Nutrient Cycling — Carbon Cycle

NCERT grounding

The NCERT Class 12 chapter Ecosystem closes its discussion of ecosystem function with nutrient cycling. It states that the storage and movement of nutrient elements through the various components of an ecosystem is called nutrient cycling, and that nutrients are repeatedly used through this process. The chapter further classifies nutrient cycles into two types — gaseous and sedimentary — and names the atmosphere or hydrosphere as the reservoir for the gaseous type, with carbon given as the example, while the Earth's crust is the reservoir for the sedimentary type, with phosphorus given as the example.

The NIOS supplement Principles of Ecology reinforces this. It defines the cycling of nutrients in the biosphere as the biogeochemical or nutrient cycle, notes that unlike energy — which flows unidirectionally — nutrients are continuously exchanged between organisms and their environment, and lists photosynthesis, respiration, decomposition and combustion as the important carbon-cycle processes. Because the planet receives no fresh input of these elements, the same carbon atoms are used over and over again.

The carbon cycle in depth

Carbon is the structural backbone of every organic molecule — carbohydrates, proteins, lipids and nucleic acids are all carbon skeletons. Carbon constitutes about 49 per cent of the dry weight of living organisms, making it second only to water among the constituents of the body of an organism. The carbon cycle is the set of pathways by which carbon atoms move between the atmosphere, the oceans and living and dead organisms. It is the standard NEET example of a gaseous-type cycle because the bulk of its actively cycling carbon sits in the atmosphere as carbon dioxide.

Carbon cycling proceeds through the atmosphere, the ocean and through living and dead organisms. Atmospheric carbon dioxide is the common source of carbon. It is highly soluble in water, so the oceans hold large quantities of dissolved carbon dioxide and bicarbonate ions, forming a second major mobile reservoir. The living portion of the cycle begins when producers fix gaseous carbon dioxide and ends when respiration, decomposition or combustion oxidises that organic carbon back to carbon dioxide. According to one estimate, 4 × 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually — fixed carbon then moves along food chains as biomass from producers to consumers.

Because the carbon cycle has an atmospheric reservoir, it is fast-cycling — carbon released by a respiring organism can be re-fixed by a leaf within the same growing season. This is the central contrast NEET draws with sedimentary cycles. A sedimentary cycle such as the phosphorus cycle locks most of its element in rock and releases it only slowly by weathering.

Carbon reservoirs and pools

A reservoir pool is a large, slow-moving store of an element; an exchange pool is a smaller, fast-moving fraction organisms draw on directly. Four carbon reservoirs matter for NEET — the atmosphere, the ocean, the biosphere and the sediments including fossil fuels. Recognising which reservoir a process draws from or adds to is the quickest route to a correct answer.

Fossil fuel is the product of complete or partial decomposition of plants and animals subjected to heat and pressure within the Earth's crust over millions of years. Until humans began burning it, this sedimentary store was isolated from the fast atmospheric cycle. Some carbon is also continually lost to sediments and removed from circulation as the calcareous shells of marine organisms settle to the ocean floor and lithify into limestone.

The fluxes that move carbon

A flux is a transfer of carbon between two reservoirs. The carbon cycle is best memorised as a balance sheet: one process moves carbon out of the atmosphere, and several move it back in. NEET phrases questions around naming these fluxes and identifying their direction.

Photosynthesis — the fixing flux

Terrestrial and aquatic plants utilise carbon dioxide for photosynthesis, converting the inorganic form of carbon into organic matter in the presence of sunlight and chlorophyll. Part of this carbon is used by plants for their own life processes, and the rest is stored as biomass available to heterotrophs as food. This is the only flux that moves carbon out of the atmospheric reservoir on a large scale, accounting for the estimated 4 × 10¹³ kg fixed each year.

Respiration and decomposition — the biological return

Respiration is essentially the reverse of photosynthesis: food is oxidised to liberate energy, releasing carbon dioxide and water. Through the respiratory activities of producers and consumers, atmospheric carbon dioxide is continually recovered. After organisms die, decomposers break down the dead organic matter and release the leftover carbon. Together, respiration and decomposition return a considerable amount of carbon and keep the cycle turning year on year.

Combustion and volcanic activity — the abiotic return

A large quantity of carbon is added to the atmosphere through the burning of wood, forest fires and the combustion of fossil fuels and other organic matter. Volcanic activity also returns carbon dioxide stored within the crust. Forests act like carbon reservoirs because the carbon they fix cycles very slowly through their long lives — but forest fires release that stored carbon rapidly.

Human impact on the carbon cycle

Human activities have significantly influenced the carbon cycle. The natural cycle was roughly balanced — photosynthetic fixation was offset by respiration, decomposition and natural combustion. Two human activities have tilted that balance toward net release of carbon dioxide.

First, rapid deforestation removes a major carbon-fixing sink and, when the cleared vegetation is burned, releases its stored carbon at once. Second, the massive burning of fossil fuels for energy and transport transfers carbon from the slow sedimentary reservoir, where it had been locked for millions of years, into the fast atmospheric reservoir within moments. Industrialisation, urbanisation and the increased use of automobiles have all accelerated this release.

The result is a steady rise in atmospheric carbon dioxide. Because carbon dioxide is a greenhouse gas, this increase intensifies the greenhouse effect and drives global warming — and NEET pairs this with the fact that carbon dioxide and methane are the gases mainly responsible for the greenhouse effect.

Worked examples

Common confusion & NEET traps

Most carbon-cycle errors at NEET come from mixing up the two cycle types, misreading flux direction, or confusing the carbon reservoir with the phosphorus reservoir. The cluster below isolates the recurring mistakes.