Soil emissions of HONO and other nitrogen-containing gases: Insights into microbial pathways and moisture effects

Nitrous acid (HONO) greatly impacts tropospheric chemistry by producing hydroxyl radical (OH) through photolysis, and yet our knowledge about sources of HONO remains elusive. Emissions of nitrogen (N) containing gases from soils have long been a subject of research in biogeochemistry. Soil emissions of HONO have received greater attention recently, helping explain a missing source of observed atmospheric HONO. We conducted laboratory experiments to simultaneously measure emission fluxes of HONO along with other N containing gases including nitric oxide (NO), nitrous oxide (N₂O), and ammonia (NH₃) from lateritic red soil samples, and evaluated the contributions of microbiological processes to HONO emissions by conducting process inhibiting experiments. Results from monitoring emissions during a full wet-drying cycle showed that the emissions of HONO, NO and N₂O have a strong dependance on soil water content, with maximum fluxes for HONO (125 ± 17 ng N m⁻² s⁻¹), NO (115 ± 11 ng N m⁻² s⁻¹) and N₂O (453 ± 100 ng N m⁻² s⁻¹) observed at 17 % (HONO), 42 % (NO) and 94 % (N₂O) water filled pore space (WFPS), respectively, while NH₃ emission remains at ∼16 ng N m⁻² s⁻¹ in majority of the WFPS range. Results from process inhibiting experiments suggested ammonia oxidation to be the dominant pathway for HONO production in the low water-content range, while reduction of nitrate to NO₂⁻ appeared dominant in the high water-content range. Our study demonstrates the feasibility of studying emissions of HONO along with other N containing gases as connected network of processes as a whole.