Soil fertility depletion is a main constraint to food production in sub-Saharan Africa. This thesis concerns the potential of N2-fixing trees to increase nitrogen inputs to agroforestry systems and accordingly to improve crop production. The suitability of five tropical tree species (including two N2-fixing species, Leucaena leucocephala and Prosopis chilensis) for tree fallows in Tanzania were evaluated by comparing their leaf chemistry, their effects on soil properties and on maize growth. After five years fallow, the per cent total soil N was higher under Prosopis compared to under other tree species. Maize biomass production was higher on soils from Leucaena or Prosopis compared to grass fallow. Prosopis contributed 11% to the total soil C over a period of 8 years. Field experiments in Kenya were performed to test a low-level 15N-tracer technique to estimate biological nitrogen fixation in Sesbania sesban over an 18-months period, and to compare the effects of short-duration tree fallows on two subsequent maize harvests with natural fallow and continuous cropping. We estimated the N derived from atmosphere by Sesbania after 18 months to between 500 and 600 kg ha-1, depending on which plant parts were used for 15N data and on the choice of reference species. We consider the 15N dilution method to be appropriate for quantifying N2 fixation in improved fallows in studies of young trees with high N2-fixing ability. In an experiment examining the effects of tree fallows on subsequent maize crops approximately 70-90% of the N in Sesbania, and 50-70% in Calliandra calothyrsus, was derived from N2-fixation. The quantity of N added by N2-fixation, 280-360 kg N ha-1 for Sesbania and 120-170 kg N ha-1 for Calliandra, resulted in a positive N balance after two cropping seasons of 170-250 kg N ha-1 and 90-140 kg N ha-1 respectively. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. The substantial accumulation of N in planted Sesbania demonstrated its potential to increase the sustainability of crop production on N-limited soils
