Question 78 from UPSC CSE 2021 - Environment

Question

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?

Accepted Answer

Correct Option: C. Phosphorus cycle

Strategy: Identify the biogeochemical cycle that primarily relies on geological sources, specifically the weathering of rocks. Eliminate cycles with significant atmospheric components (carbon, nitrogen, sulphur) and focus on the phosphorus cycle, which is characterized by rock weathering as its main source.

Key Info: ## Phosphorus Cycle
The phosphorus cycle is unique among major biogeochemical cycles because it lacks a significant atmospheric component.

- **Source:** Its primary natural source is the weathering of phosphate-containing rocks.
- **Role:** Phosphorus is a key nutrient essential for DNA, ATP, and cellular membranes.
- **Process:** Weathering releases inorganic phosphate ions into soils and water, where plants absorb them and the nutrient enters the food web.
- **Return Path:** Phosphorus is eventually returned to rock deposits through sedimentation over geological time scales.
- **Limitation:** Phosphorus availability often limits freshwater productivity due to its slow geological cycle.
- **Human Impact:** Human activities, such as phosphate fertilizer use and mining, have accelerated phosphorus movement, but the natural supply remains dependent on rock weathering.

### Other Biogeochemical Cycles
- **Carbon Cycle:** Primarily involves photosynthesis, respiration, and CO2 exchange with the ocean and atmosphere.
- **Nitrogen Cycle:** Dominated by atmospheric nitrogen fixation, nitrification, and denitrification.
- **Sulphur Cycle:** Involves both atmospheric sources (e.g., volcanic emissions) and lithospheric sources (rock weathering), but significant flux occurs through gaseous intermediates, making rock weathering a secondary source at the ecosystem level.

Answer

Option A: Carbon cycle

Answer

Option B: Nitrogen cycle

Answer

Option D: Sulphur cycle