Nitrous oxide production via ammonium oxidation at low oxygen availability in agricultural soils

Background/Question/Methods

When aiming to develop adequate mitigation strategies for emission of nitrous oxide (N₂O) from soils, an accurate understanding of biochemical N₂O production pathways is advantageous. Much previous research has been done on N₂O emissions under different agricultural soils and practices, most notably comparing different sources of fertilizer nitrogen and moisture conditions. These results can be attributed to changes in the pathways by which N₂O is produced. The availability of oxygen is the main factor affecting the amount and pathway of N₂O production; it has only been directly considered in several studies. The present work was designed to describe how oxygen availability together with nitrogen inputs influences the relative importance of pathways producing N₂O. We used three distinct agricultural soils varying in texture. Short-term incubations were carried out under a range of oxygen availability (0 to 21%) and with two forms of fertilizer, urea and ammonium sulfate. Acetylene inhibition was used to quantify the contributions of denitrification and ammonium oxidation to N₂O production, and an isotope tracer experiment provided complementary data to reinforce these results.

Results/Conclusions

In all soils, ammonium oxidation was the dominant source of nitrous oxide production under reduced oxygen availability (0.5 to 3%), accounting for 51 to 92, 64 to 90, and 35 to 75% of the total nitrous oxide produced in a loam, clay loam, and sandy clay loam, respectively. As the concentration of oxygen was reduced from 21 to 0.5%, the rate of nitrification decreased, and the amount of N₂O as well as the proportion of oxidized ammonium emitted as N₂O increased. Input of urea to all soils increased both the rate of nitrification and the proportion of nitrified nitrogen emitted as nitrous oxide, resulting in a 10 to 15% greater emission of nitrous oxide as compared to ammonium sulfate application. These results contribute to a more complete understanding of the mechanisms of nitrous oxide production in soils and the specific conditions under which different pathways operate. This information in turn makes it possible to better understand and use empirical data, including predictions of the effects of changes in nitrogen fertilization and conditions controlling oxygen availability.