than natural fallow and continuous maize. Wood production at the end of the five-year growing period ranged from 30 to 90 Mg ha_1 while C sequestered in the aboveground biomass during the same period ranged between 13 to 30 Mg ha-1 . The rate of biomass production was highest for Acacia species while the foliage nutrient accumulation was highest for Leucaena species. The wood component varied greatly among species, ranging from 32 to 85% of the total tree biomass. Mai

Mots clés : AGROFORESTRY / ARBORICULTURE / ROTATIONS / WOOD / SOIL FERTILITY / SUSTAINABILITY / INTERCROPPING / FALLOW / NUTRIENTS / WATER USE / TANZANIA / OVEREXPLOITATION

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