Nitrogen Fixation by Blue-Green Algal Soil Crusts in Nigerian Savanna
Part of this material, in different form, was presented at the Symposium on "The potentials for nitrogen fixation in the tropics", Rio de Janeiro, Brazil, 18-25 July, 1977: "Nitrogen fixation by soil algae of temperate and tropical soils' '. (Stewart, W.D.P., Sampaio, M.J., Isichei, A.O. & Sylvester- Bradley, R.).
Blue-green algae, many of which are known to be nitrogen fixers, occur on the surface of the soil as crusts. Crusts are masses of algal filaments that grow on top of each other. These blue-green algal crusts were collected from all of the savanna zones of Nigeria in order to estimate the quantitative role they may play in the nitrogen economy of savanna ecosystems. Algae of the genus Scytonema, which are nitrogen fixers, were dominant in all the crust samples collected. Using the acetylene reduction assay, it was found that the crust samples fixed nitrogen 24 h after rewetting and were affected by pH, temperature, light and moisture variations. If sufficient light were available for near maximum photosynthesis, with an algal cover of the soil surface of about 30 % and mean to maximum-fixation during 70 %of the rainy season of 180 days of 10-hour day-length, from 3.3 to 9.2 kg ha-' yr-' of nitrogen would be fixed. This amount would replace much of the nitrogen lost from the grass standing crop as a result of annual burning of the savanna.