Soil-plant interactions during early vegetation growth govern HONO, NO and N2O emissions through moisture, temperature, and mineral N pools in boreal agricultural soils

Abstract

Background and aims: Reactive nitrogen gases (HONO, NO, N2O) are key to atmospheric chemistry and climate forcing, yet their simultaneous in-situ emissions from boreal agricultural soils are poorly known. We aimed to quantify these gases during early crop growth, a period when emerging soil–plant interactions begin to shape soil microclimate and N cycling and to identify the main environmental and vegetation drivers controlling their fluxes. Methods: We quantified in-situ fluxes of HONO, NO, and N2O simultaneously and key soil variables across oat, barley and bare-soil plots in eastern Finland. Further, we applied piecewise structural equation modelling (pSEM) to determine the direct and indirect effects environment and vegetation on gas emissions. Results: Daily emissions (µg N m⁻2 h⁻1) were strongly dominated by N2O (13.5–229), followed by NO (1.5–38.2) and HONO (1.0–13.0). Cumulative fluxes confirmed this hierarchy, with HONO ~ 9–11 times lower than N2O and NO ~ 3 times lower than N2O. pSEM showed that soil temperature and moisture were the main regulators, with HONO and NO emissions increasing with temperature and decreasing with moisture, while N2O was driven solely by temperature. Vegetation indirectly suppressed emissions by increasing soil moisture and lowering soil pH, which reduced nitrite and thus HONO and NO precursor availability, consistent with the lower mean NO fluxes from vegetated compared to bare soil surfaces (17.1 vs. 22.6 µg N m−2 h−1). Conclusion: Our findings show that during early crop growth, soil–plant interactions are strongly shaped by physical drivers especially temperature and moisture which outweigh chemical controls.