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.

Microbial mechanisms and ecosystem flux estimation for aerobic NO y emissions from deciduous forest soils

“…Furthermore, Caranto and Lancaster (22) showed that NO is a precursor to NO 2ˉi n AOB via the NH 2 OH/NO obligate intermediate mechanism, indicating a possible biogenic pathway for aerobic-derived NO. Evidence for AOB contributing to NO release comes from culture-based assays showing that AOB produces significantly more NO than AOA (14,21). However, this phenomenon has yet to be demonstrated in a soil matrix, leading to questions regarding the environmental significance of this proposed mechanism.…”

“…Most studies have implicated abiotic mechanisms associated with NO x (NO and NO 2 ) chemistry as the primary driver of HONO production. However, it has recently been suggested that a portion of the NH 2 OH produced via NH 3 oxidation is released from the soil as HONO (14,33), assuming that certain conditions associated with soil pH, water content, and surface area are met (34,35). Additionally, biologically produced NO 2ˉm ay be protonated to form HONO.…”

“…The authors used flux measurements and amplicon sequencing to determine links between the relative abundances of AOA, AOB, and NOB and HONO production; they noted that, in near-neutral pH soils, HONO flux was highest and AOB were most abundant, whereas in acidic soils, HONO was lower and AOA dominated, possibly indicating a pHinfluenced source of biogenic HONO. Studies of pure cultures of AOA, AOB, and NOB indicate that lineages of AOA and AOB can potentially produce HONO; however, AOB seem to be the dominant contributors in laboratory cultures (14). The discovery by Caranto and Lancaster (22) that NO is a necessary intermediate of AOB nitrification may have direct implications on HONO production.…”

“…However, the degree to which AOA vs. AOB influence NO y emissions from soil is unknown (13) and may depend on the fate of NH 2 OH. NH 2 OH can decompose via abiotic or enzymatic pathways to nitrogen oxides and NO 2 ( 14). NO 2ˉc an then be volatilized as HONO or oxidized to NO 3v ia nitrite-oxidizing bacteria (NOB), such as Nitrobacter (15).…”

The role of HONO in O<sub>3</sub> formation and insight into its formation mechanism during the KORUS-AQ Campaign

“…not only atmospheric particles but also ground soils, with the report of direct soil emission (Meusel et al, 2018;Bhattarai et al, 2018). These heterogeneous conversions and emissions from soil are controlled by several conditions such as pH, moisture, and microbials and mainly affect daytime HONO concentrations (Ermel et al, 2018;Wu et al, 2019). For these reactions, the measurement of active surface area hinders the accurate estimation of HONO formation (Romer et al, 2016).…”