Increasing importance of deposition of reduced nitrogen in the United States

Human activities have greatly increased emissions of reactive forms of nitrogen to the atmosphere. This perturbation to the nitrogen cycle has produced large increases of nitrogen deposition to sensitive ecosystems. Over recent decades, attention has focused on wet and dry deposition of nitrate stemming from fossil fuel combustion emissions of nitrogen oxides. Successful decreases in nitrogen oxides emissions in the United States have substantially decreased nitrate deposition. By contrast, emissions of ammonia, an unregulated air pollutant, and resulting deposition of ammonium have grown. Expanded observations demonstrate that deposition of reactive nitrogen in the United States has shifted from a nitrate-dominated to an ammonium-dominated condition. Recognition of this shift is critical to formulating effective future policies to protect ecosystems from excess nitrogen deposition.