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Nitrogen cycling involves fixation by bacteria, nitrification, uptake by plants, and mineralization. Nitrogen is often the limiting nutrient in terrestrial ecosystems because atmospheric N₂ is unavailable without fixation. Legume-Rhizobium symbiosis fixes atmospheric nitrogen; excess nitrogen from fertilizers causes eutrophication in aquatic systems.
From your study of biogeochemical cycles, you know that elements move between living organisms and the physical environment in continuous loops. Nitrogen is one of the most important of these cycles — and one of the most counterintuitive. The atmosphere is 78% nitrogen gas (N₂), so it might seem strange that nitrogen is the nutrient most often limiting plant growth in terrestrial ecosystems. The problem is that the triple bond in N₂ is extraordinarily strong, making atmospheric nitrogen essentially inert to most organisms. Plants cannot use N₂ directly; they need nitrogen in reactive forms like ammonium (NH₄⁺) or nitrate (NO₃⁻).
Nitrogen fixation is the process that converts atmospheric N₂ into biologically usable ammonium, and only certain prokaryotes can do it. The most ecologically important fixers are Rhizobium bacteria living in root nodules of legumes (beans, clover, alfalfa) and free-living soil bacteria like *Azotobacter*. These organisms use the enzyme nitrogenase to break the triple bond — an energetically expensive reaction requiring 16 ATP per molecule of N₂ fixed. Cyanobacteria fix nitrogen in aquatic systems and in symbiosis with some plants. Lightning also fixes small amounts of nitrogen abiotically, but biological fixation accounts for the vast majority of new reactive nitrogen entering ecosystems.
Once nitrogen enters the biological pool as ammonium, it moves through several transformations. Nitrification — carried out by specialized soil bacteria — converts ammonium to nitrite and then nitrate, the form most readily absorbed by plant roots. Plants incorporate this nitrogen into amino acids and proteins, which move through food webs as animals eat plants and each other. When organisms die or excrete waste, decomposers break down organic nitrogen back to ammonium through ammonification (also called mineralization), completing the cycle. Denitrification closes the loop by converting nitrate back to N₂ gas, returning nitrogen to the atmosphere. Each step is performed by different microbial specialists, making the nitrogen cycle a relay race among microorganisms.
The practical significance is enormous. Because nitrogen fixation is slow and energetically costly, nitrogen availability limits productivity in most natural terrestrial ecosystems — adding nitrogen fertilizer dramatically increases plant growth, which is why the Haber-Bosch process (industrial nitrogen fixation) revolutionized agriculture. But excess reactive nitrogen from fertilizer runoff enters waterways and causes eutrophication: nitrogen-fueled algal blooms deplete dissolved oxygen when they decompose, creating dead zones in lakes, estuaries, and coastal oceans. Humans now fix more nitrogen industrially than all natural processes combined, fundamentally altering the global nitrogen cycle with consequences for water quality, biodiversity, and atmospheric chemistry.