Nitrogen is an extremely abundant element in the Earth’s atmosphere. In the form of the N2 molecule, nitrogen is harmless, making up 78 per cent of every breath we take. The two nitrogen atoms are held together by a strong triple bond (N N), making it extremely stable and chemically unreactive. The planet benefits because N2 allows a safe atmosphere in which life can flourish, while avoiding the flammable consequences of too much oxygen. The environmental interest in nitrogen focuses on the conversion of N2 into other chemically reactive forms. For simplicity, scientists refer to all other nitrogen forms as “fixed” or “reactive nitrogen” (Nr).