UPSC Prelims 2021 Q27: Biogeochemical Cycles and Rock Weathering
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?
- Carbon cycle
- Nitrogen cycle
- Phosphorus cycle
- Sulphur cycle
Answer: (c) Phosphorus cycle
Statement-wise explanation
- a: incorrect — Carbon follows a gaseous cycle. It enters living systems mainly from atmospheric and dissolved CO2 through photosynthesis, and goes back through respiration, decomposition and combustion. NCERT Class 12 Biology notes that about 71% of the Earth's carbon is dissolved in the oceans. Weathering of carbonate and silicate rocks is part of the long-term carbon cycle, but it is a slow geological flow and not the main release.
- b: incorrect — The main reservoir of nitrogen is the atmosphere, which is about 78% N2. Nitrogen enters the living world through biological fixation by microbes such as Rhizobium, through lightning, and through industrial fixation (the Haber–Bosch process). Rocks supply only a minor amount of nitrogen, so weathering is not the main way it enters the cycle.
- c: correct — NCERT Class 12 Biology (Ecosystem chapter) says the natural reservoir of phosphorus is rock that holds phosphorus as phosphate, mostly the mineral apatite. When these rocks weather, small amounts of phosphate dissolve in soil water and plant roots absorb them. Gaseous exchange of phosphorus is negligible, so weathering is effectively its only natural entry point.
- d: incorrect — Sulphur is a sedimentary cycle with a large gaseous phase. Weathering of rocks such as pyrite and gypsum releases sulphate. But sulphur also enters the cycle as SO2 from volcanoes and fossil-fuel burning, as H2S from decomposition, and as dimethyl sulphide (DMS) given off by ocean plankton. With so many atmospheric routes, weathering is one input among several, not the main one.
How to eliminate
Step 1 uses basic knowledge. Carbon and nitrogen both have large atmospheric reservoirs: CO2 used in photosynthesis, and N2 at 78% of the air. Their main inputs come from the air, not from rock, so you can drop both. Step 2 needs one fact. Phosphorus and sulphur are both sedimentary cycles, and to choose between them you have to know that phosphorus has no significant gaseous phase. Sulphur also enters through volcanic SO2, H2S and marine DMS. The fact to remember is that the natural reservoir of phosphorus is rock (phosphate), and weathering is how it enters the soil and plants.
Know more
Gaseous vs sedimentary cycles
Biogeochemical cycles move nutrients between living things and the non-living environment. They fall into two types by where the main reservoir sits:
- Gaseous cycles (carbon, nitrogen, oxygen) have their reservoir in the atmosphere or ocean. They respond fast and buffer themselves fairly well.
- Sedimentary cycles (phosphorus, sulphur, calcium, potassium) have their reservoir in the Earth's crust. Nutrients come out slowly through weathering and erosion.
| Cycle | Type | Main reservoir | Main natural input | Significant gaseous phase? |
|---|---|---|---|---|
| Carbon | Gaseous | Oceans (dissolved), atmosphere, fossil deposits | CO2 fixed by photosynthesis | Yes (CO2, CH4) |
| Nitrogen | Gaseous | Atmosphere (78% N2) | Biological and lightning fixation | Yes (N2, N2O, NH3) |
| Sulphur | Sedimentary | Rocks, ocean sulphate | Weathering, volcanoes, DMS, decomposition | Yes (SO2, H2S, DMS) |
| Phosphorus | Sedimentary | Phosphate rock (apatite) | Weathering of rocks | Negligible |
Why phosphorus is special
- It has no meaningful gaseous form, so there is no quick route back through the atmosphere. Unlike carbon, it is not released by respiration.
- Phosphate washed into the sea settles in ocean sediments. It comes back only when geological uplift exposes those rocks again, which takes millions of years. This is why the phosphorus cycle is often called 'imperfect' or slow.
- Because the supply is slow, phosphorus is often a limiting nutrient. Extra phosphate from fertiliser and detergent runoff drives eutrophication and algal blooms.
- Farms depend on mined rock phosphate, which is used to make DAP and NPK fertilisers. Rock phosphate is non-renewable, and India imports most of what it uses.
NCERT Concepts to Revise
- Chemical weathering: The question turns on which nutrient enters the cycle mainly when chemical reactions break down its rock reservoir.