Isotopic variability of N2O emissions from tropical forest soils

We report measurements of the N-15 and O-18 signature of N2O emitted from t ropical rain forest soils at the La Selva Biological station in Costa Rica and in the Fazenda Vitoria in the State of Para, Brazil. The delta(15)N val ues ranged from -34 to 2 parts per thousand with respect to atmospheric N-2 , while delta(18)O values had a smaller range, from -4 to 18 parts per thou sand with respect to atmospheric O-2. We attribute these large variations t o differences in microbial production, consumption, and transport of N2O. I n general the delta(15)N of N2O emissions from an Oxisol soil in Brazil wer e consistently enriched by similar to 20 parts per thousand in N-15 compare d 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 N-15 of nitrate was similar in both locations. We attribute the o verall variability in emitted N-15 to differences in the ratio of N2O:N-2 e scaping from the soil to the atmosphere, with a larger fraction of the N2O reduced to N-2 at the Brazilian sites. We found light delta(15)N-N2O values associated with high N2O emissions in a fertilized agricultural site in Co sta 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 in crease the fractionation associated with N2O production. Overall, the Brazi lian Oxisol soils had the most enriched delta(15)N-N2O emissions yet measur ed from soils. if these are more representative of tropical soil emissions than the Costa Rica emissions, then the globally averaged delta(15)N-N2O tr opical rain forest soil source is more enriched than previously estimated. The large variations in isotopic signature for N2O emissions demonstrate th e potential utility of stable isotopes as tools for understanding the proce sses of N2O production and consumption in soils.