Nitrogen makes up 78% of the atmosphere, but plants can’t use it in its gaseous form. It must first be converted into ammonia or nitrate, compounds that roots can actually absorb. This conversion happens three ways: through bacteria in the soil, through lightning strikes, and through industrial manufacturing. Together, these processes produce over 400 teragrams of usable nitrogen every year, fueling virtually all plant growth on Earth.
Why Plants Can’t Use Atmospheric Nitrogen
The two nitrogen atoms in atmospheric nitrogen gas are locked together by one of the strongest bonds in nature, a triple bond that requires enormous energy to break. Plants lack the biochemical machinery to crack this bond. They need nitrogen delivered in simpler, soluble forms: ammonium or nitrate. These dissolved compounds can pass through root cell membranes using specialized transporter proteins, with different transporters handling each form depending on how much is available in the soil.
Biological Fixation: Bacteria That Do the Heavy Lifting
The largest natural source of usable nitrogen comes from bacteria equipped with a special enzyme called nitrogenase. This enzyme splits atmospheric nitrogen and combines it with hydrogen to produce ammonia. The process is energy-intensive: for every single electron transferred to break the nitrogen bond, the bacteria burn through two molecules of ATP, their cellular energy currency. Electrons move one at a time from one part of the enzyme complex to another in a rapid sequence that takes roughly 200 milliseconds per cycle. The full conversion of one nitrogen molecule requires multiple rounds of this cycle.
Nitrogenase has a critical vulnerability: oxygen destroys it. This means nitrogen-fixing bacteria need strategies to keep oxygen away from the enzyme while still getting enough to survive. The solution varies depending on whether the bacteria live independently in soil or partner with plants.
Symbiotic Fixation in Root Nodules
The most efficient biological nitrogen fixation happens inside the root nodules of legumes, plants like soybeans, clover, peas, and alfalfa. These plants form a partnership with soil bacteria commonly known as rhizobia. The process begins when the plant’s roots release chemical signals into the soil. Rhizobia respond by producing signaling molecules called Nod factors, which trigger a cascade of changes in the root.
First, root hair cells curl around a small cluster of bacteria, forming a tube called an infection thread that guides the bacteria inward. Meanwhile, cells deeper in the root begin dividing to form a nodule, a small, rounded growth. Once the bacteria reach the nodule, they’re released from the infection threads and taken inside individual plant cells, where they become enclosed in a plant-made membrane. These wrapped bacteria function almost like tiny organs inside the cell, dedicated to converting nitrogen gas into ammonia.
The oxygen problem gets solved here by a protein called leghemoglobin, which is chemically similar to the hemoglobin in your blood. Leghemoglobin has a remarkably high affinity for oxygen, about ten times greater than the myoglobin in muscle tissue. It binds oxygen tightly enough to keep concentrations low around the nitrogenase enzyme, preventing damage, while still shuttling just enough oxygen to the bacteria so they can generate energy. This protein gives active nodules a distinctive pink or reddish color when you slice them open.
Nitrogen-fixing crops contribute roughly 60 teragrams of nitrogen to agricultural soils each year. This is why farmers rotate legumes with other crops: the nodules enrich the soil with nitrogen that the next planting can use.
Free-Living Nitrogen Fixers
Not all nitrogen-fixing bacteria need a plant partner. Free-living species scattered through the soil also convert atmospheric nitrogen into ammonia on their own. These include a wide range of bacteria found in croplands worldwide, and they’ve shown meaningful benefits for cereal crops like maize, rice, and wheat. A 50-year assessment of non-symbiotic fixation in these farming systems found an average contribution of about 15.5 kilograms of nitrogen per hectare. Under favorable conditions, some studies have measured contributions up to 60 kilograms per hectare per year, though typical rates fall between 0.3 and 15 kilograms.
Free-living fixers protect their nitrogenase from oxygen through various strategies: some produce thick, mucus-like coatings that slow oxygen diffusion, others fix nitrogen only at night or in waterlogged soils where oxygen levels are naturally low. Across all natural terrestrial ecosystems, biological nitrogen fixation contributes a median estimate of about 88 teragrams of nitrogen per year, with a likely range of 52 to 130 teragrams.
Lightning: Nature’s Other Fixer
Lightning provides a smaller but constant source of fixed nitrogen. The extreme heat of a lightning bolt, reaching around 30,000 degrees Celsius, supplies enough energy to rip apart nitrogen and oxygen molecules in the atmosphere. These fragments recombine to form nitric oxide gas, along with trace amounts of other nitrogen-oxygen compounds. As these gases dissolve in rainwater, they react with water to form nitrate and nitrite, both of which wash into the soil in a form plant roots can absorb directly. The rain also carries small amounts of nitric and nitrous acid. While lightning contributes far less total nitrogen than bacteria or industrial processes, it distributes fixed nitrogen across every ecosystem on the planet, including remote areas with little biological fixation.
The Haber-Bosch Process: Industrial Fixation
Since the early twentieth century, humans have been fixing nitrogen on a massive industrial scale using the Haber-Bosch process. This method combines atmospheric nitrogen with hydrogen gas under extreme pressure and temperature, using a metal catalyst to produce ammonia. The ammonia is then processed into synthetic fertilizers.
The scale of this process is staggering. In 2010, the Haber-Bosch process produced 120 teragrams of nitrogen per year as ammonia, making it the single largest source of new usable nitrogen on Earth. That’s roughly double the 63 teragrams contributed by all natural terrestrial biological fixation. Overall, about half of the 413 teragrams of usable nitrogen entering ecosystems each year now comes from human activity. This industrial nitrogen is what allows modern agriculture to feed billions of people, but it also means the global nitrogen cycle has been fundamentally altered from its pre-industrial state.
From Ammonia to Nitrate: The Soil Conversion Step
Whether nitrogen arrives in soil from bacteria, lightning, or fertilizer, much of it starts as ammonia or ammonium. Most plants prefer to absorb nitrogen as nitrate, so soil bacteria perform one more critical conversion called nitrification. This happens in two stages, each handled by different specialist bacteria.
In the first stage, ammonia-oxidizing bacteria convert ammonia into nitrite. In the second stage, nitrite-oxidizing bacteria convert that nitrite into nitrate. The conversion is one-to-one: each molecule of ammonia yields one molecule of nitrite, which yields one molecule of nitrate. Both stages require oxygen, which is why waterlogged or compacted soils with poor aeration tend to have slower nitrification and can accumulate ammonium instead.
Temperature plays a significant role in how fast these conversions happen. Both ammonification (the release of ammonium from organic matter) and nitrification increase substantially as soil temperatures rise from 5°C to 35°C, with ammonification rates climbing exponentially across that range. Soil moisture matters too, though the bacteria are relatively flexible: moisture levels between 60% and 100% of the soil’s water-holding capacity support nitrification without significant limitation.
How Plant Roots Absorb Fixed Nitrogen
Once nitrogen is in its usable forms, plants pull it in through their roots using two families of transporter proteins, one for nitrate and one for ammonium. When nitrate concentrations in the soil are low, roots switch on high-affinity transporters that can grab nitrate even at trace levels. When nitrate is abundant, a different set of low-affinity transporters handles the bulk of uptake.
Ammonium absorption works similarly. Plants deploy a family of high-affinity ammonium transporters in their roots, with two of them responsible for 60 to 70% of the total uptake capacity when nitrogen is scarce. A third transporter picks up another 20% by retrieving ammonium from the water flowing between root cells. Some of these transporters are sensitive to soil acidity, working faster in more acidic conditions. Others do double duty, triggering signals that change root architecture, encouraging the plant to branch its roots toward nitrogen-rich patches in the soil.
This final uptake step completes the journey from inert atmospheric gas to the amino acids, proteins, chlorophyll, and DNA that plants build from nitrogen. Every leaf, seed, and fruit depends on this chain of fixation, conversion, and absorption working in sequence.