source of human-produced fixed nitrogen in the terrestrial ecosystem. ammonia is a required precursor to fertilizers, explosives, and other products. the haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °c), which are routine conditions for industrial catalysis. this process uses natural gas as a hydrogen source and air as a nitrogen source. the ammonia product has resulted in an intensification of nitrogen fertilizer globally and is credited with supporting the expansion of the human population from around 2 billion in the early 20th century to roughly 8 billion people now. homogeneous catalysis much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of lowering energy requirements. however, such research has thus far failed to approach the efficiency and ease of the haber process. many compounds react with atmospheric nitrogen to give dinitrogen complexes. the first dinitrogen complex to be reported was ru(nh3)5(n2)2+. some soluble complexes do catalyze nitrogen fixation. lightning lightning heats the air around it breaking the bonds of n2 starting the formation of nitrous acid. nitrogen can be fixed by lightning converting nitrogen gas (n2) and oxygen gas (o2) in the atmosphere into nox (nitrogen oxides). the n2 molecule is highly stable and nonreactive due to the triple bond between the nitrogen atoms. lightning produces enough energy and heat to break this bond allowing nitrogen atoms to react with oxygen, forming nox. these compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form no2, which in turn reacts with water to produce hno2 (nitrous acid) or hno3 (nitric acid). when these acids seep into the soil, they make no3 (nitrate), which is of use to plants.