Isotopic variability of N 2 O emissions from tropical forest soils

We report measurements of the 15N and 18O signature of N2O emitted from tropical rain forest soils at the La Selva Biological station in Costa Rica and in the Fazenda Vitoria in the State of Pará, Brazil. The δ15N values ranged from −34 to 2‰ with respect to atmospheric N2, while δ18O values had a smaller range, from −4 to 18‰ with respect to atmospheric O2. We attribute these large variations to differences in microbial production, consumption, and transport of N2O. In general the δ15N of N2O emissions from an Oxisol soil in Brazil were consistently enriched by ∼20‰ in 15N compared to those from Ultisol and Inceptisol soils in Costa Rica. Denitrification is the most likely source of N2O in both locations during the rainy season, and the 15N of nitrate was similar in both locations. We attribute the overall variability in emitted 15N to differences in the ratio of N2O:N2 escaping from the soil to the atmosphere, with a larger fraction of the N2O reduced to N2 at the Brazilian sites. We found light δ15N-N2O values associated with high N2O emissions in a fertilized agricultural site in Costa Rica and in a “hot spot” of high emissions in the forest site in Brazil. This result suggests that the increase of substrate availability might increase the fractionation associated with N2O production. Overall, the Brazilian Oxisol soils had the most enriched δ15N-N2O emissions yet measured from soils. If these are more representative of tropical soil emissions than the Costa Rica emissions, then the globally averaged δ15N-N2O tropical rain forest soil source is more enriched than previously estimated. The large variations in isotopic signature for N2O emissions demonstrate the potential utility of stable isotopes as tools for understanding the processes of N2O production and consumption in soils