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.
Understanding Biology: The Nitrogen Cycle
The hard part of nitrogen fixation is breaking the bond between the two nitrogen atoms in nitrogen gas. Certain microbes make an enzyme called nitrogenase that can do this job. Nitrogenase needs a large input of energy, so fixation is not a simple chemical change.
Some nitrogen fixing bacteria live freely in soil or water. Others live in close partnerships with plants such as peas, beans, clover, and other legumes. These bacteria grow in small root nodules.
The plant supplies sugars made by photosynthesis. In return, the bacteria provide nitrogen compounds that the plant can use. This partnership explains why farmers sometimes grow legumes between cereal crops.
When a plant or animal dies, decomposers break down its proteins and other nitrogen containing molecules. Bacteria and fungi release ammonium during this process, which is called ammonification. This keeps nitrogen moving through soil instead of leaving it trapped in dead material.
Nitrifying microbes then use ammonium as an energy source. They need oxygen, so nitrification is strongest in loose, well aerated soil. Their activity can make soil more acidic over time.
Nitrate is useful to plants, but it dissolves easily in water. Heavy rain can carry it below the root zone or into streams before plants absorb it.
Denitrifying microbes become important in wet soil, mud, ponds, and compacted fields where oxygen is scarce. In these conditions, some bacteria use nitrate instead of oxygen during respiration. This releases nitrogen back to the air.
Denitrification can prevent too much nitrate from building up in some habitats, but it can reduce the nitrogen available to crops. One intermediate product is nitrous oxide.
This gas warms the atmosphere much more strongly than carbon dioxide over a shorter time period. Wet fertilized fields can therefore affect both water quality and climate.
The nitrogen cycle is not a neat circle that moves at one steady speed. Different forms can build up or disappear depending on moisture, temperature, oxygen, soil pH, and the organisms present. A healthy plant needs enough nitrogen, yet extra nitrogen can create serious problems.
Fertilizer spread just before intense rain is more likely to be lost in runoff. Algae in lakes and coastal water can grow rapidly when they receive too much nitrate. When the algae die, decomposers use oxygen while breaking them down, leaving too little for fish and other animals.
When studying this topic, track the form of nitrogen at each step. Notice whether the change is driven by microbes, plant uptake, decay, or movement of water. This makes the pathways easier to understand.
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.