The nitrogen cycle is a set of microbial and chemical transformations that convert atmospheric nitrogen gas into forms living organisms can use, then eventually return it to the atmosphere. What makes it remarkable is that no single organism runs the whole show. Instead, dozens of specialized microbes handle different steps, passing nitrogen-containing molecules from one to the next in a relay that spans soils, oceans, freshwater, and even the tissues of plants. Some of these steps have been understood for over a century; others were discovered only in the last two decades and are still reshaping how scientists think about nitrogen’s movement through the planet.
Nitrogen Fixation Is the Starting Gate
Atmospheric nitrogen (Nâ‚‚) is extraordinarily stable. The triple bond holding its two atoms together makes it nearly inert, which is why roughly 78% of the air is nitrogen yet most organisms cannot use it directly. Nitrogen fixation is the process that cracks that bond, converting Nâ‚‚ into ammonia (NH₃), which cells can absorb. In biological systems, the enzyme nitrogenase does this work. It requires a large energy input and accumulates reducing power in stages, storing it as metal-bridging hydrides within an iron-molybdenum cofactor. Spectroscopic analysis has shown that in the critical intermediate state, two iron-bridging hydride fragments hold reducing equivalents in a way that resists premature loss as hydrogen gas, allowing the enzyme to build up enough energy to finally split Nâ‚‚.1Chemical Reviews. Mechanism of Nitrogen Fixation by Nitrogenase: The Next Stage – Section: E4: The “Janus Intermediate”
Nitrogenase is ancient. Phylogenetic reconstructions suggest it evolved from maturase-like predecessor proteins that helped assemble metal cofactors but could not themselves fix nitrogen.2Genome Biology and Evolution. Reconstruction of Nitrogenase Predecessors Suggests Origin from Maturase-Like Proteins Structural studies tracing the enzyme’s history over billions of years show that while its core architecture has stayed conserved, it has picked up modular features aligned with major environmental shifts on Earth, from an oxygen-free atmosphere to the oxygen-rich one we have today.3eLife. Structural evolution of nitrogenase over 3 billion years The enzyme’s extreme sensitivity to oxygen is likely a relic of those early anaerobic origins, and organisms that fix nitrogen today have evolved elaborate ways to keep oxygen away from their nitrogenase.
Who Does the Fixing
The best-known nitrogen fixers are rhizobium bacteria living inside root nodules of legumes like soybeans, clover, and peas. This partnership is triggered by a bacterial signal molecule called Nod factor, which activates a signaling pathway in the plant that is evolutionarily much older than the legume-rhizobium relationship itself. That pathway was originally used for mycorrhizal fungal symbiosis, and rhizobium bacteria essentially co-opted it to establish their own endosymbiotic interaction.4PubMed Central. Evolutionary origin of rhizobium Nod factor signaling The plant provides carbon from photosynthesis; the bacteria provide fixed nitrogen. It is one of the tidiest exchanges in biology.
But symbiotic fixation in legume nodules is only part of the picture. Free-living nitrogen fixers, collectively called diazotrophs, are globally widespread in cropland soils and represent a key natural source of nitrogen in ecosystems that lack legumes.5PubMed Central. Nitrogen Fixing Azotobacter Species as Potential Soil Biological Enhancers for Crop Nutrition and Yield Stability Species of Azotobacter, for instance, live independently in the soil and fix nitrogen without needing a plant host at all. Then there are endophytic nitrogen fixers, bacteria that live inside plant tissues without forming nodules. Researchers have isolated endophytic nitrogen-fixing bacteria from the leaves, stems, and roots of sugarcane plants, confirming nitrogenase activity in all isolates.6PubMed Central. Isolation and Characterisation of Endophytic Nitrogen Fixing Bacteria in Sugarcane These endophytes are particularly interesting for crops like sugarcane and rice that are not legumes but still benefit from biological nitrogen input.
Nitrification Turns Ammonia Into Nitrate
Once ammonia enters the soil or water, a different set of microbes takes over. Nitrification is a two-step oxidation process. First, ammonia-oxidizing organisms convert ammonia to nitrite. Then nitrite-oxidizing bacteria convert nitrite to nitrate. This matters because nitrate is the form of nitrogen most readily taken up by plant roots, and it is also the form most vulnerable to being lost from ecosystems through leaching or denitrification.
The ammonia-oxidizing step was long thought to belong exclusively to a few groups of bacteria. Then researchers discovered ammonia-oxidizing archaea, and more recently, “comammox” bacteria, complete ammonia oxidizers that perform both steps of nitrification in a single organism. These comammox organisms belong to the genus Nitrospira, previously known only as nitrite oxidizers, and their genomes encode the full enzymatic toolkit for oxidizing ammonia all the way to nitrate.7PubMed Central. Complete nitrification by Nitrospira bacteria The discovery of comammox challenged the long-standing assumption that nitrification always requires a handoff between two different microbial groups. The cultivation and genome sequencing of these ammonia-oxidizing bacteria, archaea, and comammox organisms has also revealed new mechanisms for tolerating low pH and low-oxygen conditions, and mechanisms for producing nitrous oxide, a potent greenhouse gas.8PubMed. Insights into the physiology of ammonia-oxidizing microorganisms
Among the nitrite oxidizers, the genus Nitrospira and the genus Nitrobacter have distinctly different ecological strategies. Nitrospira species are adapted to environments with very low nitrite concentrations, while Nitrobacter thrives when nitrite is abundant and can sustain much higher maximum oxidation rates. A third group, “Candidatus Nitrotoga arctica,” appears to compete well in cold habitats.9PubMed Central. Comparison of oxidation kinetics of nitrite-oxidizing bacteria: nitrite availability as a key factor in niche differentiation Genomic analysis of Nitrospira has shown that these bacteria differ dramatically from other known nitrite oxidizers in their key enzymes, respiratory chains, and carbon fixation pathways, suggesting that the ability to oxidize nitrite evolved independently more than once.10PubMed Central. A Nitrospira metagenome illuminates the physiology and evolution of globally important nitrite-oxidizing bacteria
Ammonification Recycles Organic Nitrogen
Living things build nitrogen into proteins, nucleic acids, and other organic molecules. When organisms die or excrete waste, that organic nitrogen needs to be broken back down before it can re-enter the cycle. Ammonification, also called nitrogen mineralization, is the microbial decomposition of organic nitrogen compounds into ammonia. This step is what keeps the nitrogen cycle from being a one-way street: it regenerates the ammonia that nitrifiers then oxidize and that plants can directly absorb. Ammonification and nitrification together are the two primary microbial processes controlling how much ammonium is available in soil for plant growth.11Functional Ecology. Soil gross N ammonification and nitrification from tropical to temperate forests in eastern China
The rates of these processes vary with climate and latitude. In forests across China, for example, the abundance of microbial nitrogen-cycling genes for ammonification, nitrification, and denitrification was significantly higher in subtropical forests than in temperate forests, and this pattern tracked with shifts in the dominant microbial communities.12PubMed Central. Soil Nitrogen Mineralization Is Driven by Functional Microbiomes Across a North–South Forest in China Warmer, wetter conditions generally speed up organic matter decomposition and nitrogen turnover.
Denitrification and the Return to the Atmosphere
Denitrification is the process that closes the loop, returning fixed nitrogen to the atmosphere as Nâ‚‚ gas. It is an anaerobic process used by some bacteria for energy generation, carried out in four enzymatic steps: nitrate is reduced to nitrite, then to nitric oxide, then to nitrous oxide, and finally to dinitrogen.13PubMed. Metalloenzymes of the denitrification pathway Each step involves a different metalloenzyme and a different electron donor.14PubMed. Structural aspects of denitrifying enzymes Denitrification is the main mechanism for permanent nitrogen removal in soils and aquatic sediments. It is why waterlogged soils tend to lose nitrogen, and why wetlands are effective at stripping nitrogen from agricultural runoff.
One concern with denitrification is that it does not always run to completion. When the final step stalls, the end product is nitrous oxide (Nâ‚‚O) rather than harmless Nâ‚‚. Nitrous oxide is a greenhouse gas roughly 270 times more potent than carbon dioxide over a century timescale. A related process called nitrifier denitrification also produces nitrous oxide: ammonia-oxidizing bacteria can reduce nitrite through nitric oxide and nitrous oxide all the way to Nâ‚‚, but the nitrous oxide intermediate can escape.15Soil Biology and Biochemistry. Role of nitrifier denitrification in the production of nitrous oxide – Section: What is nitrifier denitrification? Understanding what controls these incomplete reactions is a major research priority for climate science.
DNRA Keeps Nitrogen in the System
Denitrification permanently removes nitrogen by converting it to gas. But there is a competing process called dissimilatory nitrate reduction to ammonium (DNRA) that does something very different: it converts nitrate back to ammonium, effectively retaining nitrogen in the ecosystem rather than letting it escape.16ACS ES&T Water. Dissimilatory Nitrate Reduction to Ammonium (DNRA) Can Undermine Nitrogen Removal Effectiveness of Persistently Reducing Riparian Sediments This matters enormously for water quality. In riparian zones and wetlands designed to strip nitrogen from agricultural runoff, if DNRA dominates over denitrification, the nitrogen stays biologically available instead of being removed.
What tips the balance between denitrification and DNRA? The ratio of dissolved organic carbon to nitrate is one factor, but it is not as simple as a single threshold. Modeling studies calibrated to laboratory experiments have found that at low dissolved organic carbon concentrations, DNRA can exceed denitrification at modest carbon-to-nitrate ratios, but at higher carbon concentrations the shift does not occur in the same way.17Journal of Geophysical Research: Biogeosciences. Modeling the Competition and Controls for Denitrification and Dissimilatory Nitrate Reduction to Ammonium (DNRA) in Riparian Sediments The nitrate concentration itself also plays a regulatory role, with evidence that nitrate represses the nitrite-to-ammonium step of DNRA, adding another layer to the competition between these two pathways.18PubMed Central. Involvement of NO(3)(-) in Ecophysiological Regulation of Dissimilatory Nitrate/Nitrite Reduction to Ammonium (DNRA) Is Implied by Physiological Characterization of Soil DNRA Bacteria Isolated via a Colorimetric Screening Method
Anammox and Feammox Add More Wrinkles
Anaerobic ammonium oxidation, or anammox, was one of the biggest surprises in nitrogen-cycle research. In this process, bacteria in the phylum Planctomycetes oxidize ammonium using nitrite as the electron acceptor, producing Nâ‚‚ gas under completely oxygen-free conditions. Anammox has become valuable in wastewater engineering because it removes ammonium-rich waste without needing oxygen, which cuts energy costs dramatically. Related processes like partial nitritation-anammox and completely autotrophic nitrogen removal over nitrite (CANON) have been developed and shown to work at the lab scale.19PubMed Central. Anaerobic ammonium oxidation for treatment of ammonium-rich wastewaters
Even more recently, researchers have identified feammox: anaerobic ammonium oxidation coupled to iron(III) reduction. In this pathway, ammonium is oxidized to Nâ‚‚ while ferric iron is reduced to ferrous iron, linking the nitrogen and iron cycles in a way that was previously unknown.20Environmental Pollution. Nitrogen loss from anaerobic ammonium oxidation coupled to Iron(III) reduction in a riparian zone An enrichment culture has demonstrated that this reaction works with both chelated iron and poorly soluble iron-bearing minerals across a wide pH range, and the dominant nitrogen end product is Nâ‚‚ gas.21PubMed Central. Anaerobic ammonium oxidation coupled to iron(III) reduction catalyzed by a lithoautotrophic nitrate-reducing iron(II) oxidizing enrichment culture Feammox may be an important but previously hidden route for nitrogen removal in iron-rich wetlands and riparian soils.
How Nitrogen Moves Through the Ocean
Marine nitrogen cycling operates under different constraints than terrestrial cycling. In the open ocean, the supply of iron and phosphorus determines where nitrogen fixation can happen. Where iron is locally available and there is excess phosphorus relative to nitrogen, diazotrophs fix Nâ‚‚ to make up the deficit. This pattern has been mapped using geochemical tracers: the isotopic signature of the widespread tropical diazotroph Trichodesmium is distinctly lighter than that of deep ocean nitrate, and when newly fixed organic matter breaks down, it lowers nitrate isotope values while raising the nitrogen-to-phosphorus ratio above the ocean’s average of about 16:1.22Oceanography. Advances in Understanding the Marine Nitrogen Cycle in the GEOTRACES Era – Section: Biological N2 Fixation
A twist emerged when genome-resolved metagenomics revealed that not all Trichodesmium species fix nitrogen. Nondiazotrophic species within this genus, lacking genes for nitrogen fixation and hydrogen recycling, turn out to be abundant and widespread in the open ocean.23PubMed Central. Discovery of nondiazotrophic Trichodesmium species abundant and widespread in the open ocean This is a significant complication for models that estimate global marine nitrogen fixation based on Trichodesmium abundance. If a large fraction of what looks like a nitrogen fixer is actually living a different functional lifestyle, the ocean’s nitrogen budget needs recalculating.
Mycorrhizal Fungi as Nitrogen Brokers
Mycorrhizal fungi are famous for helping plants absorb phosphorus, but they also move nitrogen. In arbuscular mycorrhizal symbiosis, the fungus picks up inorganic nitrogen from the soil outside the root zone, incorporates it into the amino acid arginine, and transports that arginine through its network of filaments to the root interior. Inside the root, arginine is broken down and ammonium is released to the plant, but without transferring carbon along with it.24PubMed. Nitrogen transfer in the arbuscular mycorrhizal symbiosis Stable isotope labeling experiments confirmed that arginine was the predominant free amino acid in the fungal tissue outside the root, with over 90% of labeled nitrogen found in arginine, and that carbon from labeled arginine did not end up in the plant.25PubMed. The uptake, metabolism, transport and transfer of nitrogen in an arbuscular mycorrhizal symbiosis The fungus keeps its carbon and hands over nitrogen as ammonium. It is a clean division of resources.
How Cells Know When Nitrogen Is Scarce
All of these nitrogen transformations would be wasteful if organisms could not sense how much nitrogen was available and adjust accordingly. Across bacteria, archaea, and even plant chloroplasts, a family of signal transduction proteins called PII proteins serves as the central nitrogen sensor. These small proteins detect the cell’s internal nitrogen status and relay that information to regulate nitrogen uptake, assimilation, and fixation pathways.26PubMed. P(II) signal transduction proteins: nitrogen regulation and beyond PII proteins are among the most widely distributed signaling molecules in nature, underscoring how universal the problem of nitrogen management is for living cells.
On the assimilation side, once ammonium enters a cell, it is typically incorporated into organic molecules through the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle, which represents a central metabolic step in nitrogen assimilation for both plants and microbes.27PubMed Central. The Role of Glutamine Synthetase (GS) and Glutamate Synthase (GOGAT) in the Improvement of Nitrogen Use Efficiency in Cereals In tobacco plants, isotope tracing showed that ammonium assimilation consistently ran through GS/GOGAT regardless of leaf age or light conditions, and did not depend on an alternative enzyme called glutamate dehydrogenase.28PubMed Central. Glutamine synthetase-glutamate synthase pathway and glutamate dehydrogenase play distinct roles in the sink-source nitrogen cycle in tobacco The GS/GOGAT pathway is a bottleneck for crop productivity, and improving its efficiency is an active area of breeding and biotechnology research.
What Happens When Humans Overload the System
Industrial fertilizer production, through the Haber-Bosch process, roughly doubles the amount of reactive nitrogen entering terrestrial ecosystems compared to natural biological fixation alone. This excess nitrogen cascades through the cycle in ways that disrupt its natural regulation. A study of soils from forests, grasslands, and residential yards across the contiguous United States found that higher nitrogen deposition increased the fraction of mineralized nitrogen that was nitrified, even in soils where high carbon content would normally suppress nitrification. The 2020 drop in atmospheric nitrogen deposition during the COVID-19 pandemic did not reverse this effect, suggesting that past nitrogen inputs leave a lasting imprint.29Global Change Biology. Nitrogen Deposition Weakens Soil Carbon Control of Nitrogen Dynamics Across the Contiguous United States
Excess nitrogen also reshapes the soil microbial community itself. Long-term nitrogen fertilization in grasslands shifted microbial communities toward bacteria and away from fungi, and specifically reduced the abundance of arbuscular mycorrhizal fungi.30Soil Biology and Biochemistry. Increased N availability in grassland soils modifies their microbial communities and decreases the abundance of arbuscular mycorrhizal fungi A broader synthesis across biome types found that nitrogen addition consistently decreased the ratio of fungi to bacteria and increased the ratio of gram-positive to gram-negative bacteria, driven primarily by enhanced nitrogen availability rather than the soil acidification that often accompanies it.31Soil Biology and Biochemistry. Patterns and mechanisms of responses by soil microbial communities to nitrogen addition Since mycorrhizal fungi are critical nitrogen brokers for plants, as described earlier, suppressing them with excess fertilizer is an ironic feedback: by adding nitrogen, you undermine one of the biological systems that delivers nitrogen to roots.
Engineering Nitrogen Fixation Into Crops
One of the most ambitious goals in agricultural biotechnology is to transfer symbiotic nitrogen fixation into cereal crops like wheat, rice, and maize, which currently depend on industrial fertilizer. The fact that cereals already form arbuscular mycorrhizal associations, which share the common signaling pathway with legume nodule symbiosis, provides a potential entry point. Researchers have proposed that introducing specific genes from the legume nodulation pathway into cereals could create a second generation of self-fertilizing crops, potentially reducing nitrogen fertilizer use by roughly 40% to 90%.32PubMed Central. Biological nitrogen fixation in cereal crops: Progress, strategies, and perspectives – Section: Second-generation self-fertilizing cereal crops: Transferring symbiotic N fixation into cereal plants
Two broad strategies are being pursued. The first is plant engineering: either expressing nitrogenase genes directly in plant cells, or engineering the nodulation signaling pathway into cereals so they can form their own nitrogen-fixing nodules. The second is bacterial engineering: modifying nitrogen-fixing bacteria to excrete ammonium into the root zone, to protect their nitrogenase from oxygen, or to colonize cereal roots more effectively.33Plant Physiology and Biochemistry. Nitrogen fixing cereal: A rising hero towards meeting food security Both approaches face enormous technical hurdles. Nitrogenase is oxygen-sensitive, and cereal root cells are aerobic environments. The signaling required to build a functional nodule involves a complex dialogue between plant and bacterium that took evolution tens of millions of years to refine. Still, even partial success could dramatically cut fertilizer demand and the environmental damage that comes with it.
Nitrogen Cycling at the Extremes
The nitrogen cycle is not limited to temperate soils and ocean surfaces. Extremophilic microorganisms, those adapted to extreme temperatures, pH values, pressures, or nutrient scarcity, play key roles in nitrogen transformations in habitats ranging from deep-sea hydrothermal vents to hypersaline lakes and acidic hot springs. These organisms have evolved molecular adaptations affecting their metabolism, cell signaling, and enzyme stability that allow them to carry out nitrogen fixation, nitrification, and denitrification under conditions that would destroy conventional enzymes. Archaea, in particular, are prominent players in nitrogen cycling at environmental extremes. The study of these organisms is expanding our understanding of how the nitrogen cycle operated on early Earth, when conditions were far harsher than today, and it helps explain why nitrogen-cycle genes show such deep evolutionary roots across all domains of life.