Correct Option
The phosphorus cycle is a sedimentary biogeochemical cycle. Its primary reservoir is the Earth's crust, where phosphorus exists as phosphate minerals within rocks. The weathering of these phosphate-rich rocks is the main natural process that releases phosphorus (in the form of phosphate ions) into the soil and water. From there, it becomes available for absorption by plants, initiating its movement through the food web. This cycle is distinct as it lacks a significant gaseous phase, making rock weathering the predominant source of its entry into the active biological cycle.
Incorrect Options
Carbon cycle: The primary reservoirs of carbon are the atmosphere (as CO₂), oceans, and fossil fuels. Carbon enters the active cycle mainly through photosynthesis (atmospheric CO₂) and is released via respiration, decomposition, and combustion. Rock weathering is not the primary source for its entry into the active cycle.
Nitrogen cycle: The atmosphere, containing N₂ gas, constitutes the largest reservoir of nitrogen. Nitrogen enters the biosphere predominantly through biological nitrogen fixation by microorganisms and atmospheric fixation (e.g., lightning), which convert atmospheric N₂ into usable forms. Rock weathering is not a significant primary source for the nitrogen cycle.
Sulphur cycle: Sulphur is found in the atmosphere (as SO₂), oceans (as sulphates), and in the Earth's crust in mineral deposits. While some sulphur can be released through rock weathering, major natural inputs to the atmosphere and active cycle also include volcanic eruptions, decomposition of organic matter, and sea spray. Rock weathering is not the predominant initial release mechanism for the global sulphur cycle.