In case of which one of the following biogeochemical cycles, the weathering of rocks is the main source of release of nutrient to enter the cycle?

Answer: (c) Phosphorus Cycle

Phosphorus has no gaseous phase. It is locked in phosphate rock and enters the living world only as rocks weather into soluble phosphate.

The idea

Gaseous cycles (carbon, nitrogen, oxygen) have their reservoir in the air. Sedimentary cycles (phosphorus, and largely sulphur) have it in rocks.

Phosphate rock → weathering → PO₄ in soil → plants → animals → decomposers → sea sediment → uplift

Why the others are wrong

(a) Carbon enters mainly as CO₂ from the air — photosynthesis, respiration, burning.

(b) The main entry is the air: 78% of it is N₂, fixed by bacteria and lightning.

(d) Sulphur has a big atmospheric route (SO₂ from volcanoes and fuel burning), so weathering is not its MAIN source.

The trap

Sulphur is also partly sedimentary, so it tempts. But it has a large gas phase; phosphorus has none.

Also know

With no gas phase, phosphorus that washes into the sea does not return quickly, so it is often the nutrient in shortest supply for plants. Farmers add it as phosphate fertiliser made from rock phosphate, most of which India imports. Too much phosphate in lakes, from fertiliser and detergents, causes algal blooms.