Once created, a molecule of reactive nitrogen has a cascading impact on people and ecosystems as it contributes to a number of environmental issues. Molecular terms represent oxidized forms of nitrogen primarily from fossil fuel combustion (such as nitrogen oxides, NO_x), reduced forms of nitrogen primarily from agriculture (such as ammonia, NH₃), and organic forms of nitrogen (N_org) from various processes. NO_y is all nitrogen-containing atmospheric gases that have both nitrogen and oxygen, other than nitrous oxide (N₂O). NH_x is the sum of ammonia (NH₃) and ammonium (NH₄). (Figure source: adapted from EPA 2011;346e77a5-93b9-4c37-bd95-d7db44091a4c Galloway et al. 2003;c713eeb5-4e17-41e2-b65d-da453bbf9f97 with input from USDA. USDA contributors were Adam Chambers and Margaret Walsh).