Ecosystem

Phosphorus cycling is classified as a sedimentary cycle because the great reservoir of phosphorus lies in rocks, which contain phosphorus in the form of phosphates. Weathering of these rocks releases phosphate ions into the soil and water, where producers absorb them; there is no appreciable respiratory release of phosphorus into the atmosphere. This contrasts sharply with the carbon cycle, which is a gaseous cycle with a large atmospheric reservoir of carbon dioxide. The option naming the atmosphere as gaseous oxides is incorrect because phosphorus has no stable gaseous form that participates meaningfully in the cycle. The deep oceans hold phosphorus as dissolved phosphates in sediments, not as dissolved gases, so that option is wrong. Living biomass contains organic phosphate but represents only a small, temporary pool, not the major reservoir. As described in NCERT Class 12, Chapter 14 (Ecosystem), nutrient cycles are of two types, gaseous and sedimentary, with phosphorus exemplifying the sedimentary type. The cycling of phosphorus through living organisms is also much slower than that of carbon, and most phosphate that washes into the sea is locked into ocean sediments for long geological periods before returning to land. Plausibility check: the absence of a gaseous phase logically follows from a rock-bound reservoir, making the sedimentary-rock answer internally consistent.