Reactive nitrogen ("Nr"), also known as fixed nitrogen, refers to all forms of nitrogen present in the environment except for molecular nitrogen ().  While nitrogen is an essential element for life on Earth, molecular nitrogen is comparatively unreactive, and must be converted to other chemical forms via nitrogen fixation before it can be used for growth.  Common Nr species include nitrogen oxides (), ammonia (), nitrous oxide (), as well as the anion nitrate ().
Biologically, nitrogen is "fixed" mainly by the microbes (eg., Bacteria and Archaea) of the soil that fix  into mainly  but also other species. Legumes, a type of plant in the Fabacae family, are symbionts to some of these microbes that fix .  is a building block to Amino acids and proteins amongst other things essential for life.  However, just over half of all reactive nitrogen entering the biosphere is attributable to anthropogenic activity such as industrial fertilizer production.  While reactive nitrogen is eventually converted back into molecular nitrogen via denitrification, an excess of reactive nitrogen can lead to problems such as eutrophication in marine ecosystems.