N2O AND NO EMISSIONS FROM SOILS OF THE NORTHERN PART OF THE GUAYANA SHIELD, VENEZUELA

Soil fluxes of N2O and NO were measured in a scrub grass savanna and a nearby semideciduous forest in the savanna climatic region of Venezuela, during October 1988. The forest soil produces significantly larger emissions of N2O (ranging from 5.7 to 25.0 X 10(9) molecules cm-2 s-1) than the savanna soil (-3.6 to 14.0 X 10(9) molecules cm-2 s-1). This suggests that "long-term" deforestation could produce a reduction of the N2O emissions to the atmosphere. In comparison with previous results from another site in the Venezuelan savanna region, very low emissions of NO, of the order of 1 to 2 X 10(9) molecules cm-2 s-1, were recorded. Considering that the NO/N2O emission ratio was much lower than 1, that practically no changes in the emission were observed after the fertilization with NH4Cl, and that very low concentration of NO3- is found in the natural soil, it is likely that under undisturbed conditions N2O and NO are mainly produced by denitrification and that there is a lack of nitrifying bacteria in these soils. Both N2O and NO emissions increased after application of NaNO3. Since the emission of NO remained larger than the N2O emission (with a ratio of approximately 4/1), a significant change in the production pattern must have occurred after fertilization with nitrate.