N2O is not only a potent greenhouse gas, but also a major contributor to ozone depletion. In the late 1960s, it was found that part of the N2O emitted from the soil and other sources can reach all the way into the stratosphere. Here, it is partly converted to nitrogen oxides (NO, NO2), a process linked to ozone depletion. Proven in 1970, Paul Crutzen received for these findings the Nobel Prize in Chemistry in 1995.
He further noted that the increasing use of fertilizers might have led to increasing N2O emissions over the natural background, which would in turn result in a further depletion of ozone in the stratosphere. Throughout the 1970s, the prevailing thought was that N2O is produced in soils only through denitrification of nitrate (NO3–) under anaerobic conditions.
This landmark paper, led by Alfred Blackmer at Iowa State University, challenged that assumption. In 1978, the authors had already shown in a paper in Science that N2O emissions from well-aerated soils treated with ammonium forms of nitrogen exceeded those from soils treated with nitrate. N2O emissions were shown to be reduced by adding nitrapyrin, a compound that inhibits nitrification – the conversion of ammonium to nitrate in soils by soil microorganisms.
In this paper from 1980, they provide more conclusive evidence for this process. They added ammonium sulfate to sterilized soil and inoculated it with pure cultures of ammonia-oxidizing microorganisms. They concluded that, at least under some conditions, most of the N2O evolving from soils treated with ammonium-based fertilizers is generated by nitrifying microorganisms during the oxidation of ammonium to nitrite. This provides a strong justification for utilizing technologies such as nitrification inhibitors or controlled release fertilizers, or for N sources and application practices that avoid high ammonium concentrations in the root zone.
