Capturing the externalities: National and watershed scale damages from release of reactive nitrogen beyond the farm, factory, tailpipe and table

Human demand for food, fuel, and industrial products results in the release of 61% of the newly fixed anthropogenic N to the environment in the US each year. This 15.8 Tg N yr^-1 input to air, land and water has important social, economic and environmental consequences, yet little research clearly links this N release to the full suite of effects. Here we connect the biogeochemical fluxes of N with existing data on N-associated damages in order to quantify the externalities of N release related to human health, ecosystems and climate regulation for the US at national and watershed scales. Release of N to the environment was estimated circa 2000 with models describing N inputs by source, nutrient uptake efficiency, leaching losses, and gaseous emissions at the scale of 8-digit US Geologic Survey Hydrologic Unit Codes (HUC8s). Potential damages or benefits of anthropogenic N leaked to the environment were calculated by scaling specific N fluxes with the costs associated with human health, agriculture, ecosystems, and the climate system. For the US, annual damage costs of anthropogenic N leaked to the environment in 2000 totaled $289 billion USD. Approximately 57% of the total damages were associated with fossil fuel combustion, driven by the human respiratory health impacts of NO_x as a precursor of ozone and a component of particulates. Another 37% of the damage costs were associated with agricultural N. Damages associated with agriculture were $85.5 billion, largely through eutrophication and harmful effects on aquatic habitat. Through aggressive but tangible improvements in atmospheric emissions, agricultural N use and wastewater treatment, we could reduce N export to the coast by nearly 25% within 30 years. These improvements would reduce the externalities associated with the leakage of N beyond its intended uses in agriculture, transportation and energy with minimal impact to these sectors dependent on anthropogenic N fixation.