Canopy Resistance Suppresses Fertilization-Driven Soil HONO Emissions in Intensively Managed Greenhouse Cultivation

Fertilization-derived emissions from soil are an important source of atmospheric nitrous acid (HONO), yet emissions from rapidly expanding greenhouse cultivation remain poorly constrained. We conducted continuous HONO flux observations in a vegetable greenhouse in southwestern China, spanning two fertilization events, crop growth and harvest. Using an air-exchange-based flux approach, we quantified fertilizer-induced HONO pulses from soil (mean peak flux 0.92 ± 0.55 ng N m^–2 s^–1) and showed that soil temperature and mineral nitrogen jointly modulated emission strength. As the crop canopy developed, daytime HONO fluxes decreased by up to an order of magnitude relative to postharvest bare-soil conditions, indicating strong canopy resistance to soil-derived HONO. Complementary leaf-chamber experiments yielded HONO uptake coefficients for vegetable leaves that are consistent with the canopy reduction factors inferred from the field, confirming the role of foliage as an efficient chemical sink. Our results demonstrated that greenhouse cultivation can be a substantial but highly dynamic HONO source, with canopy growth critically governing the temporal pattern and magnitude of emissions. These findings highlight the need to account for canopy resistance when representing soil HONO sources in regional air quality and atmospheric nitrogen cycling models.