Department of Earth System Science

Emissions of NO from soils in the Amazon rain forest were measured at 66 locations (224 measurements) using an enclosure technique, and continuous vertical profiles of NO and O₃ were measured between the ground and 41-m altitude. Fluxes of NO averaged 8.9 (±1.5) × 10⁹ molecules cm⁻² s⁻¹ from the dominant (yellow clay) soils of the region, with larger fluxes (33.7 (±6.5) × 10⁹ molecules cm⁻² s⁻¹) observed from adjacent white sand soils. Fluxes from clay soils were lower by more than a factor of 5 than fluxes observed during the dry season at a nearby site. Low soil emission rates were reflected in lower concentrations of NO at the top of the forest canopy in the wet season, only 30–50 pptv (parts per trillion by volume) during the daytime. The measured fluxes are consistent with chemical mass balances for NO within the forest canopy, calculated from the NO and O₃ profiles taken at night, and with observations of NO between 150 and 5000 m altitude (A. L. Torres and K. R. Hooks, unpublished manuscript, 1989). Measurements of NO emission rates from soil plots fertilized using NaNO₃, NH₄Cl or sucrose indicated that a reductive pathway (denitrification) may have been primarily responsible for production of the NO released by both clay and sand soils.