13.5: Nitrous oxide

Like carbon dioxide and methane, microorganisms can produce and consume nitrous oxide \(\left(\text{N}_{2}\text{O}\right)\), also known as laughing gas. The main pathways by which microorganisms produce nitrous oxide are denitrification and nitrification. Microorganisms can consume nitrous oxide by catalyzing the terminal step of denitrification. This chapter summarizes some information about these microbial reactions, though additional details are provided in Chapter 5 (Sections 5.2.5 and 5.2.3).

During nitrification, ammonium\(\left(\text{NH}_{4}^{+}\right)\) is oxidized to hydroxylamine \(\left(\text{NH}_{2} \text{OH}\right)\) and then to nitrite \(\left(\text{NO}_{2}\right)\) and nitrate \(\left(\text{NO}_{3}^{-}\right)\) (see Fig. \(5.3\)). As described by Glass and Orphan (2012), nitrous oxide is produced from two offshoots of nitrification: nitrifier denitrification and hydroxylamine oxidation. Nitrifier denitrification is a somewhat confusing term. Essentially, some nitrifiers can not only nitrify but also denitrify as well (Ritchie and Nicholas, 1972), which is referred to as nitrifier denitrification (Wrage et al., 2001). Through this pathway, nitrite is reduced to nitric oxide \((\text{NO})\) and then to nitrous oxide or dinitrogen \(\left(\text{N}_{2}\right)\) instead of being oxidized to nitrate (Glass and Orphan, 2012). In the hydroxylamine pathway, hydroxylamine is oxidized to nitric oxide and then reduced to nitrous oxide (Glass and Orphan, 2012). For both pathways, ammonium is aerobically oxidized, and thus nitrification produces nitrous oxide in oxic environments.

During denitrification, nitrate is reduced to nitrite, and then nitric oxide, nitrous oxide, and ultimately dinitrogen under favorable conditions (see Fig. \(5.3\)). However, the process is leaky and some intermediates, including nitrous oxide, are typically released as well, with proportions of nitrous oxide generation sensitive to pH and oxygen concentration (Glass and Orphan, 2012). Denitrification is thought to primarily occur in anoxic environments. However, some bacteria can use the reaction in oxic environments (Ji et al., 2015).

In terms of a microbial nitrous oxide sink, the terminal step of denitrification, oxidation of nitrous oxide to dinitrogen, appears to be the main pathway. Intracellular nitrous oxide as well as nitrite can exchange with external pools, creating the possibility that they take in more than they produce and thus serve as a net sink for nitrous oxide (Bourbonnais et al., 2017; Sun et al., 2021).

To place these microbial activities in an environmental context, we first consider nitrous oxide production in soil. Significant emissions of nitrous oxide occur from terrestrial environments and the oceans, but the largest source overall is terrestrial soils (Gruber and Galloway, 2008; Tian et al., 2016). As an example study, Davidson and Swank (1986) and Davidson et al. (1986) examined gaseous nitrogen losses from nitrification and denitrification in soils collected from two forested watersheds. They found nitrous oxide generation by nitrification was most important in well-drained sites of a disturbed watershed where nitrate concentrations were relatively high (Fig. \(13.7\)). In contrast, denitrification was most important in poorly drained sites within the riparian zones of the streams in each study watershed. At those locations, the soils were wetter than they were upslope and also richer in organic matter, increasing the likelihood of oxygen depletion. Similar relationships between soil moisture and denitrification have also been observed in agricultural soils (e.g., Ryden, 1983; Tiedje et al., 1984). Taken together, these results illustrate how the moisture content of soils can impact oxygen levels and ultimately the pathway of nitrous oxide production.

One thing to bear in mind for these results is that nitrous oxide is not necessarily produced by one pathway or the other at a particular location within an environment. In fact, both nitrification and denitrification can occur even within the same soil aggregate (Stevens et al., 1997). Environmental controls that determine the rate and pathway of nitrous oxide production include soil moisture content, oxygen concentration, temperature, pH, and substrate availability (Butterbach-Bahl et al., 2013). The status of these controls can vary significantly over short distances in soils.

As a second example, we consider a study by Beaulieu et al. (2014), who examined nitrous oxide generation in the hypolimnion of a thermally stratified reservoir in a watershed dominated by agricultural land use. The hypolimnion is the lower layer of water in a stratified lake or reservoir, which is cooler and often contains lower oxygen concentrations than water near the lake surface. Their findings indicate that denitrification was a consistent sink for nitrate and source of dinitrogen gas while the lake was stratified. However, whether it was a source or sink of nitrous oxide varied over time. Production of nitrous oxide relative to dinitrogen ranged from -3.4% (net \(\text{N}_{2} \text{O}\) sink) to 19.5% (net \(\text{N}_{2} \text{O}\) source) during the study period. Their results suggest that this variation was not caused by changes in denitrification rates, but rather changes in the extent to which denitrification was able to complete its final step, oxidation of nitrous oxide to dinitrogen. The authors concluded that the mechanisms responsible for the observed variation include variation in environmental controls on the activity of nitrous oxide reductase, the enzyme that catalyzes the final step of denitrification.

Figure \(13.7\): Schematic illustration of the distribution of nitrous oxide \(\left(\text{N}_{2} \text{O}\right)\) sources in one of the watersheds studied by Davidson and Swank (1986) and Davidson et al. (1986). Both nitrification and denitrification occurred in the soils at the study area, but nitrification was most important in well-drained upslope areas and denitrification was most important in poorly drained areas in the riparian zone, the interface between the terrestrial and aquatic ecosystems. Concentrations of nitrate measured in the stream and in soil water collected from a lysimeter are shown. The difference in nitrate concentrations between those sampling locations is consistent with nitrate removal by denitrification as subsurface water (groundwater and soil water) flows toward the stream. This figure is based on figure 5 of Davidson and Swank (1986).
Image source: https://commons.wikimedia.org/wiki/File:Nitrous_oxide_sources.jpg