Biogeochemical Cycles
NCERT Class 9 • Science • Chapter "Natural Resources" | NCERT Class 12 • Biology • Chapter "Ecosystem"
Biogeochemical Cycles
Unlike energy, which flows one way and is lost, matter circulates through an ecosystem again and again. The pathways by which elements move between living organisms and the non living environment are called biogeochemical cycles, and they are what make life on a finite planet possible.
The Water and Carbon Cycles
- The water cycle moves water between the ocean, atmosphere and land through evaporation, transpiration, condensation and precipitation.
- Water also moves underground through infiltration and returns as springs and river flow.
- The cycle distributes fresh water, shapes the land and moves dissolved minerals and nutrients.
- The carbon cycle moves carbon between the atmosphere, living organisms, soil, ocean and fossil fuel.
- Photosynthesis removes carbon dioxide from the air while respiration, decomposition and combustion return it.
- Carbon is stored as coal, petroleum, limestone and organic matter, which are its long term reservoirs.
- Burning fossil fuel releases carbon that was locked away for millions of years, which is why the atmospheric share has risen.
The Nitrogen and Oxygen Cycles
- Nitrogen makes up about 78 per cent of the atmosphere but cannot be used by plants in its gaseous form.
- Nitrogen fixation converts atmospheric nitrogen into usable compounds, and it is done by rhizobium in root nodules, by blue green algae and by lightning.
- Nitrification converts ammonia into nitrite and then nitrate, which plants absorb through their roots.
- Animals get nitrogen by eating plants, and decomposers return it to the soil as ammonification.
- Denitrification converts nitrate back into nitrogen gas, completing the cycle.
- The oxygen cycle runs on photosynthesis, which releases oxygen, and on respiration and combustion, which consume it.
- Excess fertiliser, sewage and burning of fuel have disturbed these cycles and are the root of several pollution problems.
| Cycle | Adds to Atmosphere | Removes from Atmosphere |
|---|---|---|
| Carbon | Respiration, decay, burning | Photosynthesis |
| Oxygen | Photosynthesis | Respiration, combustion |
| Nitrogen | Denitrification | Fixation by bacteria and lightning |
Nitrogen fixation — the conversion of atmospheric nitrogen gas into compounds that plants can absorb and use.
Hindi me samjhein
वायुमंडल में नाइट्रोजन बहुत है पर पौधे उसे गैस रूप में नहीं ले सकते। नाइट्रोजन स्थिरीकरण उसे उपयोग योग्य यौगिक में बदलता है, और यही काम राइजोबियम, नील हरित शैवाल और तड़ित करते हैं।
Exam me kaise aata hai
- Nitrogen forms about what share of the atmosphere — 78 per cent
- Which bacterium fixes nitrogen in root nodules — rhizobium
- Which process removes carbon dioxide from air — photosynthesis
- Conversion of nitrate back to nitrogen gas is — denitrification
UPSC/State PSC ke liye
- Matter cycles while energy flows, which is the single most important structural difference between the two in any ecosystem.
- Human activity disturbs cycles mainly by moving reservoirs faster than nature does, as in burning in decades what took millions of years to store.
Yahan confuse hote hain
✗ Plants absorb nitrogen directly from the air | They absorb it as nitrate from the soil after fixation | ✓
✗ Energy also cycles like matter | Energy flows one way; only matter cycles | ✓
✗ Photosynthesis is part of the carbon cycle only | It is central to both the carbon and oxygen cycles | ✓
Ek nazar me
- Matter cycles through ecosystems while energy flows one way.
- The water cycle moves water between ocean, air and land.
- Photosynthesis removes carbon dioxide; respiration and burning return it.
- Nitrogen must be fixed before plants can use it, and denitrification completes the cycle.