Nutrient and water dynamics in rotational woodlots: A case study in western Tanzania
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Date
2004
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Wageningen University
Wageningen University
Wageningen University
Abstract
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
trcc-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 waler 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.
Maize yields were higher after growing trees than after natural fallow and continuous
maize when no fertilizers were applied. The benefits of tree fallows compared to natural
fallows were modest, in terms of maize yield increases. The increase in crop yields after
woodlots was attributed in part to higher soil inorganic N. Maize responded to fertiliser N and
P. The agronomic efficiency was about 30 kg grain per kg N applied at a rate of 50 kg
fertilizer N, and 15 kg kg-1 between 50 and 100 kg N applied. Application of more than 20 kg
ha"1 of P or K did not significantly increase maize yields. The benefit of woodlots expressed
in terms of maize yield was more pronounced at mid and lower slope positions than at upper
slopes. The significance of rotational woodlots in improving soil fertility is limited by
substantial accumulation of nutrients in wood. Nutrient mining by wood exportation istherefore a major threat for the sustainability of woodlots, when the exported nutrients are not
supplemented via external sources. Medium-term rotational woodlots have the potential to
meet the domestic and industrial wood needs and at the same time to reduce deforestation in
Sub-Saharan Africa.
Description
PhD Thesis
Keywords
Nutrient, Water dynamics, Rotational woodlots, Western Tanzania, Acacia, Aboveground biomass, Decomposition, Deforestation, Fuel-wood