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?

Answer: C. The PHOSPHORUS CYCLE is the biogeochemical cycle where weathering of rocks is the main source. Biogeochemical cycles are classified by where the bulk reservoir of the nutrient is held: GASEOUS cycles (atmospheric reservoir) or SEDIMENTARY cycles (lithospheric reservoir). PHOSPHORUS CYCLE (C) — CORRECT. Phosphorus has NO STABLE GASEOUS PHASE under natural environmental conditions. Its principal reservoir is PHOSPHATIC ROCKS (apatite minerals). PHOSPHORUS ENTERS THE BIOGEOCHEMICAL CYCLE primarily through the WEATHERING OF PHOSPHATE-BEARING ROCKS, which releases phosphate ions (PO4 3-) into soil and water. From there phosphorus is taken up by plants, moves through food chains, and eventually returns to sediments. It is the classic example of a SEDIMENTARY CYCLE. This makes phosphorus a critical limiting nutrient and a non-renewable resource on human timescales — there is no significant atmospheric flux. Distractors: (A) CARBON CYCLE — predominantly GASEOUS. Atmospheric CO2 is the main exchangeable reservoir; photosynthesis and respiration drive flux. Weathering of carbonate rocks is a smaller, slower component. (B) NITROGEN CYCLE — GASEOUS. Atmospheric N2 (78%) is the main reservoir. Biological nitrogen fixation (Rhizobium, cyanobacteria), lightning, and industrial Haber-Bosch process bring N2 into the cycle. Rocks are NOT the main nitrogen source. (D) SULPHUR CYCLE — MIXED (both gaseous and sedimentary). It involves both atmospheric (SO2, H2S, DMS) and sedimentary (gypsum, pyrite weathering) fluxes. Volcanic emissions and biological reduction are significant gaseous components alongside rock weathering. Source: NCERT Class 12 Biology Chapter 'Ecosystem' (nutrient cycling section).