The Nitrogen Cycle

Nitrogen exists in the environment across a spectrum of oxidation states, from −3 (in ammonia, NH₃, and organic amines) through 0 (in N₂ gas) up to +5 (in nitrate, NO₃⁻). Biological molecules — amino acids, nucleotides, chlorophyll — use nitrogen almost exclusively in its reduced, −3 form. The nitrogen cycle is fundamentally a sequence of oxidation and reduction reactions that shuttle nitrogen atoms between these states, and each shuttle step is catalyzed by a different, often taxonomically narrow, group of microorganisms. Five processes define the cycle. Nitrogen fixation reduces inert N₂ gas to biologically usable ammonia (NH₃/NH₄⁺), a reaction performed only by certain bacteria and archaea possessing the enzyme nitrogenase. Assimilation is the uptake of ammonium or nitrate by plants and microbes and its incorporation into amino acids and nucleic acids — this step, unlike the others, is performed by all life, not just prokaryotes. Ammonification (mineralization) is the release of ammonium from organic nitrogen during decomposition, carried out by the same heterotrophic bacteria and fungi responsible for general decay. Nitrification is the sequential oxidation of ammonium to nitrite and then nitrate, performed by chemoautotrophic bacteria and archaea that harvest energy from this oxidation. Denitrification is the reduction of nitrate back to gaseous nitrogen (mostly N₂, with N₂O and NO as intermediates), performed by facultatively anaerobic bacteria that use nitrate as an alternative electron acceptor when oxygen is scarce. Crucially, only fixation and denitrification cross the gaseous/dissolved boundary — they are the cycle's "doors" between the atmosphere and the biosphere. Everything else (ammonification, nitrification, assimilation) simply recycles nitrogen among dissolved and organic pools within an ecosystem. Because the doors are narrow — biological nitrogen fixation is metabolically expensive and denitrification requires anoxic microsites — nitrogen tends to be retained and intensively recycled within ecosystems rather than lost, and its supply commonly limits net primary production, a theme picked up directly in Nutrient Limitation and Eutrophication.

Step 1 — Nitrogen fixation. The enzyme complex nitrogenase catalyzes the reaction N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pi. Nitrogenase has two components: the iron-molybdenum protein (MoFe protein, or dinitrogenase) that binds and reduces N₂ at a cofactor called the FeMo-co cluster, and the iron protein (Fe protein, or dinitrogenase reductase) that delivers electrons to it one at a time, consuming two ATP per electron transferred. Because nitrogenase is irreversibly inactivated by oxygen, fixing organisms must protect it: free-living cyanobacteria sequester the enzyme in thick-walled cells called heterocysts; symbiotic rhizobia inside legume root nodules are shielded by the host protein leghemoglobin, which mops up O₂ while still allowing enough diffusion for respiration. Nitrogen fixers include free-living soil bacteria (Azotobacter), symbiotic rhizobia in legume nodules, actinomycete symbionts (Frankia) in non-legume trees like alder, and cyanobacteria (Anabaena, Nostoc) both free-living and in symbiosis with lichens and the water fern Azolla.

Step 2 — Assimilation. Plants take up NH₄⁺ or NO₃⁻ through root transporters; NO₃⁻ must first be reduced back to NH₄⁺ inside the plant via nitrate reductase and nitrite reductase, costing additional ATP and reducing power — one reason plants preferentially take up ammonium where available. The ammonium is then incorporated into glutamate via the glutamine synthetase/glutamate synthase (GS-GOGAT) pathway, generating the amino acid backbone from which all other nitrogenous biomolecules are built.

Step 3 — Ammonification. When organisms die, or when animals excrete nitrogenous waste (urea, uric acid, ammonia itself), decomposer bacteria and fungi break down the organic nitrogen and release it as ammonium — the same detrital pathway covered under Decomposition and Detritus. This step returns fixed nitrogen to the soil solution in a form nitrifiers and plants can both use.

Step 4 — Nitrification. This is a two-stage oxidation carried out by distinct microbial guilds. Ammonia-oxidizing bacteria and archaea (AOB/AOA, e.g. Nitrosomonas, and the archaeal group Thaumarchaeota) oxidize NH₄⁺ to nitrite (NO₂⁻) using the enzyme ammonia monooxygenase, harvesting a small amount of energy. A second, unrelated group of nitrite-oxidizing bacteria (NOB, e.g. Nitrobacter, Nitrospira) then oxidizes nitrite to nitrate (NO₃⁻) via nitrite oxidoreductase. Both steps require oxygen — nitrification is an aerobic process — and both groups are chemoautotrophs, fixing their own carbon from CO₂ using the energy released by nitrogen oxidation rather than from sunlight or organic matter.

Step 5 — Denitrification. In anaerobic or low-oxygen microsites (waterlogged soils, wetland sediments, the interior of soil aggregates), facultative anaerobic bacteria (Pseudomonas, Paracoccus, and others) use nitrate as a terminal electron acceptor in place of oxygen, running a stepwise reduction: NO₃⁻ → NO₂⁻ → NO → N₂O → N₂. Each step is catalyzed by a distinct reductase enzyme (nitrate reductase, nitrite reductase, nitric oxide reductase, nitrous oxide reductase). The end product, N₂ gas, escapes to the atmosphere, completing the cycle and balancing fixation over geological time.

The two nitrification steps deserve special emphasis because they illustrate a general principle in microbial ecology: metabolic pathways that look like a single continuous reaction to a chemist are often split between organisms that never share the whole pathway. Neither Nitrosomonas nor Nitrobacter can complete the ammonia-to-nitrate conversion alone; each survives only because the other exists nearby to consume its waste product, a form of syntrophy.

Nitrogenase and the discovery of nitrogen fixation's cost. The basic phenomenon of biological nitrogen fixation was demonstrated in the 1880s by Hermann Hellriegel and Hermann Wilfarth, who grew legumes in sterilized versus unsterilized sand and showed that only plants with root nodules — and only when the sand contained the right soil bacteria — could grow without added nitrogen fertilizer, gaining dry weight and nitrogen content far beyond what the seed alone supplied. Martinus Beijerinck then isolated the causative bacterium, naming it Bacillus radicicola (later reclassified as Rhizobium), establishing the symbiosis as a specific, transmissible microbial trait rather than a property of the plant itself. The biochemical mechanism came much later. In the 1960s, Robert Burris and colleagues used the acetylene reduction assay: nitrogenase, it turns out, will also reduce acetylene (C₂H₂) to ethylene (C₂H₄) — a side reaction that is easy to measure by gas chromatography and became the standard proxy for nitrogenase activity in field and lab studies for decades. Structural biology later resolved the enzyme itself: X-ray crystallography of nitrogenase (Kim and Rees, 1992, and subsequent work) revealed the FeMo-cofactor's exact composition — 7 iron atoms, 9 sulfur atoms, 1 molybdenum atom, and a central light atom later identified as carbon — explaining why the enzyme is so oxygen-sensitive and so energetically expensive: each N₂ reduced consumes roughly 16 ATP, a cost that helps explain why nitrogen fixation is not more widespread among plants despite the obvious advantage of self-sufficiency.

Nitrification as a distinct biological process — Winogradsky's enrichment cultures. In the 1890s, Sergei Winogradsky set out to isolate the bacteria responsible for converting ammonia to nitrate in soil, a transformation agricultural chemists had already documented but could not explain mechanistically. Using an entirely inorganic (mineral salts) culture medium with ammonium as the sole energy source and no organic carbon at all, Winogradsky isolated bacteria that grew and oxidized ammonia to nitrite purely chemoautotrophically — proving, against the prevailing assumption that all bacteria needed organic carbon, that some microbes derive their energy entirely from inorganic chemical oxidation. He then isolated a second, physiologically distinct organism that oxidized nitrite to nitrate but could not touch ammonia at all. This was the first clean demonstration that nitrification is a two-step process carried out by two separate microbial specialists — a finding confirmed and refined by every subsequent culture and, a century later, by 16S rRNA gene sequencing and metagenomics that placed ammonia-oxidizing archaea as major, previously unrecognized players in the first step (Könneke et al., 2005, cultured the first ammonia-oxidizing archaeon, Nitrosopumilus maritimus).

Tracing nitrogen flow with ¹⁵N. Because nitrogen has a stable, non-radioactive heavy isotope (¹⁵N, versus the common ¹⁴N), ecologists can label a fertilizer or substrate with enriched ¹⁵N and track exactly where those atoms go — into plant tissue, into microbial biomass, into denitrified gas — using mass spectrometry. Classic tracer experiments (for example whole-ecosystem ¹⁵N tracer additions at the Hubbard Brook Experimental Forest and in Arctic tundra streams) showed that added inorganic nitrogen is assimilated by microbes and plants within meters to tens of meters of its point source, revealing just how tightly ecosystems retain nitrogen and how quickly biological demand outstrips supply — direct, quantitative evidence for nitrogen limitation, the theme that leads into Nutrient Limitation and Eutrophication.

Denitrification enzymes and the discovery of N₂O as an intermediate. Early 20th-century soil chemists observed that nitrate disappeared from waterlogged soils and gaseous nitrogen appeared, but the pathway was only worked out through the isolation of the individual reductase enzymes in denitrifying bacteria (from the 1950s onward) and confirmed with ¹⁵N-labeled nitrate: labeled N₂ gas recovered from incubated anoxic soils confirmed nitrate, not atmospheric N₂, as the gas's immediate source. The identification of nitrous oxide (N₂O) as an obligate intermediate — rather than a side product — became critically important once N₂O was recognized as both a potent greenhouse gas and a stratospheric ozone-depleting agent, linking a microbiological detail directly to global atmospheric chemistry.

Example 1 — Nitrogen budget of a fertilized maize field. A farmer applies ammonium nitrate fertilizer supplying 200 kg N/ha to a maize field. Suppose field measurements show:

• 40% of applied N is taken up by the crop (assimilation)
• 25% is immobilized into soil organic matter by microbes
• 15% is lost via denitrification to the atmosphere
• 10% leaches as nitrate into groundwater
• 10% is lost via ammonia volatilization

Compute the mass of nitrogen (kg/ha) lost from the field in forms unavailable to the crop this season, and identify which loss pathway is environmentally most concerning downstream. Reasoning: Total applied = 200 kg N/ha. "Unavailable to the crop" excludes the 40% assimilated (80 kg N/ha); everything else is either immobilized (temporarily unavailable, but not lost from the field) or genuinely lost from the system.

• Immobilized: 0.25 × 200 = 50 kg N/ha (retained in soil, not "lost")
• Denitrified: 0.15 × 200 = 30 kg N/ha (lost to atmosphere as N₂/N₂O)
• Leached: 0.10 × 200 = 20 kg N/ha (lost to groundwater as NO₃⁻)
• Volatilized: 0.10 × 200 = 20 kg N/ha (lost to atmosphere as NH₃)

Total lost from the field-plant-soil system = 30 + 20 + 20 = 70 kg N/ha, i.e., 35% of what was applied. Of these, nitrate leaching (20 kg N/ha) is the pathway of greatest downstream ecological concern, because dissolved NO₃⁻ moves with groundwater into streams, lakes, and estuaries where it can drive algal blooms and hypoxia — the mechanism developed fully in Nutrient Limitation and Eutrophication. Denitrification, while also a "loss," is in one sense the more benign pathway ecologically (it returns nitrogen to an inert atmospheric form), though the N₂O fraction of that 30 kg contributes to greenhouse warming.

Example 2 — Comparing legume and non-legume nitrogen economies. A legume crop (soybean) fixes its own nitrogen via Rhizobium symbiosis at a typical rate of 100–150 kg N/ha/season, while requiring roughly 6–8 g of root-nodule biomass and a photosynthate cost estimated at about 12 g of carbon fixed by the plant for every gram of nitrogen fixed (covering the ~16 ATP-per-N₂ nitrogenase cost plus nodule maintenance). A non-fixing cereal (wheat) must instead obtain its ~150–200 kg N/ha/season entirely from soil mineralization or fertilizer. Question: If a farmer rotates soybean with wheat instead of continuously cropping wheat, roughly how much fertilizer nitrogen can be saved in the wheat year, and why is the benefit not simply "zero fertilizer needed forever after"? Reasoning: Soybean residue (roots, nodules, fallen leaves) left in the field after harvest decomposes via ammonification — the Decomposition and Detritus pathway — releasing perhaps 30–60 kg N/ha of the fixed nitrogen into the soil ammonium/nitrate pool for the following wheat crop (soybean also exports a large share of its fixed N in the harvested, nitrogen-rich seed itself, so not all 100–150 kg N/ha stays in the field). A following wheat crop might therefore need 50–70 kg N/ha less fertilizer than it would after continuous wheat. The benefit is not permanent or complete because: (1) most of the fixed nitrogen leaves the field in harvested soybean seed rather than remaining as residue; (2) mineralized nitrogen not immediately taken up is itself subject to nitrification and then leaching or denitrification losses, just like fertilizer nitrogen; and (3) wheat's own nitrogen demand in subsequent years still exceeds what one rotation cycle of residue can supply, so rotation reduces but does not eliminate the need for external nitrogen input. This illustrates why the "door" of biological fixation, however valuable, does not by itself decouple agriculture from the broader nitrogen cycle's leaks.

Sergei Winogradsky worked at a time when the dominant view, associated with Louis Pasteur's tradition, held that essentially all bacteria required organic carbon compounds to grow — heterotrophy was assumed to be universal. Working first in Strasbourg and then in St. Petersburg through the 1880s–90s, Winogradsky built simple mineral-salts media containing no organic carbon at all, added ammonium as the only potential energy source, and inoculated with soil. Bacteria grew anyway, and as they grew, they oxidized the ammonium to nitrite. This was direct, reproducible proof of chemoautotrophy — life powered by inorganic chemical energy rather than sunlight or organic carbon — a concept as foundational to microbiology as photosynthesis is to plant biology. He went on to isolate a second organism converting nitrite to nitrate, definitively splitting nitrification into two microbial steps, and later did comparable pioneering work on sulfur-oxidizing bacteria, establishing the broader principle of chemolithotrophy across multiple elemental cycles.

Agriculture and the Haber–Bosch legacy. Industrial ammonia synthesis (the Haber–Bosch process, developed 1909–1913) artificially replicates nitrogen fixation at industrial scale by combining N₂ and H₂ under high heat and pressure with an iron catalyst, and now underlies the fertilizer supply that sustains roughly half of global food production. This single chemical process has arguably done more to enable human population growth than any other twentieth-century technology, but it has also roughly doubled the total amount of nitrogen entering the biologically active cycle each year compared to pre-industrial times.

Eutrophication and dead zones. Nitrate that escapes crop uptake and denitrification leaches into rivers and coastal waters, fueling algal blooms whose decomposition consumes dissolved oxygen — the mechanism responsible for hypoxic "dead zones" such as the one recurring each summer in the Gulf of Mexico, fed by nitrogen runoff from the Mississippi River basin. This connects the nitrogen cycle directly forward to Nutrient Limitation and Eutrophication.

Greenhouse gas and ozone chemistry. Incomplete denitrification releases nitrous oxide (N₂O), a greenhouse gas roughly 270–300 times more potent than CO₂ per molecule over a century timescale, and also a major contributor to stratospheric ozone depletion — making agricultural soil management a genuine climate policy lever, not merely an agronomic detail.

Biotechnology and engineering nitrogen fixation. Researchers are actively working to extend nitrogenase-based fixation, or nodulation-like symbioses, into cereal crops such as wheat, rice, and maize — none of which naturally host nitrogen-fixing symbionts — with the goal of reducing fertilizer dependence, cost, and runoff. Approaches range from engineering nitrogenase gene clusters into non-symbiotic microbes that colonize cereal roots, to attempts at transferring nodulation signaling pathways from legumes.

Wastewater treatment. Municipal and industrial wastewater treatment plants deliberately engineer nitrification–denitrification sequences (aerobic tanks followed by anoxic tanks) to strip nitrogen from sewage before discharge, directly applying the same two microbial guilds — ammonia oxidizers, nitrite oxidizers, and denitrifiers — that Winogradsky first isolated, now harnessed at engineering scale to prevent eutrophication of receiving waters.

Conservation and restoration ecology. Restoring nitrogen-fixing pioneer species (alder, lupine, certain legumes) is a standard technique in reclaiming degraded or post-glacial/post-volcanic soils, since primary succession on nitrogen-poor substrates is often bottlenecked precisely by the absence of a fixation pathway until such colonizers arrive.

Ask the tutor: How do wetland and riparian buffer restorations use the denitrification step of this cycle to reduce nitrogen pollution before it reaches lakes and coastal waters?