Site-specific N2O isotopic compositions from Brazilian Amazon soils and their implications for the global N2O isotope budget

The site-specific ¹⁵N signature of N₂O (i.e., the ¹⁵N isotopic composition at the central or terminal position, expressed as δ ¹⁵N^α or δ ¹⁵N^β ) from natural sources (e.g., tropical rain forest soils) remains poorly explored, despite indications from the few measurements available that the signature can be used for distinguishing microbiological N₂O production mechanisms in soils and for reducing uncertainties in the global isotope budget. We present measurements of site-specific δ ¹⁵N^α , δ ¹⁵N^bulk (i.e., the average of the two N atom positions), and δ ¹⁸O of N₂O of soil gas samples collected in March 2002 during the rainy season in the Tapajos National Forest (TNF), Para, Brazil and on N₂O emissions from soil incubation experiments using the TNF soils and soils from Nova Vida Farm, Rondonia, Brazil. The microbiological and physical processes that could play roles in the observed distribution and variation of N₂O isotopologues from the tropical forest soils will be discussed and compared with results for N₂O produced in laboratory cultures of denitrifying bacteria and with stratospheric N₂O observations. We then use the combination of data from tropical soils, bacterial cultures and the stratosphere to constrain the contribution of the tropical forest soils to the atmospheric N₂O isotope budget.