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

Nitrous acid (HONO) plays a key role in atmospheric chemistry by contributing to hydroxyl radical (OH) formation. Despite its importance, a strong daytime source of HONO remains unexplained in ambient air measurements. Soils have been proposed as a potential source, but the mechanisms behind HONO emissions are not fully understood. Ammonia-oxidizing bacteria (AOB) have been suggested as possible contributors, but the exact pathways remain unclear. Prior research has identified HONO's role in OH formation, but the microbial sources and conversion processes are still under investigation. This gap motivated the need to explore microbial contributions to HONO emissions. No prior work had resolved the specific role of hydroxylamine (NH2OH) in this process. Understanding how HONO is produced in soils could clarify its atmospheric impact. This study addresses the uncertainty surrounding microbial HONO emissions.

The aim of this study was to investigate the microbial sources of HONO emissions from soils. Researchers sought to determine whether ammonia-oxidizing bacteria (AOB) and other nitrifying organisms could release hydroxylamine (NH2OH), a precursor to HONO. The study aimed to clarify the mechanisms behind HONO formation in soils. By using a dynamic soil-chamber system, the researchers tested whether NH2OH emissions from nitrifying cultures could lead to HONO production. The study also aimed to assess the role of ammonia-oxidizing archaea (AOA) in this process. The motivation was to resolve the unknown daytime HONO source in ambient air. This approach could help explain atmospheric HONO levels. The findings may contribute to a better understanding of microbial contributions to atmospheric chemistry.

The study employed a dynamic soil-chamber system to measure gaseous emissions from nitrifying pure cultures. Researchers analyzed emissions from ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). The system allowed for controlled conditions to observe NH2OH and HONO production. Glass bead surfaces were used to simulate heterogeneous reactions with water vapor. The researchers monitored gas-phase HONO formation from NH2OH emissions. They tested whether NH2OH could convert to HONO through surface reactions. The study included multiple microbial species to assess variability in emissions. This approach provided direct evidence of HONO formation from microbial activity.

The study found that nitrifying pure cultures emitted hydroxylamine (NH2OH), which then converted to HONO on glass bead surfaces. Ammonia-oxidizing bacteria (AOB) were confirmed as a source of NH2OH emissions. Ammonia-oxidizing archaea (AOA) also released HONO, indicating a broader microbial contribution. The conversion of NH2OH to HONO occurred via a heterogeneous reaction with water vapor. The findings suggest that HONO emissions are not limited to AOB but extend to AOA. Diverse organisms, including fungi and heterotrophic nitrifiers, may also contribute. The study provides direct evidence of HONO formation from microbial activity. These results support the idea that soil emissions significantly impact atmospheric HONO levels.

The authors concluded that hydroxylamine (NH2OH) emissions from nitrifying microorganisms are a precursor to HONO formation in soils. The study provides direct evidence of NH2OH release from ammonia-oxidizing bacteria (AOB) and archaea (AOA). The conversion of NH2OH to HONO occurs via a heterogeneous reaction with water vapor. These findings suggest that HONO emissions are not restricted to AOB but also involve AOA. The study implies that diverse microbial groups contribute to HONO formation. The results support the idea that soil emissions are a significant source of atmospheric HONO. The authors propose that biogenic NH2OH from various organisms may drive HONO emissions. These conclusions align with the observed atmospheric HONO levels and their unknown daytime source.