Reactive nitrogen

Reactive nitrogen ("Nr") is a term used for a variety of nitrogen compounds that support growth directly or indirectly. Representative chemical species of Nr include the gases nitrogen oxides (NO_x), ammonia (NH₃), nitrous oxide (N₂O), as well as the anion nitrate (NO₃⁻). Most of these species are used in intensive farming . Nitrogen makes up about 75% of the atmosphere in an unreactive ("unfixed") form N₂. Reactive nitrogen is "fixed" mainly by the microbes (eg., Bacteria and Archaea) of the soil that fix N2 into mainly NH3 but also other species. Legumes, a type of plant in the Fabacae family, are symbiants to some of these microbes that fix N2. NH3 is a building block to Amino acids and proteins amongst others things essential for life. However, in 1915 the Haber-Bosch process was invented. It uses fossil fuels to capture N2 and along with Hydrogen (H2) heats them up to 500 degree Celsius and under 250 bars of pressure fixes N2 that is then converted into fertilizers like Urea. This urea increases crop yields and has freed thousands of people from hunger. However, agriculture is under pressure to feed an extra 2 billion mouths by 2050 that they are using too much N fertilizers and it is polluting: leaching from soils into waterways, depleting oxygen (eutrophication), known as dead zones, in fresh and marine waters.[1] Nr is remains in the nitrogen cycle but undergoes Denitrification by mibrobes.

A schematic representing the marine nitrogen cycle.

Eutrophication, which is often caused by overabundance of reactive nitrogen, is apparent as increased turbidity in the northern part of the Caspian Sea, imaged from orbit.