EMISSION OF NITROGEN MONOXIDE FROM AFRICAN TROPICAL ECOSYSTEMS - CONTROL OF EMISSION BY SOIL CHARACTERISTICS IN HUMID AND DRY SAVANNAS OF WEST-AFRICA

Heterogeneity of soil characteristics and potential microbial activiti es were measured in several humid tropical West African ecosystems and in one dry savanna ecosystem in order to explain the high spatial var iability of NO fluxes observed in the field. At the landscape scale, t he variability of NO emission rates in the humid zone is mainly a func tion of topographic position and vegetation type. The highest NO fluxe s occur in the downslope nutrient-rich environment (forest) which exhi bits high denitrification potential and high total mineral N accumulat ion. 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 cyclin g relative to their surroundings. Ammonium is quickly converted to nit rate; the soil has high potential rates of denitrification with corres pondingly high NO emissions. NO fluxes are also high in dry savanna so ils with high nitrification potentials but low denitrification potenti als, suggesting that NO at these sites was produced during nitrificati on rather than denitrification. These results underscore the importanc e of considering the mechanisms driving spatial variability and the ne ed to account for them in large-scale extrapolations. Among the soil c haracteristics studied, denitrification potentials provide a reliable indication of high NO fluxes for the humid sites, but not for the dry sites.