Nitrogen is an essential element in proteins, DNA, RNA, and chlorophyll, but most organisms cannot use the nitrogen gas that makes up about 78% of the atmosphere. The nitrogen cycle describes how nitrogen moves between the air, soil, water, plants, animals, and microbes. This cycle matters because it controls soil fertility, ecosystem productivity, and the growth of crops.
Without microbial transformations, most nitrogen would remain locked in forms that plants and animals cannot use.
The cycle depends on bacteria and archaea that change nitrogen from one chemical form to another. Nitrogen fixation converts N2 gas into ammonia or ammonium, nitrification converts ammonium into nitrate, assimilation moves nitrogen into living tissue, and denitrification returns nitrogen gas to the atmosphere. Fertilizers add usable nitrogen to fields, but excess nitrate can wash into waterways and cause algal blooms and low oxygen zones.
Understanding the nitrogen cycle helps explain both natural ecosystem balance and human impacts on land and water.
Key Facts
- Atmospheric nitrogen is mostly N2, which has a strong triple bond and cannot be used directly by most plants or animals.
- Nitrogen fixation converts N2 into ammonia: N2 + 3H2 -> 2NH3.
- In soil water, ammonia often becomes ammonium: NH3 + H+ -> NH4+.
- Nitrification occurs in two main steps: NH4+ -> NO2- and NO2- -> NO3-.
- Assimilation is the uptake of NH4+ or NO3- by plants to build amino acids, proteins, and nucleic acids.
- Denitrification converts nitrate back to nitrogen gas under low oxygen conditions: NO3- -> NO2- -> N2O -> N2.
Vocabulary
- Nitrogen fixation
- The process that converts atmospheric nitrogen gas into ammonia or ammonium that can enter food webs.
- Nitrification
- The aerobic microbial process that changes ammonium into nitrite and then nitrate.
- Assimilation
- The process by which plants or microbes take up inorganic nitrogen and build it into organic molecules.
- Denitrification
- The anaerobic microbial process that converts nitrate into nitrogen gases released to the atmosphere.
- Eutrophication
- The overgrowth of algae and aquatic plants caused by excess nutrients such as nitrate or phosphate in water.
Common Mistakes to Avoid
- Thinking plants use N2 gas directly, which is wrong because most plants must absorb nitrogen mainly as NH4+ or NO3- from soil.
- Mixing up nitrification and nitrogen fixation, which is wrong because fixation starts with N2 while nitrification starts with NH4+.
- Forgetting that bacteria drive most major nitrogen transformations, which is wrong because enzymes in microbes make many of these reactions possible under natural conditions.
- Assuming more fertilizer is always better, which is wrong because excess nitrate can leach into water, harm ecosystems, and waste nutrients.
Practice Questions
- 1 A farmer applies 120 kg of nitrogen fertilizer to a field. If 35% is taken up by crops, how many kilograms of nitrogen are absorbed by the plants?
- 2 A soil sample contains 80 mg of nitrate. After a heavy rain, 25% of the nitrate leaches away. How many milligrams of nitrate remain in the soil?
- 3 A wetland has waterlogged soil with very little oxygen. Explain which nitrogen cycle process is likely to increase and how it affects the amount of nitrogen returned to the atmosphere.