N-15 AND O-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 N-15/N-14 and O-18/O-16 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 N-15 and O-18 relative to mean tropospheric N2O. A major source of heavy N2O, enriched in both N-15 and O-18, 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 N-15 and O-18 by photolysis and chemistry. However these return fluxes of N-15 and O-18 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.