Heterogeneity of soil characteristics and potential microbial activities were measured in several humid tropical West African ecosystems and in one dry savanna ecosystem in order to explain the high spatial variability of NO fluxes observed in the field. At the landscape scale, the variability of NO emission rates in the humid zone is mainly a function of topographic position and vegetation type. The highest NO fluxes occur in the downslope nutrient-rich environment (forest) which exhibits high denitrification potential and high total mineral N accumulation. NO fluxes are low in humid savanna environments which exhibit no detectable potential denitrification. Local structures such as mounds colonized by hypogeous termites exhibit an accelerated nitrogen cycling relative to their surroundings. Ammonium is quickly converted to nitrate; the soil has high potential rates of denitrification with correspondingly high NO emissions. NO fluxes are also high in dry savanna soils with high nitrification potentials but low denitrification potentials, suggesting that NO at these sites was produced during nitrification rather than denitrification. These results underscore the importance of considering the mechanisms driving spatial variability and the need to account for them in large-scale extrapolations. Among the soil characteristics studied, denitrification potentials provide a reliable indication of high NO fluxes for the humid sites, but not for the dry sites. ¿ American Geophysical Union 1995