Nitrogen Cycle: Nature's Way of Recycling Nitrogen Explained
The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, water, and living organisms. Though nitrogen is abundant in the air, most org
TL;DR: The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, water, and living organisms. Though nitrogen is abundant in the air, mos…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
Nitrogen Cycle: Nature's Way of Recycling Nitrogen: The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, water, and living organisms. Though nitrogen is abundant in the air, most org
Nitrogen Cycle: Nature's Way of Recycling Nitrogen Explained Simply
The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, water, and living organisms. Though nitrogen is abundant in the air, most org This is a key Biology concept for Class 10.
🎬 Watch the animated, step-by-step Nitrogen Cycle: Nature's Way of Recycling Nitrogen diagram →
Forms of Nitrogen in the Cycle
Atmospheric nitrogen (N2) makes up 78% of the air but is unusable by most organisms. Ammonia (NH3) is produced when nitrogen is fixed and can be absorbed by plants. Nitrate (NO3-) is another form usable by plants. Nitrite (NO2-) is an intermediate form. Proteins containing nitrogen are made by organisms. Dead organic matter and animal waste return nitrogen to the soil.
Nitrogen Fixation
Nitrogen fixation is the conversion of atmospheric N2 to ammonia (NH3). Free-living bacteria in soil and water perform this function. Symbiotic bacteria in root nodules of legumes (beans, peas, clover) fix nitrogen in exchange for sugars from the plant. Lightning also causes nitrogen fixation in the atmosphere, creating nitrogen oxides that fall as rain. This is why farmers rotate crops with legumes to naturally replenish soil nitrogen.
Nitrification and Assimilation
Nitrification is the conversion of ammonia to nitrite by bacteria, and nitrite to nitrate by other bacteria. Plants absorb nitrate and use it to synthesize proteins and nucleic acids. Animals obtain nitrogen by eating plants or other animals. When organisms die or produce waste, decomposers break down proteins, releasing ammonia back to the soil. This closes the loop between organic and inorganic nitrogen.
Denitrification and Return to Atmosphere
Denitrification is the conversion of nitrate back to nitrogen gas by anaerobic bacteria in waterlogged soil. This occurs when oxygen is scarce. Some nitrogen is also lost through soil erosion and water runoff. A small amount of nitrogen is fixed by industrial processes for fertilizers. Without denitrification, nitrate would accumulate forever, breaking the cycle.
Human Impact on the Nitrogen Cycle
Humans have doubled atmospheric nitrogen input through industrial fertilizer production and fossil fuel combustion. Excess nitrate from fertilizers leaches into groundwater, polluting drinking water. Nitrogen runoff causes eutrophication in lakes and coastal waters, killing fish. Agricultural practices have increased denitrification, releasing more N2O, a potent greenhouse gas. Understanding the nitrogen cycle is essential for sustainable agriculture.
Worked Examples
Example 1. Why do farmers plant legumes in fields after growing wheat?
Solution: Wheat depletes soil nitrogen. Legumes have symbiotic nitrogen-fixing bacteria in their roots that convert atmospheric nitrogen to ammonia. When the legume crop is plowed back into the soil, its nitrogen-rich biomass enriches the soil for the next wheat crop.
Example 2. Why does excessive fertilizer runoff kill fish in lakes?
Solution: Excess nitrogen causes algae to bloom explosively. When algae die, decomposers consume oxygen, creating dead zones. Fish cannot survive in water depleted of oxygen. This eutrophication transforms balanced ecosystems into barren waters.
Where You See This in Real Life
- Indian farmers plant pulses like lentils and chickpeas after grain crops because nitrogen-fixing bacteria in their roots restore soil nitrogen naturally.
- Cow dung in composting heaps contains nitrogen that bacteria convert to forms plants can absorb.
- In rice paddies with standing water, denitrification occurs, slowly converting soil nitrate back to atmospheric nitrogen.
- Heavy fertilizer use in Indian agriculture has led to high nitrate levels in groundwater, a concern in states like Punjab.
Common Mistakes Students Make
- Thinking all organisms can use atmospheric nitrogen: Only nitrogen-fixing bacteria and some archaea can convert N2 to usable forms.
- Believing denitrification is bad: Denitrification is essential for completing the cycle. Without it, nitrogen would accumulate indefinitely.
- Assuming fertilizers solve nitrogen problems forever: Excess fertilizer creates pollution and does not improve long-term soil health.
Frequently Asked Questions
Why cannot plants use atmospheric nitrogen directly?
The N2 triple bond is extremely strong and requires massive energy to break. Most organisms lack the enzymes to accomplish this. Only specialized bacteria with the nitrogenase enzyme can fix nitrogen.
How do lightning fix nitrogen?
Lightning provides extreme heat and energy that breaks the N2 triple bond. This creates nitrogen oxides that dissolve in rain and fall to the soil as nitrate, entering the nitrogen cycle.
What happens in poorly drained soil where denitrification occurs?
Waterlogged conditions force denitrifying bacteria to use nitrate as an electron acceptor since oxygen is unavailable. This produces nitrogen gas that escapes to the atmosphere, and soil loses nitrogen content.