Natural Cycles: Water, Carbon and Nitrogen

Probe and Ponder — The Never-Ending Recycler

Think of the glass of water you drank this morning. Some of its molecules may have once flowed in the Ganga, been sipped by a dinosaur 100 million years ago, or floated as a cloud over the Himalayas. Nothing on Earth is ever truly created from scratch — nature reuses the same materials again and again, in astonishing loops called natural cycles.

12.6 The Water Cycle — Nature's Familiar Loop

You already know this cycle from earlier classes. Heat from the Sun evaporates water from oceans, rivers, lakes, wet soil and even plant leaves (transpiration). The invisible water vapour rises, cools, and condenses into tiny droplets that form clouds. When the droplets become heavy, they fall as rain, snow or hail. Some water runs off into rivers and returns to the ocean; some seeps underground to form groundwater. And the loop begins again.

12.7 The Carbon Cycle — The Breath of the Planet

Carbon is the backbone of every living thing. It travels through Earth in a beautiful loop called the carbon cycle. Two opposite processes keep it moving:

• Photosynthesis — green plants pull carbon dioxide (CO₂) from the air and lock carbon into sugars, starch, wood and other food.
• Respiration — plants, animals and microbes break these sugars down again, releasing CO₂ back into the air.

When wood or petrol burns (combustion), and when dead bodies decompose, more CO₂ also returns to the atmosphere. Oceans quietly swap CO₂ with the air. Slow geological processes turn some dead organisms into coal, oil and natural gas — these are our fossil fuels.

12.8 The Oxygen Cycle — The Gas of Life

Oxygen and carbon are twin travellers. The same photosynthesis that removes CO₂ releases oxygen, while respiration and burning remove oxygen and release CO₂. Together, plants and the oceans keep the air at roughly 21% oxygen — just right for us to breathe. Tropical forests like those of the Western Ghats and the Amazon are often called the "lungs of the Earth" because they release enormous amounts of oxygen every day.

12.9 The Nitrogen Cycle — The Invisible Fertiliser Factory

Air is 78% nitrogen — there is more nitrogen over our heads than anything else. Plants need nitrogen to build proteins and DNA, and animals need it too (from plants). Yet plants cannot use the nitrogen gas (N₂) in the air directly — the two nitrogen atoms are bonded so tightly that the molecule is almost inert. So, how does nitrogen get into living things?

Nitrogen Fixation — The Hidden Helpers

Tiny nitrogen-fixing bacteria — most famously Rhizobium, which lives inside the root nodules of pulse plants such as moong, tur and gram — convert N₂ gas into nitrates (NO₃⁻) that plants can absorb. Lightning flashes also "fix" a small amount of nitrogen during thunderstorms.

The Full Nitrogen Loop

1. Fixation: Bacteria (or lightning) turn N₂ into nitrates usable by plants.
2. Uptake: Plants absorb nitrates through roots and build proteins.
3. Consumption: Animals eat plants and gain nitrogen.
4. Decomposition: Dead organisms and wastes are broken down; bacteria release ammonia, then nitrates back into the soil.
5. Denitrification: A special group of bacteria returns some nitrogen to the air as N₂, completing the cycle.

12.10 How Cycles Keep Nature in Balance

These cycles are not separate — they run all together, every second, across the whole planet. Water carries nutrients. Carbon and oxygen feed every breath and every meal. Nitrogen builds every protein. When the cycles run smoothly, air stays breathable, rain keeps falling, and soil stays fertile. This is what we mean by nature in harmony.

12.11 Human Disruption of the Cycles

For thousands of years, humans lived within the cycles. But in the last 200 years, industries, vehicles and deforestation have begun to push the cycles out of their usual balance.