Fertilizer-intensive agriculture is a leading source of reactive nitrogen (Nr) emissions, including nitric oxide (NO), nitrous acid (HONO), and ammonia (NH3), that damage air quality, climate, and human health. As air pollutants, NO and NH3 contribute to the formation of ozone (O3) and particulate matter (PM), with impacts that depend on spatiotemporally variable atmospheric conditions. However, N2O, as a potent greenhouse gas, has climate impacts that are independent of when and where it is emitted. Additionally, the impacts of control strategies on Nr emissions vary widely and may introduce trade-off effects. Therefore, there is a strong need for a comprehensive assessment that considers the emissions, impacts, and mitigation strategies for Nr.

I established an integrated assessment framework that connects a process-based agroecosystem model with reduced-complexity air quality and health models, along with the social cost of greenhouse gas. This framework enables us to consistently estimate Nr emissions, spatially track their contributions to air pollution, jointly quantify the associated damages using monetized damages as the metric, and assess the efficacy of control strategies.

I applied this framework to investigate Nr emissions from U.S. croplands, their associated adverse impacts, and their responses to two mitigation strategies: carbon amendments and nitrification inhibitors (NIs). This study revealed that the Nr emissions are highest in regions with intensive fertilizer use. NH3 is the most damaging, especially in densely populated regions. Impacts of carbon amendments on Nr emissions would vary widely, with net effects depending mostly on how nitrification is affected. Biochar amendments are most likely to mitigate emissions if applied at high rates in acidic soils with low organic carbon and inorganic nitrogen content. Adding NIs to fertilizers can mitigate N2O and NO emissions but inadvertently stimulate NH3 emissions. Impacts of NIs are largest in regions with moderate soil temperatures. Net impacts of NIs would be negative across most regions as the health harms from NH3 stimulation outweigh the benefits of N2O and NO mitigation.

This study provides valuable insights into Nr emissions mitigation. However, the current agroecosystem model, enhanced with carbon amendments and NIs algorithms, needs more field measurements for further evaluation and refinement.