Integrated Environmental Assessment of Agricultural Emissions of Reactive Nitrogen

Abstract:

Fertilizer-intensive agriculture dramatically boosts crop yields, yet it also imposes a variety of adverse environmental impacts on water, air, and climate. Almost half of added nitrogen is lost to the environment in multiple reactive forms, including the water pollutant nitrate (NO₃^-), the air pollutants ammonia (NH₃) and nitric oxide (NO), and a potent greenhouse gas, nitrous oxide (N₂O). Currently, agriculture has become the leading source of these reactive nitrogen (Nr) emissions. Managing nitrogen in croplands is thus essential to reduce Nr emissions and mitigate their impacts on the environment. Some farming practices could reduce emissions of all forms of Nr species, while others could generate trade-offs among different species. Integrated assessment of agricultural systems can help illuminate the relationships between farming practices and environmental impacts and identify practices that could yield synergies for addressing water, air, and climate concerns.

Previous assessments of agriculture systems using agroecosystem modeling have focused mainly on climate change and water quality. Since agroecosystem modeling cannot track the air pollutants, such assessments fail to estimate the spatiotemporal variation of NO and NH₃ and their contributions to secondary air pollutants, particulate matter (PM) and ozone (O₃). As agriculture has become a leading contributor to PM, it is essential to consider both biogeochemical processes in the ground and photochemical processes in the air. Here we develop an integrated environmental assessment framework of agricultural systems by linking the enhanced agroecosystem modeling called FEST-C and the reduced complexity atmosphere modeling called APEEP. We update the nitrogen cycling schemes in FEST-C modeling to predict Nr emissions across the U.S. on regional scales and estimate how Nr emissions vary with farming practices. The APEEP modeling is used to track the transport of NO and NH₃ and their contributions to ozone and PM. To measure the overall environmental impacts, we monetize the adverse environmental effects based on the damage cost associated with the Nr. This framework enables us to quantify the environmental impacts of agricultural systems in the U.S., and also identify practices that could mitigate adverse environmental impacts.