Nitrogen-15 and oxygen-18 characteristics of nitrous oxide: a global perspective

The global budget of N(2)O 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 (15)N/(14)N and (18)O/(16)O ratios in two major tropospheric sources of N(2)O, tropical rain forest soils and fertilized soils, show that soil N(2)O from a tropical rain forest in Costa Rica and from sugar-cane fields in Maui is strongly depleted in both (15)N and (18)O relative to mean tropospheric N(2)O. A major source of heavy N(2)O, enriched in both (15)N and (18)O, must therefore be present to balance the light N(2)O from soils. One such source is the back-mixing flux of N(2)O from the stratosphere, which is enriched in (15)N and (18)O by photolysis and chemistry. However these return fluxes of (15)N and (18)O are so great that a large oceanic flux of N(2)O is required to balance the heavy isotope-enriched stratospheric flux. All these effects will be reflected in climatically related isotopic variations in trapped N(2)O in polar ice cores.