Q5.
What would happen to plants and animals on Earth if the biogeochemical cycles were disrupted and stopped? Explain by giving a few examples.
Answer
Life would end, because the Earth has no outside supply of matter. Every atom of water, carbon, nitrogen and oxygen a plant or animal uses has been used before by some other organism. Stop the recycling and the supply simply runs out.
What happens cycle by cycle:
- Water cycle stops. No evaporation, no condensation, no rain. Rivers, lakes and soil moisture dry up. Plants cannot draw water for photosynthesis or transport minerals; animals have nothing to drink. Land life ends within days to weeks.
- Carbon cycle stops. Plants keep drawing CO2 out of the air for photosynthesis but decomposition and respiration return none. The atmosphere's CO2 is exhausted, photosynthesis halts, and every food chain that depends on plants collapses. Meanwhile dead bodies and leaves pile up because decomposers are not returning their carbon.
- Nitrogen cycle stops. Atmospheric N2 is unreactive and plants cannot use it directly. Without nitrogen fixation there is no ammonia, without nitrification no nitrate, so plants cannot make proteins or nucleic acids. Growth, repair and reproduction all fail. Animals, which get their nitrogen only by eating, starve of protein.
- Oxygen cycle stops. Respiration and combustion keep consuming O2, but photosynthesis no longer restores it. Atmospheric oxygen falls until aerobic respiration is impossible.
Earth = open system for energy (sunlight in, infrared out)
Earth = closed system for matter (no new C, N, O or H2O arrives)
Therefore matter must be recycled, or the supply is finite and life stops.
Earth = closed system for matter (no new C, N, O or H2O arrives)
Therefore matter must be recycled, or the supply is finite and life stops.
A concrete example to write down: the nitrogen in the dal you eat today was, not long ago, N2 gas in the air. Rhizobium in the root nodules of the pulse crop fixed it into ammonia, nitrifying bacteria turned it into nitrate, the plant assimilated it into protein — and when you digest that protein and later return the waste to the soil, decomposers ammonify it and hand it back. Break any one link in that chain and the field stops feeding you.