Future riverine nitrogen export to coastal regions in the United States: prospects for improving water quality.

Nitrogen (N) fluxes generated by an increasing human population have the potential to increase coastal riverine N loading, with implications for areas already degraded by elevated nutrient loads. Here we examine contemporary (year 2005) and future (year 2030) loading of total dissolved N (TDN) in the continental United States using the Nutrient Export from WaterSheds model (NEWS2-TDN). Model-derived TDN estimates compared well with measured export of 29 catchments that represent 65% of land surface area for the continental United States (Nash-Sutcliffe efficiency = 0.83). Future output is based on scenarios that reflect future population growth and "business as usual" (BAU) and "ambitious" (AMB) approaches to nutrient management. Model-derived TDN export was 2.1 Tg N yr in 2005 and 2.2 and 1.6 Tg N yr in 2030 for the BAU and AMB scenarios, respectively. Depending on year and scenario, agriculture supplies 44 to 48% of coastal TDN, atmospheric N deposition supplies 14 to 17%, human sewage supplies 13 to 18%, and background sources supply 21 to 29%. The AMB scenario suggests that reducing nutrient loads to coastal areas will require aggressive actions, including a 25% improvement in agricultural nutrient use efficiency, a 20% reduction in N runoff from croplands, a 30% reduction in ammonia emissions from agriculture, and a 40% reduction in nitrogen oxide emissions from vehicles. Together, these aggressive actions could reduce year 2030 TDN export by 24% from 2005 levels, even with a 20% larger population.