¹⁵ N Kinetic Analysis of N ₂ O Production by Nitrosomonas europaea : an Examination of Nitrifier Denitrification

Abstract

A series of 15 N isotope tracer experiments showed that Nitrosomonas europaea produces nitrous oxide only under oxygen-limiting conditions and that the labeled N from nitrite, but not nitrate, is incorporated into nitrous oxide, indicating the presence of the “denitrifying enzyme” nitrite reductase. A kinetic analysis of the m/z 44, 45, and 46 nitrous oxide produced by washed cell suspensions of N. europaea when incubated with 4 mM ammonium (99% 14 N) and 0.4 mM nitrite (99% 15 N) was performed. No labeled nitrite was reduced to ammonium. All labeled material added was accounted for as either nitrite or nitrous oxide. The hypothesis that nitrous oxide is produced directly from nitrification was rejected since (i) it does not allow for the large amounts of double-labeled ( m/z 46) nitrous oxide observed; (ii) the observed patterns of m/z 44, 45, and 46 nitrous oxide were completely consistent with a kinetic analysis based on denitrification as the sole mechanism of nitrous oxide production but not with a kinetic analysis based on both mechanisms; (iii) the asymptotic ratio of m/z 45 to m/z 46 nitrous oxide was consistent with denitrification kinetics but inconsistent with nitrification kinetics, which predicted no limit to m/z 45 production. It is concluded that N. europaea is a denitrifier which, under conditions of oxygen stress, uses nitrite as a terminal electron acceptor and produces nitrous oxide.

A review of chemical reactions of nitrification intermediates and their role in nitrogen cycling and nitrogen trace gas formation in soil

“…Hooper & Terry (1979) had already stated that incomplete oxidation of NH 2 OH can lead to the development of N 2 O. This matches the 'nitrification-unstable intermediate' hypothesis formulated, but abandoned, by Poth & Focht (1985), in which N 2 O is produced during nitrification by various reactions of intermediates formed during NH 3 oxidation. Nitrogen trace gas production, other than that through classical denitrification, nitrifier denitrification or co-denitrification, does not have to be entirely chemical per se.…”

“…Nitrifier denitrification is a facultative pathway of nitrifiers in which NO 2 − is reduced anaerobically to N 2 by NO and N 2 O by the same autotrophic microorganism (Wrage et al, 2001). The pathway from NH 4 + through NO 2 − to N 2 O was introduced by Poth & Focht (1985), and the further step to N 2 by Poth (1986). The NH 3 oxidizers might use NO 2 − as an alternative electron acceptor for O 2 when oxygen is deficient (Ritchie & Nicholas, 1972).…”

Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes

“…This is an oversimplification, because oxidation decreases as soils become very dry and microbial activity is inhibited (e.g. Dobbie & Smith, 1996 Nitrification is an aerobic process, but when the supply of O 2 is limited by diffusional constraints the nitrifying bacteria can use nitrite as an electron acceptor and reduce it to NO and N 2 O (Poth & Focht, 1985;Bollmann & Conrad, 1998). The process may be simply represented as follows:…”

Processes Regulating Soil Emissions of NO and N^2O in a Seasonally Dry Tropical Forest

“…wetting event. Nitrifying bacteria are known to produce N 2 0 directly by reducing N0 2 - (Poth and Focht 1985). Enhanced N 2 0 production when N0 2 -was added to live soil and lack ofN 2 0 production when N0 2 -was added to autoclaved soil (Fig.…”