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Abstract

Nitrous acid (HONO) is a precursor of the hydroxyl radical (OH), a key oxidant in the degradation of most air pollutants.
Field measurements indicate a large unknown source of HONO during the day time. Release of nitrous acid (HONO) from
soil has been suggested as a major source of atmospheric HONO. We hypothesize that nitrite produced by biological nitrate
reduction in oxygen-limited microzones in wet soils is a source of such HONO. Indeed, we found that various contrasting
soil samples emitted HONO at high water-holding capacity (75
–
140%), demonstrating this to be a widespread phenomenon.
Supplemental nitrate stimulated HONO emissions, whereas ethanol (70% v/v) treatment to minimize microbial activities
reduced HONO emissions by 80%, suggesting that nitrate-dependent biotic processes are the sources of HONO. High-
throughput Illumina sequencing of 16S rRNA as well as functional gene transcripts associated with nitrate and nitrite
reduction indicated that HONO emissions from soil samples were associated with nitrate reduction activities of diverse
Proteobacteria
. Incubation of pure cultures of bacterial nitrate reducers and gene-expression analyses, as well as the analyses
of mutant strains de
fi
cient in nitrite reductases, showed positive correlations of HONO emissions with the capability of
microbes to reduce nitrate to nitrite. Thus, we suggest biological nitrate reduction in oxygen-limited microzones as a hitherto
unknown source of atmospheric HONO, affecting biogeochemical nitrogen cycling, atmospheric chemistry, and global
modeling.