Comprehensive Reactive Nitrogen Fluxes from in-Use Agricultural Soils Using Dynamic Chambers: Observations from Two Canadian Campaigns (Invited Presentation)

There is significant pressure on the Earth system through humanity’s agricultural activities. A substantial portion of that pressure is from the perturbation of elemental cycles, such as that for nitrogen (N). Disruption of the N cycle has numerous negative effects, e.g., interference with ecosystem function through eutrophication, negative impacts on air quality, and production of the potent greenhouse gas (GHG) nitrous oxide (N₂O). Here we consider atmospheric reactive nitrogen (N_r) to include all N-containing formulae, except N₂. Global fixation of ammonia (NH₃) and release of N_r through fertilizer use, as well as combustion of fuels have led to never-before-seen N_r levels, as humans fix N at double the rate of natural systems. Fixation and transport of N-related products is central to the success of Canadian agriculture but also contributes to its climate burden. Residual soil N has been increasing in Canadian soils, and globally N-use efficiency (NUE) has been decreasing for decades. This is often attributed to inefficiently utilized N_r resources, where, instead of contributing to crop yields, N is leached, denitrified, or accumulated in root zones of agricultural soil. Improving agricultural N-use by growers in Canada remains a challenge with major knowledge gaps, which we have begun to address with comprehensive field scale measurements with newly developed dynamic flux chambers for reactive gases, so that the agronomy sector may improve its NUE alongside reducing its GHG impacts. These were deployed during two MAANURE field campaigns in 2022 and 2024 to simultaneously measure fluxes of: NO, NO₂, HONO, NH₃, N₂O, CH₄, H₂O, and CO₂. In the first two-week campaign, testing of the new chambers was made alongside standard eddy covariance flux techniques to validate chamber performance for N_r gases, while also facilitating proof-of-concept capabilities for conducting in situ experimental perturbations. The second campaign was conducted over 6 weeks from bare tilled soil through to planting and germination of commercial crops, under standard use by industrial collaborators. Soil characteristics, microbial communities, and N_r stable isotopic composition were all measured to provide insight into reservoirs and controls on reactive gas exchange between biological and chemical pathways.