Prelims 2021

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

a) Carbon cycle
b) Nitrogen cycle
c) Phosphorus cycle
d) Sulphur cycle

Correct Answer: c) Phosphorus cycle

Question from UPSC Prelims 2021 GS Paper

Explanation : 

Phosphorus Cycle

The Phosphorus cycle is one of the slowest biogeochemical cycles and differs from the other major biogeochemical cycles—carbon, nitrogen, and sulfur—because it does not have a significant atmospheric component. Phosphorus is not found in a gaseous state under normal environmental conditions.

Phosphorus in Rocks

The primary source of phosphorus in the phosphorus cycle is rock. Phosphate rock contains the mineral apatite, which is composed mainly of phosphate minerals containing phosphorus in the form of phosphate ions (PO4^3-). Over time, weathering processes break down these rocks and release phosphate into the soil and water.

Phosphorus in the Soil and Ecosystem

Once in the soil, phosphorus is taken up by plants through their roots. It is an essential nutrient for plant growth, playing a key role in energy transfer within cells (as part of ATP), in genetic material (DNA and RNA), and in cell membranes (as phospholipids). Animals obtain their phosphorus by consuming plants or other animals.

Phosphorus is generally found in nature as part of phosphate ions, and it moves through the ecosystem via the food web. When plants and animals die, decomposers like bacteria and fungi break down the organic matter, releasing phosphate back into the soil or aquatic environments.

The Unique Nature of the Phosphorus Cycle

The phosphorus cycle lacks a significant atmospheric component because phosphorus does not typically enter the atmosphere in a gaseous form. Instead, it remains mostly on land and in rock and soil minerals, and its cycle is primarily driven by geological and biological processes such as weathering, erosion, and the growth and decay of organisms.

In summary, the weathering of rocks is the main source of release of phosphorus into the cycle, making the phosphorus cycle the correct answer to the question.

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