In the 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? [2021]

In the 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?

Carbon cycle
Nitrogen cycle
Phosphorus cycle
Sulphur cycle

Explanation

Option (c) is correct

Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems. Many animals also need large quantities of this element to make shells, bones and teeth. The natural reservoir of phosphorus is rock, which contains it as phosphates. When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by plant roots. Herbivores and other animals obtain this element from plants. The waste products and dead organisms are decomposed by phosphate-solubilising bacteria, which release phosphorus. Unlike carbon cycle, there is no respiratory release of phosphorus into atmosphere.

Diagram illustrating phosphate cycle in sedimentary environments, showing processes like weathering of phosphate rocks, geological uplifting, runoff, leaching, sedimentation, and roles of plants, microbes, and decomposers. Key components include labeled arrows indicating phosphate movement through soil, solution, detritus, and highlighting interactions between terrestrial and aquatic systems.

Option (a) is incorrect

The primary sources of carbon include the atmosphere (in the form of carbon dioxide), living organisms, and fossil fuels. Carbon enters the cycle through processes like photosynthesis, respiration, decomposition, and combustion. While weathering can release some carbon dioxide trapped in rocks, it’s not the primary source for the cycle.

Diagram illustrating carbon cycle showing movement of carbon through photosynthesis, respiration, decomposition, and emissions. Key components include producers, animals, decaying organisms, factories, ocean with phytoplankton, and arrows indicating carbon dioxide flow, organic carbon, and fossil fuel formation.

Option (b) is incorrect

The nitrogen cycle describes the conversion of nitrogen between different chemical forms. The primary source of nitrogen is the atmosphere, where it exists as nitrogen gas (N₂). Key processes in the nitrogen cycle include nitrogen fixation, nitrification, assimilation, ammonification, and denitrification.

Diagram illustrating nitrogen cycle showing processes and key bacteria involved. It highlights atmospheric nitrogen conversion by nitrogen-fixing bacteria, ammonium formation through decomposition, nitrification to nitrites and nitrates by nitrifying bacteria, assimilation by plants, and denitrification returning nitrogen to atmosphere.

Option (d) is incorrect

Major sources of sulphur include volcanic activity, the burning of fossil fuels, and the decomposition of organic matter. The weathering of rocks can release sulphur, but it is not the primary pathway.

Diagram illustrating sulfur cycle showing processes like mining, combustion of fossil fuels, plant uptake, decomposition, and acid precipitation. Key elements include sulfur compounds in atmosphere, soil, water, and organisms, with labeled arrows indicating transformations such as sulphate reduction, organic deposition, and dry deposition of sulfur dioxide.

Answer: (c) Phosphorus cycle; Difficulty Level: Easy

2021, PYQs