Microbial nitrate reduction to nitrite drives large HONO emissions at high soil moisture
Abstract
Nitrous acid (HONO) significantly contributes to the atmospheric hydroxyl radical (OH·) budget and therefore influences the atmospheric oxidation capacity and air quality. Comparisons between HONO concentrations measured in field campaigns and those derived by modeling suggest a large unknown HONO source during daytime. This unknown HONO source has been largely attributed to soil emissions, involving biogenic production of nitrite and subsequent exchange with the atmosphere through chemical equilibrium between soil nitrite and atmospheric HONO. The reactive uptake and acid displacement on the soil surface, emissions by ammonia-oxidizing bacteria (AOB), and a surface reaction between hydroxylamine and H2O, described for non-flooded soils, have been suggested as underlying mechanisms. Here we report large HONO emissions at high soil water-holding capacity (75-140%), which was observed for various contrasting soil samples, demonstrating this to be a widespread phenomenon. Supplemental nitrate stimulated HONO emissions, whereas ethanol (70% v/v) treatment, minimizing microbial activities, reduced HONO emissions by 80%, suggesting that nitrate-dependent biotic processes form a major source 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 deficient 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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B13N2531W
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES;
- 0469 Nitrogen cycling;
- BIOGEOSCIENCES;
- 0478 Pollution: urban;
- regional and global;
- BIOGEOSCIENCES