Hydroxylamine released by nitrifying microorganisms is a precursor for HONO emission from drying soils

Abstract

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH), the atmosphere´s primary oxidant. An unknown strong daytime source of HONO is required to explain measurements in ambient air. Emissions from soils are one of the potential sources. Ammonia-oxidizing bacteria (AOB) have been identified as possible producers of these HONO soil emissions. However, the mechanisms for production and release of HONO in soils are not fully understood. In this study, we used a dynamic soil-chamber system to provide direct evidence that gaseous emissions from nitrifying pure cultures contain hydroxylamine (NH₂OH), which is subsequently converted to HONO in a heterogeneous reaction with water vapor on glass bead surfaces. In addition to different AOB species, we found release of HONO also in ammonia-oxidizing archaea (AOA), suggesting that these globally abundant microbes may also contribute to the formation of atmospheric HONO and consequently OH. Since biogenic NH₂OH is formed by diverse organisms, such as AOB, AOA, methane-oxidizing bacteria, heterotrophic nitrifiers, and fungi, we argue that HONO emission from soil is not restricted to the nitrifying bacteria, but is also promoted by nitrifying members of the domains Archaea and Eukarya.

Figure 1

Optimum emissions (F_opt) of HONO and NO from the investigated cultures of AOB, NOB and AOA. The cell densities of the pure culture expressed as ATP concentration in µmol l⁻¹ are shown by the numbers above the corresponding bars. For NOB and AOA no ATP data is available. Measurements refer to single values as some cultures have been measured repeatedly but at different ATP concentrations (e.g. N. europaeae emitted 500 to 800 nmol m⁻² s⁻¹ for ATP between 2.5 µmol L⁻¹ and 10 µmol L⁻¹).

Figure 2

Increased porosity of the cell membrane of N. europaea by formaldehyde (CH₂O) causes instantaneous release of HONO and NO. The release of H₂O from the soil sample is represented in arbitrary units.

Figure 3

N. europaea releases NH₂OH at the same order of magnitude as HONO, but it is emitted at higher gravimetric water content, θ_g, compared to HONO. With decreasing θ_g, the number of cells with membrane damage rises (the time course of the experiment proceeds from left to right). Error bars of damaged cell numbers denote standard deviations (n = 3).

Figure 4

HONO is formed on glass bead surfaces during the surface reaction of NH₂OH (207 ppb) with water vapor (12.5 mmol mol⁻¹). The point at zero is the value measured without glass beads.