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

Authors: LUO, LINA - Rice University; COHAN, DANIEL - Rice University; GURUNG, RAM - Colorado State University; Venterea, Rodney - Rod; RAN, LIMEI - Natural Resources Conservation Service (NRCS, USDA); BENSON, VEREL - Collaborator; YUAN, YONGPING - Environmental Protection Agency (EPA)

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2024
Publication Date: 5/8/2024
Citation: Luo, L., Cohan, D., Gurung, R., Venterea, R.T., Ran, L., Benson, V., Yuan, Y. 2024. Impacts assessment of nitrification inhibitors on U.S. agricultural emissions of reactive nitrogen gases. Journal of Environmental Management. 359. Article 121043. https://doi.org/10.1016/j.jenvman.2024.121043.
DOI: https://doi.org/10.1016/j.jenvman.2024.121043

Interpretive Summary: Application of fertilizers in agriculture can result in the release of different forms of nitrogen gas to the atmosphere including nitrous oxide, nitric oxide, and ammonia. Each of these gases can have negative impacts on the environment and human health. Nitrous oxide is a potent greenhouse gas that promotes climate change, while nitric oxide can cause ozone formation, and both nitric oxide and ammonia can form particulate matter. Ozone and particulate matter can both negatively impact human health. Adding nitrification inhibitors to fertilizers can reduce nitrous oxide and nitric oxide emissions but can inadvertently stimulate ammonia emissions. The goal of this research was to quantify the net effects of these trade-offs on a national scale for the USA and to monetize the net benefits of nitrification inhibitors. For this, we used an agroecosystem model to predict emissions of each gas and then used three air quality and health models to monetize the impacts on human health. We also used results of previous gas emissions meta-analyses as inputs to the air quality and health models. The results showed that nitrification inhibitors could reduce nitrous oxide and nitric oxide emissions by an average of 11-44 percent and 16-24 percent, respectively, while stimulating ammonia emissions by 20-87 percent. Monetized impacts assessments show that the particulate matter-related damages caused by stimulating ammonia emissions outweigh the particulate matter- and ozone-related benefits of reducing nitric oxide emissions plus the climate benefits of mitigating nitrous oxide in most regions of the USA. Our study highlights the importance of considering multiple pollutants when assessing the benefits of nitrification inhibitors, and also underscores the need to mitigate ammonia emissions. Further field studies and model inter-comparisons are needed to bolster the robustness of multi-pollutant assessments. These results will be helpful to scientists, land managers, and policy makers in developing targeted practices to reduce the overall environmental and health impacts of nitrogen fertilizer application to agricultural soils.

Technical Abstract: Intensive use of fertilizers in 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 inadvertently stimulate NH3 emissions. Quantifying the net effects of these trade-offs requires spatially resolving changes in emissions and associated changes in human exposure to ozone and PM. We introduce an assessment framework for spatially resolving the impacts of NIs on emissions of N2O, NO, and NH3 from U.S. agricultural soils and associated impacts on climate, air quality, and human health. We deploy an agroecosystem model with enhanced capabilities to predict emissions of each gas and the impacts of NIs. A social cost of greenhouse gas is applied to monetize the impacts of N2O on climate. Three reduced-complexity air quality and health models are applied to connect NO and NH3 emissions changes with impacts on ozone and PM and to monetize associated impacts on human health downwind. We first evaluated the model performance in predicting the impacts of NIs on Nr emissions by comparing it with U.S. field measurements. We found that the model could capture the direction of emission changes but underestimated reductions in N2O and overestimated the increases in NH3 emissions at the few sites available for comparisons. National-scale assessment of U.S. agricultural soils in 2011 showed that NIs could reduce N2O and NO emissions for simulated areas that applied ammonia- or urea-based fertilizers by an average of 11 percent (Interquartile (IR): 6 percent to 16 percent) and 16 percent (10 percent to 22 percent), respectively, while stimulating NH3 emissions by 87 percent (51 percent to 117 percent). Impacts are largest in regions with intensive ammonia- and urea-based fertilizer use and 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 recent global meta-analysis: -44 percent for N2O, -24 percent for NO and 20 percent for NH3. Monetized impacts assessments show that the PM-related damages caused by stimulating NH3 emissions greatly outweigh the PM- and ozone-related benefits of reducing NO emissions plus the climate benefits of mitigating N2O under model-based estimates in most regions, and to a lesser extent under meta-analysis-based estimates. Our study highlights the importance of considering multiple pollutants when assessing NIs, and underscores the need to mitigate NH3 emissions. Further field studies and model intercomparisons are needed to bolster the robustness of multi-pollutant assessments.