In this study we estimated regional scale N2O fluxes over two Central Amazon sites through vertical profiles sampled since 2000 over Tapajós National Forest, near Santarém, Pará (SAN), and since 2004 over Cuieiras Biological Reserve, near Manaus, Amazonas (MAN). For this estimation we remove the influence of the tropical background using N2O measurements from the NOAA/ESRL background sites at Barbados and Ascension Island. The relative influence of each site is determined through SF6 measurements, which permitted us to estimate a background concentration for N2O.
Analysis of the seasonality of fluxes shows higher flux during wet season, possibly because of the use of N-fertilizer in the beginning of the wet season and the better conditions for microbial nitrification and denitrification in the soil. At SAN after 2004, we observe an enhancement of N2O flux during wet season, possibly related to enhance N-fertilization in the area. At this site, we calculated wet season averages of 0.45 ± 1.2 mgN2O m-² day-¹ (2001); -0.44 ± 0.6 (2002); -0.9 ± 1.3 (2003); 1.3 ± 0.8 (2004), 2.7 ± 0.8 (2005) and 0.8 ± 0.5 day-¹ for 2006. In 2006 the lower flux may be explained by a reduction in the areas where fertilizer was applied, due to political programs in Pará. Some negative fluxes presented in our calculations may be results of soil uptake or errors in our determination of the difference between Amazonian and background measurements.
The results from MAN also exhibit seasonality, with higher fluxes during the wet season. The average fluxes for each year are 1.9 ± 0.8 mgN2O m-² day-¹ (2005) and 0.6 ± 0.3 mgN2O m-² day-¹ (2006).