Nitrogen-15 and Oxygen-18 Characteristics of Nitrous Oxide: A Global Perspective

The global budget of N2O shows a significant imbalance between the known rate of destruction in the stratosphere and the estimated rates of natural and anthropogenic production in soils and the ocean. Measurements of the 15N/14N and 18O/16O ratios in two major tropospheric sources of N2O, tropical rain forest soils and fertilized soils, show that soil N2O from a tropical rain forest in Costa Rica and from sugar-cane fields in Maui is strongly depleted in both 15N and 18O relative to mean tropospheric N2O. A major source of heavy N2O, enriched in both 15N and 18O, must therefore be present to balance the light N2O from soils. One such source is the back-mixing flux of N2O from the stratosphere, which is enriched in 15N and 18O by photolysis and chemistry. However these return fluxes of 15N and 18O are so great that a large oceanic flux of N2O is required to balance the heavy isotope—enriched stratospheric flux. All these effects will be reflected in climatically related isotopic variations in trapped N2O in polar ice cores.

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