In the sudanian zone of Cameroon (annual rainfall 1050 mm) the restoration of the fertility of abandoned ferruginous soils which have been degraded by 10-15 years of continuous cotton and cereals cultivation poses a problem. This study compares nitrogen cycling in different types of fallow with Acacia polyacantha, Cassia siamea, Eucalyptus camaldulensis, a herbaceous fallow protected against fire and grazing and a non-protected, overgrazed herbaceous fallow. The roles of symbiotic nitrogen fixation, surface root development and nitrogen recycling appear to be important in the restoration process. In the A. polyacantha plot, N stock is improved by over 550 kg/ha in a seven year period due to the symbiotic nitrogen fixation. This species greatly enhances the biological nitrogen cycle and provides the highest soil organic matter accumulation in spite of the most important soil N mineralization. Additionally, its well-developed surface root biomass constitutes a store of mineralizable nitrogen for the following crop. Cassia siamea, the litter of which disapears rather quickly, shows a less favourable effect on soil organic matter due to a low N cycle. Although the protected herbaceous fallow has a very high biomass production capacity, N flows are low and, as a result, the accumulation of organic matter is rather superficial. At seven years, Eucalyptus camaldulensis has a negative soil N balance due to nitrogen immobilization in woody biomass and the forest floor litter to the detriment of the soil. Livestock damping of the soil of the degraded unprotected fallow has an unfavourable effect on the performance of the following crop.