Influence of Oxygen On Nitrous Oxide Production Via Nitrifier Denitrification.

The development of mitigation strategies to reduce nitrous oxide (N₂O) emission from soils is dependent on explaining the key biophysical factors affecting its production pathways. Nitrifier denitrification (ND), carried out by autotrophic ammonia oxidizers, produces N₂O by oxidation of ammonia to nitrite which is then reduced to N₂O. The existence of this pathway has long been established in pure culture, but has only recently been shown to be a significant source of N₂O in soil, responding characteristically to soil moisture content. The main factor affecting the amount and pathway of N₂O production, however, is the availability of oxygen (O₂); this is usually associated indirectly with soil moisture status but has rarely been considered directly. The present work examined the interaction of fertilizer nitrogen source with O₂ availability in influencing the production of N₂O from ND in agricultural soil. This was studied in short-term incubations under a range of O₂ content (21, 3, 0.5, and 0%) and two forms of fertilizer (urea and ammonium sulfate). A dual isotope (¹⁵N, ¹⁸O) approach was used to calculate the proportion of N₂O arising from several possible pathways, including ND. Except at 0% O₂, under which no N₂O was derived from ammonium, the amount of N₂O produced from ND increased as O₂ availability decreased. Under reduced O₂ availability (3 and 0.5%), ND was the dominant source of N₂O in soil applied with urea, accounting for over 48% of the total produced. This contribution was greater than that from any other pathway, including fertilizer denitrification, nitrification-coupled denitrification, and nitrifier nitrification. At 21% O₂, ND remained a significant source of N₂O, with a greater contribution from ND observed following application of urea as compared to ammonium sulfate.