The Nitrogen Cycle

Air is 78 percent nitrogen gas, N2, but that nitrogen is locked up tight. Two nitrogen atoms share a triple bond, one of the strongest bonds in chemistry, and breaking it takes huge amounts of energy. Plants have no enzyme for that job, so nitrogen sits in the air, useless to life, until something fixes it.

Fixation is the unlock step. Certain bacteria, some free-living in soil, some living inside root nodules of legumes like beans and peas, carry an enzyme called nitrogenase that can snap the triple bond and combine N2 with hydrogen to make ammonium, NH4+. From there, nitrifying bacteria run nitrification, oxidizing ammonium first to nitrite then to nitrate, NO3-. Plant roots absorb ammonium or nitrate and build it into proteins and DNA, a step called assimilation. Animals get their nitrogen by eating plants or other animals. When organisms die or excrete waste, decomposers break that organic nitrogen back down to ammonium, feeding nitrification again. Finally, denitrifying bacteria, working in oxygen-poor soil or water, convert nitrate back into N2 gas, releasing it to the atmosphere and closing the loop.

Lightning also fixes nitrogen, using heat to force N2 to react with oxygen, but lightning strikes are rare and localized compared to the constant, worldwide work of bacteria, so biology supplies far more fixed nitrogen than any spark ever could. This cycle matters beyond soil chemistry: when fertilizer adds extra fixed nitrogen to a watershed, it feeds algae instead of crops once it washes into lakes, setting up the oxygen crashes covered next under eutrophication.