Nitrogen is a macronutrient essential to all living organisms. It is an integral component of amino acids which are the building blocks of proteins; it forms part of the nitrogenous bases common to DNA and RNA; it helps make up ATP, and it is a major component of the chlorophyll molecule in plants. In essence, life as we know it cannot exist without nitrogen.

Although nitrogen is readily abundant as a gas (it comprises 79 percent of atmospheric gases by volume), most organisms cannot use it in this state. It must be converted to a chemically usable form such as ammonia (NH₃) or nitrate (NO₃) for most plants, and amino acids for all animals. The processes involved in the conversion of nitrogen to its various forms comprise the nitrogen cycle. Of all the nutrient cycles, this is considered the most complex and least well understood scientifically. The...