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?

Correct Answer: Phosphorus cycle

Explanation:

Biogeochemical cycles represent the movement of essential chemical elements between living organisms and the non-living environment (atmosphere, hydrosphere, and lithosphere). These cycles are broadly categorized into two types based on their primary reservoir:

1. Gaseous Cycles: The primary reservoir for these nutrients is the atmosphere or hydrosphere. Examples include the carbon cycle, nitrogen cycle, and oxygen cycle. In these cycles, nutrients are mainly replenished through atmospheric processes or gaseous exchanges.

2. Sedimentary Cycles: The main reservoir for sedimentary cycles is Earth's crust (rocks and soil). The primary mechanism for the release of these nutrients into ecosystems is the weathering of rocks. Examples include the phosphorus cycle and sulphur cycle, with phosphorus being the primary textbook example where weathering is the ultimate source of input.

Detailed Analysis of the Options:

• Carbon cycle: The primary reservoir is the atmosphere (as carbon dioxide gas, ${\mathrm{CO}}_2$) and oceans.

• Nitrogen cycle: The main reservoir is the atmosphere, which consists of approximately 78% nitrogen gas ($N_2$).

• Phosphorus cycle: Phosphate rocks are the natural reservoir for phosphorus. Over time, rainfall and weathering processes cause rocks to release inorganic phosphate ions into the soil and water, which are then absorbed by plants and passed through the food web. Hence, rock weathering is the primary source of nutrient release for the phosphorus cycle.

• Sulphur cycle: Although sulphur has sedimentary reservoirs, it also has a significant gaseous phase (such as sulfur dioxide and hydrogen sulfide in the atmosphere), making phosphorus the quintessential sedimentary cycle driven predominantly by rock weathering.

Therefore, the Phosphorus cycle is the correct answer.