1. Reactive nitrogen (N) is essential to life, making up compounds such as proteins and nucleic acids. Technology now allows us to create N fertilizers, increasing food production. Currently, 50% of N in the human body is synthetic N fertilizer. N also is an important component of many industrial products. Human-created N is critical for our modern society and economy. However, its overabundance in the environment harms human health, alters ecosystem function, and contributes to climate change. Projected changes in climate for the 21 st century may dampen or exacerbate human health and ecosystem effects of overabundant, human-created N. Understanding the interactions between reactive N and climate change is critical. Estimate reactive N inputs across the conterminous United States (US). Identify important reactive N sources across conterminous US watersheds. Project mid-century shifts in riverine N export due to climate change. CC-26: Mapping Loading Rates and Sources of Reactive Nitrogen across the United States suggests Regional Interactions with Climate Change Daniel J. Sobota a,b, Jana E. Compton a,c, Shweta Singh a,d, and Douglas J. Norton e a Western Ecology Division, US Environmental Protection Agency, Corvallis, OR: b Oak Ridge Institute for Science and Education; c National Health and Environmental Effects Research Laboratory; d National Research Council e Office of Water, US Environmental Protection Agency, Washington, DC Objectives Reactive nitrogen is a wicked problem Shifts in riverine N exports due to climate change Reactive nitrogen inputs and sources, circa 2000 Acknowledgements: Funding was provided by Oak Ridge Institute for Science and Education, the National Research Council, and the US Environmental Protection Agency. sobota.dan@epa.gov compton.jana@epa.gov www.whoi.edu Harmful Algal Blooms & Hypoxia Harmful Algal Blooms & Hypoxia Colin Bishop Products & Energy singh.shweta@epa.gov norton.douglas@epa.gov Colin Bishop Enhanced crop production Energy & products Water pollution Air pollution Source data for reactive N inputs Source data for reactive N inputs Changes in riverine N exports due to climate change Methods Air Land Water Humanconsumption Agriculturalproduction Energy consumption Biological N fixation The Nitrogen Cascade Galloway JN, Aber JD, Erisman JW, et al. 2003. The nitrogen cascade. Biosci 53:341-356. New human-created reactive N inputs 8-digit USGS Hydrologic Units National reactive N inputs  Human activities currently put 7.5 times more new reactive N into the US annually than do natural processes alone.  Synthetic fertilizer is clearly the largest national reactive N source. However, atmospheric deposition is important in the East and Southwest, while legumes are important in the West. Largest human-created reactive N sources 8-digit USGS Hydrologic Units National reactive N sources Reactive N sourceSpatial resolutionTime scale Synthetic fertilizerCounty1997-2001 LegumesCounty1997, 2001 Atmospheric deposition36 km grid2002 Industrial productsNational2002 - 2009 ManureCounty1997 SewageCounty2000 Trade exportNational2002 Riverine N export data  Changes in annual temperature and precipitation regimes may reduce or increase riverine N export depending on region. For data references, see: Sobota DJ, Compton JE, Harrison JA. 2013. Reactive nitrogen in US lands and waterways: How certain are we about sources and fluxes? Fron Ecol Environ 11:82-90. Spatial data were reformatted to 8-digit USGS Hydrologic Unit Codes (HUC8s) using the Zonal Statistics Tool in ArcMap 10.0 (ESRI Inc., Redlands, CA). We compiled data from 71 watersheds in the US that described riverine export of annual watershed N inputs (fractional N export): Boyer, EW, Goodale CL, Jaworski NA, Howarth RW. 2002. Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern USA. Biogeochem 57/58: 137-169 Schafer SC, Alber M. 2007. Temperature controls a latitudinal gradient in the proportion of watershed nitrogen exported to coastal ecosystems. Biogeochem 85:333-346. Schafer SC, Hollibaugh JT, Alber M. 2009. Watershed nitrogen input and riverine export on the west coast of the US. Biogeochem 93:219-233. Sobota DJ, Harrison JA, Dahlgren RA. 2009. Influences of climate, hydrology, and land use on input and export of nitrogen in California watersheds. Biogeochem 94:43-62. Mean annual temperature and precipitation were used to predict fractional N export (see equation in climate change panel). Ensemble average data (4.5 km grids) for mean annual temperature and precipitation changes for 2040 – 2059 were downloaded from the National Center for Atmospheric Research ( Ensemble average data (4.5 km grids) for mean annual temperature and precipitation changes for 2040 – 2059 were downloaded from the National Center for Atmospheric Research (https://gisclimatechange.ucar.edu). Ensemble temperature and precipitation data were scaled to HUC8s using ArcMap 10.0. Coefficients for the fractional N export~precipitation and temperature regression equation presented in the climate change panel were applied to HUC8 climate change data. ln(annual riverine export of watershed N inputs) = 1.83+0.0018*Mean Annual Temperature - 0.11*Mean Annual Precipitation n=71; p<0.0001; r 2 =0.66 Projected changes in riverine N export, 2040 -2059 8-digit USGS Hydrologic Units