Nitrogen Cycle Diagram
Free printable nitrogen cycle diagram — a labeled illustration showing nitrogen fixation, nitrification, assimilation, ammonification and denitrification. Download PDF or PNG for Letter paper.
How nitrogen moves through the cycle
The nitrogen cycle is how nitrogen moves between the air, the soil and living things. Here is what happens at each labeled part of the diagram:
| Process | What it does | Why it matters |
|---|---|---|
| Nitrogen in the atmosphere | About 78% of the air is nitrogen gas, which most living things can't use directly. | |
| Nitrogen fixation | Bacteria turn nitrogen gas from the air into forms plants can use. | Farmers have known for centuries that planting beans or clover can restore a worn-out field — they just didn't know why. The reason is fixation: nitrogen-fixing bacteria living on the roots of those plants are quietly converting useless air into the fertilizer the soil desperately needs. |
| Nitrification | Bacteria change ammonia in the soil into nitrates. | Plants can absorb ammonia only in tiny amounts, but nitrates dissolve easily in water and travel straight up through roots — nitrifying bacteria perform the conversion that makes nitrogen actually accessible at scale. Skip this step and most plants would starve even in nitrogen-rich soil. |
| Assimilation | Plants absorb nitrogen through their roots to grow. | Every gram of protein in the food you eat — the chicken, the beans, the cheese — was built by a plant that first pulled nitrogen from the soil through its roots. Without assimilation, nitrogen would cycle through the ground but never enter the food chain. |
| Ammonification | Decomposers break down dead matter, releasing nitrogen back into the soil. | When a leaf falls or an animal dies, the nitrogen locked inside those proteins would be lost forever if decomposers didn't break it back down. Decomposition is nature's recycling program, and without it the soil would slowly go bankrupt. |
| Denitrification | Bacteria return nitrogen gas to the air, completing the cycle. | If nitrogen only ever moved from the air into the soil and stayed there, it would build up as toxic levels of nitrates — poisoning waterways and eventually killing the organisms the cycle is meant to support. Denitrifying bacteria act as a pressure-release valve, returning gas to the atmosphere and keeping the whole system in balance. |
Companion worksheet
A free fill-in worksheet with an answer key, built from the labels above — students match each process to its description, then answer a couple of short questions. Great for a quick check after using the diagram.
How to print
Choose Download PDF for a ready-to-print file that fits US Letter paper (8.5 × 11") in landscape, or Download PNG for the high-resolution image.
Frequently asked questions
What is the nitrogen cycle?
The nitrogen cycle is the way nitrogen moves in a loop between the air, the soil and living things, through processes carried out mostly by bacteria.
Why do living things need nitrogen?
Nitrogen is used to build proteins and DNA, which every living thing needs to grow.
What paper size should I print this on?
The diagram is sized for standard US Letter paper (8.5 × 11") in landscape orientation. Use "Download PDF" for a print-ready file that fits the page automatically.