Impacts assessment of nitrification inhibitors on U.S. agricultural emissions of reactive nitrogen gases

Abstract

Fertilizer-intensive agriculture leads to emissions of reactive nitrogen (Nr), posing threats to climate via nitrous oxide (N₂O) and to air quality and human health via nitric oxide (NO) and ammonia (NH₃) that form ozone and particulate matter (PM) downwind. Adding nitrification inhibitors (NIs) to fertilizers can mitigate N₂O and NO emissions but may stimulate NH₃ emissions. Quantifying the net effects of these trade-offs requires spatially resolving changes in emissions and associated impacts. We introduce an assessment framework to quantify such trade-off effects. It deploys an agroecosystem model with enhanced capabilities to predict emissions of Nr with or without the use of NIs, and a social cost of greenhouse gas to monetize the impacts of N₂O on climate. The framework also incorporates reduced-complexity air quality and health models to monetize associated impacts of NO and NH₃ emissions on human health downwind via ozone and PM. Evaluation of our model against available field measurements showed that it captured the direction of emission changes but underestimated reductions in N₂O and overestimated increases in NH₃ emissions. The model estimated that, averaged over applicable U.S. agricultural soils, NIs could reduce N₂O and NO emissions by an average of 11% and 16%, respectively, while stimulating NH₃ emissions by 87%. Impacts are largest in regions with moderate soil temperatures and occur mostly within two to three months of N fertilizer and NI application. An alternative estimate of NI-induced emission changes was obtained by multiplying the baseline emissions from the agroecosystem model by the reported relative changes in Nr emissions suggested from a global meta-analysis: -44% for N₂O, -24% for NO and +20% for NH₃. Monetized assessments indicate that on an annual scale, NI-induced harms from increased NH₃ emissions outweigh (8.5-33.8 times) the benefits of reducing NO and N₂O emissions in all agricultural regions, according to model-based estimates. Even under meta-analysis-based estimates, NI-induced damages exceed benefits by a factor of 1.1-4. Our study highlights the importance of considering multiple pollutants when assessing NIs, and underscores the need to mitigate NH₃ emissions. Further field studies are needed to evaluate the robustness of multi-pollutant assessments.

Keywords: Agricultural nitrogen emissions; Ammonia (NH(3)); Monetized environmental impacts; Nitric oxide (NO); Nitrification inhibitors; Nitrous oxide (N(2)O).