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

Abstract

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.