A Nitrogen Inventory of Major Water Regions Across the USA as a Benchmark for Future Progress in Mitigating Nitrogen Pollution

We present a contemporary inventory of reactive nitrogen (Nr) inputs and, air, and surface waters throughout major water regions in the United States. Inputs of Nr to the nation and the world have been increasing, largely due to human activities associated with food production and energy consumption via the combustion of fossil fuels and biofuels. Despite the obvious essential benefits of a plentiful supply of food and energy, the adverse consequences associated with the accumulation of Nr in the environment are large. Most of the Nr created by anthropogenic activities is released to the environment, often with unintended negative consequences. The greater the inputs of Nr to the landscape, the greater the potential for negative effects, caused by greenhouse gas production, ground level ozone, acid deposition, and Nr overload that can contribute to climate change, degradation of soils and vegetation, acidification of surface waters, coastal eutrophication, hypoxia and habitat loss. Here, we present a consistent accounting method for quantifying Nr sources and transport that was used in our inventory, and discuss associated data needs for tallying Nr inputs at regional scales. The inventory is a necessary tool for exploring the role of Nr contributed to the environment from various sources (e.g., from fertilizers, manure, biological fixation, human waste, atmospheric deposition) and from various industrial sectors (e.g., from agriculture, transportation, electricity generation). Agriculture and use of fertilizers to produce food, feed, and fiber (including bioenergy and biological nitrogen fixation) and combustion of fossil fuels are the largest sources of Nr released into the environment in the USA. Our inventory can be used as a benchmark of the current Nr situation against which future progress can be assessed in varying regions of the country, amidst changing Nr inputs and implementation of policy and management strategies to mitigate Nr pollution.