Field Evidence for the Spatial Heterogeneity of HONO Exchange Fluxes in Wheat Fields of the Huai River Basin

Abstract Soils have been identified as an important HONO source, yet field studies on HONO fluxes remain limited, and the impact of soil heterogeneity on HONO fluxes has not yet been systematically evaluated. Here, an automated dynamic chamber system equipped with dual measurement chambers (MC1, MC2) was employed to measure HONO fluxes from wheat fields in the Huaihe River Basin, with alternating measurements of gas concentrations inside the chambers achieved via solenoid valve switching. Laboratory and field evaluations confirmed the feasibility of this flux measurement system. Two MCs were separated by 7 m, covering a wheat field at the seedling stage with low seedling coverage. The field results revealed similar diurnal variation patterns of HONO fluxes in MC1 and MC2, with higher fluxes observed at noon, and their mean values were 1.06 and 1.41 ng N m −2 s −1 , respectively. Correlation analysis indicated that photosensitized conversion of NO 2 on soil surfaces and microbial processes contribute comparably to HONO flux in the MC1, whereas microbial processes play a relatively more important role in the MC2. The higher inorganic nitrogen levels in the MC2 further suggest that enhanced nitrification may have increased HONO emissions. Further analysis showed that when the HONO flux difference between MC1 and MC2 is most significant, it may be caused by temperature‐induced variations in soil microbial processes, while the flux difference on some dates is likely associated with changes in NO 2 × J(NO 2 ). Our study provides the field evidence for understanding the spatial heterogeneity of HONO emissions from agricultural soils.