Cyanobacterial crusts developing on the sandy and loamy soils of fallow lands in the Sahel (Niger) were investigated for their potential to fix nitrogen. Three sites were selected in this arid environment, differing in sediment type and species composition. In the sandy sites heterocystous nitrogen-fixing cyanobacteria were present, whereas the loamy site did not contain such species. All sites showed light-dependent nitrogenase activity, starting within 2 h after re-wetting of the desiccated crust samples. Inhibition of photosystem II caused a decrease of nitrogenase activity in the samples with heterocystous cyanobacteria, but was stimulatory in the non-heterocystous crust. The results suggest that cyanobacterial crusts may be important for the improvement of the soil by enriching it with nitrogen.
