Previous Year Question

In the phosphorus cycle, weathering of rocks is the main source for the release of the nutrient.

• The phosphorus cycle is a major sedimentary biogeochemical cycle.

• The principal long-term reservoir of phosphorus is not in the atmosphere but in phosphate-bearing rocks of the Earth’s crust and marine sediments.

• Physical and chemical weathering of rocks containing phosphate minerals releases inorganic phosphate into soil and water.

• Plants absorb dissolved phosphate ions from the soil through their roots.

• Phosphorus passes from plants through the food chain to herbivores and then to carnivores.

• Excretion by organisms and decomposition of dead plants and animals return phosphate to soil or water.

• Phosphate carried by water may reach lakes and oceans and accumulate in the form of sediments.

• Over long geological periods, these sediments may form new phosphate rocks.

• Tectonic uplift brings these rocks back to the Earth’s surface, and the process of weathering begins again.

• The phosphorus cycle does not have a significant natural gaseous phase.

• Due to the absence of a gaseous phase, the phosphorus cycle is comparatively slower than the carbon or nitrogen cycle.

The carbon cycle is not the correct answer.

• The principal active reservoirs of carbon occur in the atmosphere, oceans, biosphere and soil.

• Carbon dioxide enters the biosphere through photosynthesis.

• Respiration, decomposition, combustion and ocean-atmosphere gas exchange are major processes of the carbon cycle.

• Weathering of rocks affects the carbon cycle, but it is not the main source through which carbon enters the cycle.

The nitrogen cycle is not the correct answer.

• The largest reservoir of nitrogen is the atmosphere, which contains approximately 78% nitrogen gas.

• Most organisms cannot directly use atmospheric N₂.

• Biological nitrogen fixation, lightning and industrial fixation convert N₂ into useful nitrogen compounds.

• Therefore, weathering of rocks is not the main source of the nutrient in the nitrogen cycle.

The sulphur cycle is not the correct answer.

• Sulphur occurs in rocks and sediments, and weathering can release sulphate.

• However, volcanic emissions, sea spray, biological decomposition and the combustion of fossil fuels are also important sources of sulphur.

• The sulphur cycle also includes gaseous compounds such as sulphur dioxide and hydrogen sulphide.

• Therefore, among the given options, the cycle most directly dependent upon rock weathering is the phosphorus cycle.

Biological importance of phosphorus

• Phosphorus is a component of the phosphate backbone in DNA and RNA.

• It is necessary for the storage and transfer of energy in ATP.

• It is an important component of cell membranes in the form of phospholipids.

• Calcium phosphate provides structure and hardness to bones and teeth.

• Phosphorus can be a limiting nutrient in many ecosystems.

• Excessive entry of phosphate from fertilisers and sewage into water bodies can accelerate eutrophication.