Interventions In The Nitrogen Cascade To Benefit Human Well-Being and Ecosystems.

The unintentional release of nitrogen (N) to the environment, as a consequence of human demand for food and fuel, has important consequences for human health and ecosystems.   Increases in efficiency of N use could reduce damage associated with the release of N beyond its intended uses in agriculture, transportation and energy generation.  We use existing data on N-associated damages to generate a preliminary assessment of current and future N effects on human health, ecosystems and climate regulation.  We connect the major sources of N release (agricultural N, fossil fuel combustion and sewage) to key drivers of N damages and also discuss the tools available for intervention of N releases.  Based on year 2000 N input information for the conterminous US, preliminary estimates of damages of external N release to human health, ecosystems and climate are $123, $101 and $1.40 billion US dollars, respectively.  Approximately 57% of the total damages are associated with fossil fuel combustion, largely 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 are associated with agricultural N, which includes synthetic fertilizers, crop biological N fixation, and manure application.  The damage cost associated with agriculture is $85.5 billion, largely through harmful effects on aquatic habitat and eutrophication.  In comparison, the 2002-2011 average net farm income for the US was $74 billion, indicating that the beneficial economic impacts of agriculture are slightly less than the damage costs caused by release of this N beyond the farm field.  We also use scenario analysis to conduct an assessment of the marginal impacts of changing inputs by source.  In some areas future N sources to the coastal zone are dominated by sewage (California), some by agriculture (Gulf of Mexico), and others by deposition.  This analysis can assist in identifying the most cost effective or efficient intervention required to address air and water quality goals in these areas.  Continuing the trajectory of reductions in NO_x via the Clean Air Act is expected to decrease the impact of N on human respiratory health.  In order to decrease freshwater and coastal ecosystem damages, effective approaches for reducing agricultural N release to these ecosystems are needed.