Abstract

Decomposition of, and the subsequent nutrient release from tree leaf litter as well as external inputs of nutrients are important in the traditional agroforestry parklands of West Africa. The management of nutrient resources is essential to optimize crop performance and soil fertility. To explore temporal and spatial nutrient release in such management options has been the overall objective of this study. Specifically, this thesis investigated: (i) decomposition patterns and nutrient release from litters of Faidherbia albida and Vitellaria paradoxa; (ii) limitation of microbial respiration, as a result of low nitrogen (N) and phosphorus (P) in the F. albida and V. paradoxa parklands; and (iii) the effects of applications of combined compost and urea on maize yield in the F. albida and V. paradoxa parklands. The litter of F. albida lost more mass and nutrients than litter of V. paradoxa. However, the decomposition rate of V. paradoxa increased in the presence of F. albida litter or following the application of inorganic fertilizers. Soil microbial respiration was mainly limited by P-fixation, but N was also limiting. Two microbial growth peaks were observed when excess carbon and P were present, suggesting that either two pools of N are sequentially available to microbes or that two microbial communities do exist. Respiration tests also indicated that P and N availability to microorganisms were higher under the canopy than outside it. Maize yield was similar with the addition of urea, compost, or a combination of compost and urea, if applied at an equivalent N-content. However, in the driest year, the combination of compost and urea had the greatest effect in the increase of maize yield as compared with control treatments. The maize yield under the canopies of F. albida was up to 2.3 times the yield outside the canopies, while under the canopies of V. paradoxa it was 0.6 to 0.8 times the yield outside the canopies. The yield under the canopy was relatively better in dry year. Compost-N and inorganic fertilizer-N recoveries in maize shoot biomass were low, indicating that a large proportion of maize shoot-N is derived from the soil. Also, under the canopy of F. albida, the proportion of N derived from fertilizer in maize shoot was lower compared with that outside the canopy, implying that the soil N-pool was more readily available to plants under trees. In conclusion, there is a potential to improve the decomposition rate of V. paradoxa litter by inorganic fertilizer application or by mixing it with F. albida litter. Thus, the litter of V. paradoxa can be a useful to improve soil quality, rather than being burnt as is nowadays practiced by the farmers. Application of combined compost and inorganic fertilizer offers a management option that is cheaper than only inorganic fertilizers and more manageable than only compost applications.