The nitrogen cycle
The nitrogen cycle is the movement of nitrogen through air, water, soils, rocks, microbes, plants, animals, and human systems. It explains how an abundant atmospheric element becomes usable by living things and how excess reactive nitrogen can disrupt ecosystems.
What the nitrogen cycle is
The nitrogen cycle describes how nitrogen changes form and moves among the atmosphere, living organisms, soils, waterways, and sediments. Nitrogen is essential for amino acids, proteins, DNA, RNA, chlorophyll, and many enzymes, but the biggest store of nitrogen is atmospheric N2 gas. That gas is chemically stable, so most plants and animals cannot use it until microbes or industrial processes convert it into more reactive forms.
Why usable nitrogen is limited
A field, forest, wetland, or ocean surface can be surrounded by nitrogen-rich air and still be short of usable nitrogen. The problem is access. Organisms generally need nitrogen as ammonium, nitrate, or organic nitrogen compounds. When these forms are scarce, plant growth and plankton growth can slow even when light, water, and carbon dioxide are available.
Main transformations
Nitrogen fixation converts N2 into ammonia or ammonium. Assimilation moves ammonium or nitrate into plant and microbial tissues. Ammonification returns organic nitrogen from dead material and waste to ammonium. Nitrification changes ammonium into nitrite and then nitrate in oxygen-rich conditions. Denitrification and related pathways can return nitrate or nitrite to nitrogen gases, sending nitrogen back toward the atmosphere.
Microbes do much of the work
The cycle depends heavily on microbes in soil, roots, wetlands, sediments, lakes, and the ocean. Some nitrogen-fixing microbes live freely, while others form partnerships with plants such as legumes. Other microbial groups specialize in oxidizing ammonium, reducing nitrate, or breaking down organic matter. Conditions such as oxygen, moisture, temperature, pH, and available carbon can change which pathway dominates.
How people changed the cycle
Modern agriculture and industry have made reactive nitrogen far more available. The Haber-Bosch process produces ammonia for fertilizer, and farms also cycle nitrogen through manure and nitrogen-fixing crops. Vehicles, power plants, wastewater, septic systems, and stormwater can add nitrogen compounds to air and water. These inputs can support food production, but they can also overload places that evolved with lower nitrogen supply.
Why it matters
Balanced nitrogen cycling supports soil fertility, crop yields, forests, wetlands, freshwater food webs, and marine ecosystems. Too little available nitrogen can limit growth. Too much can fuel eutrophication, harmful algal blooms, low-oxygen dead zones, nitrate contamination, air pollution, acidification, biodiversity loss, and emissions of nitrous oxide, a powerful greenhouse gas.
Connections to other cycles
The nitrogen cycle does not operate alone. Photosynthesis connects nitrogen demand to carbon uptake, while the water cycle carries nitrate and organic nitrogen through soils, rivers, groundwater, and coasts. Oxygen levels shape nitrification and denitrification, and climate affects microbial activity, crop demand, runoff, drought, wildfire, and wetland emissions. Thinking about nitrogen as part of an Earth-system network helps explain why local fertilizer choices can have downstream and atmospheric effects.