Plants and bacteria are crucial for the nitrogen cycle because they are the only organisms that can convert nitrogen into forms living things actually use. Nitrogen makes up 78% of Earth’s atmosphere, but almost all of it exists as a gas that most organisms cannot absorb. Without bacteria to pull nitrogen from the air and transform it into usable compounds, and without plants to channel those compounds into food webs, life on Earth would run out of this essential nutrient.
The Problem With Atmospheric Nitrogen
Nitrogen gas consists of two nitrogen atoms bonded together with a triple bond, one of the strongest chemical bonds in nature. Animals, plants, and most microorganisms cannot break this bond. They need nitrogen in “reactive” forms like ammonium or nitrate to build proteins, DNA, and other essential molecules. The entire nitrogen cycle exists to convert that inert atmospheric gas into these usable forms and eventually return it to the atmosphere, keeping the supply in balance.
How Bacteria Pull Nitrogen From the Air
Nitrogen fixation, the conversion of atmospheric nitrogen gas into ammonium, is performed exclusively by certain bacteria and archaea. No plant or animal can do this. These microorganisms use an enzyme called nitrogenase, which contains iron-sulfur clusters and demands enormous amounts of energy: 16 units of the cell’s energy currency for every single molecule of nitrogen gas converted. The enzyme also needs eight electrons delivered at very low electrical potential, making the whole process biochemically expensive.
Nitrogenase is extremely sensitive to oxygen, which destroys its function. Different bacteria have evolved different workarounds. Some, like species of Azotobacter, breathe so rapidly that they consume oxygen before it reaches the enzyme. Others, like Rhizobium species, rely on a partnership with plants that provides a low-oxygen environment inside root nodules. Between free-living soil bacteria and these symbiotic partners, biological nitrogen fixation is responsible for an estimated 60% of all the nitrogen made available to life on Earth, contributing roughly 52 to 130 million metric tons of reactive nitrogen per year to terrestrial ecosystems alone. Even with the massive scale of industrial fertilizer production, biological fixation still accounts for about half of all bioavailable nitrogen on the planet.
The Legume-Bacteria Partnership
One of the most productive nitrogen-fixing relationships in nature is the symbiosis between legumes (beans, peas, clover, soybeans) and Rhizobium bacteria. This partnership begins with a precise chemical conversation. The plant’s roots release compounds called flavonoids into the surrounding soil. When nearby rhizobia detect these flavonoids, they respond by producing signaling molecules known as Nod factors, which are specialized sugar-based compounds decorated with various chemical side chains.
When Nod factors bind to receptors on the plant’s root hair cells, they trigger a cascade of changes: root hairs curl, cortical cells begin dividing, and the plant forms tube-like “infection threads” that guide the bacteria deep into the root tissue. The end result is a nodule, a small, pinkish swelling on the root where the bacteria settle in and begin fixing nitrogen. Inside these nodules, oxygen levels are kept low enough to protect the nitrogenase enzyme, while the plant supplies the bacteria with sugars for energy. In return, the bacteria feed the plant a steady supply of ammonium.
This partnership is powerful enough that legume cover crops can fix between 50 and 150 pounds of nitrogen per acre, depending on growing conditions. Farmers who rotate legumes into their fields can significantly reduce how much synthetic fertilizer they need for the next crop.
Why Plants Are the Cycle’s Distribution Network
Bacteria make reactive nitrogen available, but plants are the primary way that nitrogen enters food webs. Plant roots absorb nitrogen from the soil mainly as nitrate or ammonium ions, though they can also take up amino acids from decomposing organic matter. Most plants prefer nitrate, which they must first convert to ammonium inside their cells before it becomes useful. This conversion happens in two steps: first in the cell’s main compartment, then inside the chloroplast, using a pair of specialized enzymes.
Once the nitrogen is in ammonium form, a second set of enzymes incorporates it into glutamine and glutamate, two amino acids that serve as the starting materials for building all other amino acids, proteins, chlorophyll, and DNA. Every animal that eats a plant is ultimately getting its nitrogen from this process. And every animal that eats another animal is tracing its nitrogen supply back through the same chain. Without plants converting soil nitrogen into organic molecules, there would be no pathway for nitrogen to reach the rest of the living world.
Nitrification: Bacteria Converting Between Forms
Not all soil nitrogen starts in a form that plants prefer. When ammonium accumulates in soil, whether from nitrogen fixation, decomposition, or fertilizer, a group of bacteria converts it into nitrate through a two-step process called nitrification. In the first step, bacteria such as Nitrosomonas oxidize ammonium into nitrite. In the second step, bacteria such as Nitrobacter oxidize that nitrite into nitrate. Both groups are autotrophs, meaning they actually harvest energy from these chemical conversions rather than from sunlight or organic food.
This matters for plants because nitrate is more mobile in soil water than ammonium, making it easier for roots to absorb. Nitrification effectively makes fixed nitrogen more accessible across a wider area of soil. Without these bacteria, much of the nitrogen fixed by other microorganisms would remain locked in a less available form.
Decomposition: Recycling Nitrogen From Dead Matter
When plants and animals die, or when animals excrete waste, the nitrogen locked in their proteins and nucleic acids doesn’t vanish. Bacteria and fungi break down these organic compounds and release the nitrogen as ammonium in a process called ammonification. This recycled ammonium re-enters the soil pool, where it can be taken up directly by plants, converted to nitrate by nitrifying bacteria, or used by other soil microorganisms. Without decomposers performing this step, nitrogen would accumulate in dead organic matter and become permanently unavailable. The cycle would grind to a halt within a few growing seasons.
Denitrification: Closing the Loop
The nitrogen cycle is not a one-way street from atmosphere to soil. Certain bacteria close the loop by converting nitrate back into nitrogen gas, returning it to the atmosphere. This process, called denitrification, happens primarily in waterlogged or oxygen-poor soils where bacteria use nitrate as a substitute for oxygen during respiration. The ultimate end product is nitrogen gas, though intermediate gases are also released along the way.
Denitrification prevents reactive nitrogen from building up indefinitely in soils and waterways. Without it, ecosystems would become overloaded with nitrogen, leading to problems like algal blooms in lakes and coastal dead zones. In a balanced ecosystem, the rate of denitrification roughly offsets the rate of fixation, keeping the global nitrogen budget stable over long timescales.
Why No Other Organisms Can Replace Them
The nitrogen cycle depends on plants and bacteria because the chemistry involved requires enzymes that no other groups of organisms possess. Only bacteria and archaea produce nitrogenase. Only specific bacterial groups perform nitrification. Only bacteria carry out denitrification at meaningful scales. And while fungi assist with decomposition, bacteria are responsible for the majority of ammonification in most soils. Plants, meanwhile, are the dominant bridge between soil nitrogen and the rest of the food web, channeling inorganic nitrogen into the organic molecules that animals depend on.
Remove bacteria from the equation, and nitrogen stays trapped as an inert gas in the atmosphere or locked in dead organic matter. Remove plants, and even if bacteria keep fixing and cycling nitrogen in soil, there is no efficient route for that nitrogen to reach animals and other consumers. Together, these two groups form the backbone of a cycle that every living thing depends on.