What is the Nitrogen Cycle?
The nitrogen cycle is how nitrogen moves between the atmosphere, soil, and living organisms. Although nitrogen gas makes up 78% of the air, most organisms cannot use it directly — bacteria must first convert it into forms plants and animals can absorb.
The nitrogen cycle is the natural process by which nitrogen is converted between atmospheric N2 gas and usable compounds like ammonia, nitrite, and nitrate through fixation, nitrification, assimilation, ammonification, and denitrification.
- 1.Nitrogen Fixation — Bacteria (e.g., Rhizobium) or lightning convert atmospheric N2 into ammonia (NH3).
- 2.Nitrification — Nitrifying bacteria convert ammonia into nitrite (NO2−) and then nitrate (NO3−).
- 3.Assimilation — Plants absorb nitrate and ammonium to build proteins and nucleic acids; animals get nitrogen by eating plants.
- 4.Ammonification — Decomposers break down dead organisms and waste, releasing ammonia back into the soil.
- 5.Denitrification — Denitrifying bacteria convert nitrate back into N2 gas, returning it to the atmosphere.
Step-by-step worked examples
A legume crop fixes about 150 kg of nitrogen per hectare per year via root-nodule bacteria. If a farm has 8 hectares of soybeans, how much nitrogen is fixed annually?
Nitrogen fixed = 150 kg/ha × 8 ha = 1,200 kg of nitrogen fixed per year
Air is about 78% nitrogen gas (N2) by volume. In 100 L of air, how many liters are N2?
Volume of N2 = 78% × 100 L = 0.78 × 100 = 78 L of N2
A wetland's denitrifying bacteria convert 40 kg of nitrate-nitrogen back to N2 gas each month. How much is converted in a year?
Annual denitrification = 40 kg/month × 12 months = 480 kg of nitrogen returned to the atmosphere per year
Flashcards
Quick quiz
Q1.Which process converts atmospheric N2 gas into ammonia?
Q2.Which bacteria convert ammonia into nitrite and nitrate?
Q3.What happens during denitrification?
Q4.Why is nitrogen fixation necessary for life?
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Common mistakes
Plants can absorb nitrogen gas (N2) directly from the air. — Correct: Plants can only absorb nitrogen in fixed forms like nitrate (NO3−) or ammonium (NH4+), not N2 gas.
Nitrogen fixation and nitrification are the same process. — Correct: Fixation converts N2 into ammonia; nitrification then converts that ammonia into nitrite and nitrate.
Denitrification adds nitrogen to the soil. — Correct: Denitrification removes usable nitrogen from the soil by converting nitrate back into N2 gas released to the air.
All bacteria in the nitrogen cycle do the same job. — Correct: Different bacteria specialize in fixation, nitrification, or denitrification — each is a distinct step.
FAQ
What is the nitrogen cycle?
It's the natural cycling of nitrogen between the atmosphere and the biosphere through fixation, nitrification, assimilation, ammonification, and denitrification.
What is the formula or main steps of the nitrogen cycle?
The main steps are: nitrogen fixation (N2 → NH3), nitrification (NH3 → NO2− → NO3−), assimilation (uptake by plants), ammonification (decomposition), and denitrification (NO3− → N2).
What are examples of nitrogen fixation?
Rhizobium bacteria in legume root nodules, cyanobacteria in soil and water, and lightning strikes that fix atmospheric nitrogen.
Why is the nitrogen cycle important for agriculture?
Crops need fixed nitrogen (nitrate/ammonium) to grow; farmers use nitrogen-fixing legumes or fertilizers to replace nitrogen removed by harvesting.