Carbon, Nitrogen, Phosphorus and Water Cycles in Nature

The carbon, nitrogen, phosphorus and water cycles are natural recycling systems that move essential elements between air, water, soil and living organisms. Without these cycles, nutrients would run out and waste would pile up.

Human actions like burning fossil fuels and overusing fertilisers disturb these cycles, causing climate change, algal blooms and water shortages. Learning the cycles shows where we can reduce harm.

Key Concepts

Carbon cycle moves carbon as carbon dioxide (CO2) in air, glucose in plants, and carbonates in rocks. Nitrogen cycle involves nitrogen gas (N2, 78 percent of air), ammonia (NH3), nitrites (NO2-) and nitrates (NO3-). Phosphorus cycle has no major gas phase; phosphorus moves as phosphate (PO4 3-) in rocks, soil and water. Water cycle moves H2O by evaporation, condensation, precipitation and transpiration. Reservoir is a store like ocean or atmosphere, and flux is movement rate, often in gigatonnes per year for carbon.

How It Works

1. Carbon: Plants absorb CO2 for photosynthesis; animals release CO2 by respiration; decomposers release CO2; some carbon is buried as fossil fuels.
2. Nitrogen: Nitrogen-fixing bacteria convert N2 to ammonia; nitrifying bacteria make nitrates that plants absorb; denitrifying bacteria return N2 to air.
3. Phosphorus: Weathering releases phosphate from rocks; plants absorb it; animals eat plants; decomposition returns phosphate to soil.
4. Water: Sun evaporates ocean water; vapour condenses into clouds; rain falls; water infiltrates soil or runs off and returns to oceans.

Worked Example

A tree absorbs CO2 and stores carbon in wood. Suppose the tree gains 10 kg of dry biomass in a year and about 50 percent of dry biomass is carbon.

Step 1: Carbon stored = 10 kg x 0.50 = 5 kg carbon.

Step 2: To find CO2 removed, use ratio of molecular masses: CO2 is 44, carbon is 12. CO2 = 5 x 44 / 12 = 18.3 kg.

So the tree removed about 18.3 kg of CO2 that year. If a park has 100 such trees, removal = 100 x 18.3 = 1,830 kg CO2 per year.

Common Misconceptions

• Plants can directly use nitrogen gas. Most plants cannot; they need fixed nitrogen made by bacteria or fertilisers.
• Phosphorus cycles through air like carbon. Phosphorus has almost no gaseous phase and moves mainly through rock, soil and water.
• Water cycle creates new water. The total water on Earth stays nearly constant; it only changes form and location.

Real-World Applications

Farmers rotate legumes like peas to fix nitrogen naturally and reduce fertiliser use. Wetlands are protected because they filter phosphorus and prevent lake algal blooms. Afforestation projects are valued for carbon storage. Rainwater harvesting follows the water cycle to store runoff for dry months.

Overall, carbon moves via photosynthesis and respiration, nitrogen needs bacterial fixation, phosphorus flows from rocks without a gas stage, and water circulates by evaporation and precipitation, all keeping life supplied.