Nitrogen Cycling in West African Ecosystems MICROBIOLOGICAL CONSIDERATIONS OF THE NITROGEN CYCLE IN WEST AFRICAN ECOSYSTEMS

The review is an attempt to specify particular features of the biological processes involved in the nitrogen cycle as they occur in the following categories of West African ecosystems : flooded rice .fields, rain-fed agroecosystems, and forests. In rice fields in Senegal the nitrogen input through blue-green algae was reported to be in the range -, of 1-30 kg ha-' yr-'. Figures for heterotrophic rhizosphere Nn-fixation in the rice rhizosphere should be reassessed. In rain-fed ecosystems, symbiotic Nz-fixation is often impeded by such limiting factors as moisture stress (in semi-arid areas), nematode attacks, soil acidity and toxicity, mineral deficiencies (especially phosphorus deficiency), inadequacy of Rhizobium populations and competition between native and introduced strains. Inoculation with Rhizobium is futile if even one of the above-mentioned limiting factors is still operating. No reliable evaluation has been published thus far of Nzfixation in forests. Nitrification and denitrification are limited by soil acidity in rice soils as well as in non-fertilized rain-fed agrosystems. However, when nitrogen fertilizers are applied to the soil, losses through denitrification were reported to be ca. 30 %, whereas losses through leaching were only 10 %. In West African forest ecosystems, nitrification is potentially much more active than in temperate conditions. Data on denitrification is lacking. Mineralization rates are high, except in acid rice fields, as long as the soil is flooded. In arid conditions, mineralization is slowed down but still persists as long as the soil pF is not higher than 5.2, which corresponds to relatively dry conditions. ' Introduction A voluminous amount of literature has been accumulated concernhg nitrogen transformations in soil, but, unfortunately, most of the information is limited to temperate conditions. Many of the general concepts applicable to temperate soils are applicable to tropical soils, but the special conditions which prévail in tropical environments lead to considerable modifications of the transformation rates and nitrogen transfers. The present paper is a review of our current knowledge of the role of microorganisms in nitrogen transformations occurring in different ecosystems which are typical of West Africa, attempting to specify the particular features of the biological processes in such conditions. m R "% .T,Q ~ Pgf EI Table 1. Distribution of bacteria, fungi and actinomycetes in different fractions of a typical sandy soil (Dior) from Central Senegal (Panthier & Feller, unpublished data) Fractions Weight Bacteria Fungi Actinomycetes (%I (%I (%I (%I