Comparison of nitrogen monoxide emissions from several African tropical ecosystems and influence of season and fire

NO emission rates from soils were measured for twelve major African ecosyst ems in four countries (Congo, Niger, Ivory Coast, and South Africa) and wit hin four major phytogeographic domains: the Guineo-Congolese, Guinean, Sahe lian, and Zambezian domains. Measurements were performed during wet and/or dry seasons. All the measurements were made with the same dynamic chamber d evice, which allowed true comparisons to be made. This study showed that em ission rates strongly differed between ecosystems and exhibited a marked te mporal variability. Ecosystem effect was highly significant during both the dry and wet seasons. Emission rates were low (<0.6 ng NO-N m(-2) s(-1)) in Hyparrhenia and Loudetia savannas of the Guinean or Guineo-Congolese domai ns. Intermediate NO fluxes were obtained in rain forest and gallery forest ecosystems, in a broad-leafed savanna and in a seasonally wetted grassland (sandy soil) of the Zambezian domain, and in a dry fallow savanna of the Sa helian domain. Emission rates were maximum (>7 ng NO-N m(-2) s(-1)) in a se asonally wetted grassland (site 2) and in particular sites subjected to var ious disturbances, for example soil fauna activity (termite mounds) or past human disturbance (Acacia patches-settlement site). Microbial activity pot entials (i.e., carbon mineralization, nitrification, denitrification, and t otal net N mineralization) were determined for most of the soils where NO f luxes were measured. In some sites, these potential activities were useful to identify the major processes controlling NO emission rates. Denitrificat ion potential was very low and could not explain substantial NO fluxes from broad-and fine-leafed savannas and Hyperthelia savannas of the Zambezian d omain. Very low potentials of both nitrification and denitrification could be related to the low NO fluxes for the three Guinean savanna sites studied . NO fluxes were significantly higher during the wet season than the dry se ason in both savanna and forest ecosystems. Emission rates in savanna ecosy stems were significantly increased within a few hours after fire. The measu rements presented here provide a unique, consistent database which can be u sed to further analyze the processes involved in the spatial and temporal v ariations of NO emissions.