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Most students meet nutrient cycles as a pair of diagrams with arrows. That is enough to recognise the cycles but not to answer a question that gives you numbers, or asks why phosphorus behaves so differently from carbon. This post explains both cycles, uses the latest published carbon budget for a worked example, and shows how to avoid the unit errors that trip people up.
Where is this topic in the syllabus?
The current NCERT ecosystem chapter keeps nutrient cycling only in its summary, which says that nutrient cycling is of two types, gaseous and sedimentary, with the atmosphere or hydrosphere as the reservoir for carbon and the Earth's crust for phosphorus. The NEET (UG) 2026 syllabus does not list nutrient cycles in its ecosystem line. The HBCSE syllabus for NSEB and INBO does, along with ecological succession. So learn the NCERT summary for school and NEET, and the detail below for olympiads and for understanding climate news. Check the current syllabus for your exam.
How does the carbon cycle work?
Carbon moves between four large stores: the atmosphere (mainly as CO2), the oceans (dissolved CO2 and carbonates), living things and soil, and rocks and fossil fuels.
Into living things: photosynthesis fixes CO2 into sugars. See our post on photosynthesis for how.
Back to the air: respiration by plants, animals and decomposers, plus fires.
Ocean exchange: CO2 dissolves in sea water and is released again.
Slow paths: carbon buried as sediments and fossil fuels stays out of the cycle for millions of years, unless it is burned.
Humans have added a fast path from the slow store to the air. The Global Carbon Budget 2024, published in Earth System Science Data, gives these figures for 2023, in billions of tonnes of carbon (GtC):
Fossil fuel emissions: 10.1 GtC.
Land-use change: 1.0 GtC.
Total human emissions: 11.1 GtC, or 40.6 billion tonnes of CO2.
Growth in the atmosphere: 5.9 GtC, equal to 2.79 ppm.
Taken up by the ocean: 2.9 GtC; by land: 2.3 GtC.
The same paper reports that atmospheric CO2 averaged 419.31 ppm in 2023, and preliminary data put 2024 at 422.45 ppm, 52 per cent above the pre-industrial level of about 278 ppm.
Worked example: where did 2023's carbon go?
Fraction staying in the air: 5.9 / 11.1 = 0.53. About 53 per cent of the year's emissions stayed in the atmosphere.
Ocean and land: 2.9 / 11.1 = 26 per cent; 2.3 / 11.1 = 21 per cent. The three add to 100 per cent, because the budget balances almost exactly in 2023.
Carbon per ppm: 5.9 GtC / 2.79 ppm = about 2.1 GtC for each ppm of CO2 in the atmosphere.
Carbon to CO2: multiply by 44/12, the ratio of the molar masses of CO2 and C. 11.1 x 44/12 = 40.7, which matches the paper's 40.6 within rounding.
Rise since 1750: 422.45 / 278 = 1.52, a 52 per cent increase.
Why is the phosphorus cycle so different?
Phosphorus has no significant gas phase, so it does not cycle through the air. Its reservoir is phosphate in rocks.
Release: weathering of rocks frees phosphate into soil and water.
Uptake: plants absorb phosphate; animals get it by eating plants. It ends up in DNA, ATP, membranes, bones and teeth.
Return: decomposers release phosphate from dead matter back to the soil.
Loss: phosphate washed into rivers and the sea settles into sediments, which become rock again only over geological time.
Because it returns to land so slowly, phosphorus mined for fertiliser is effectively a one-way flow. The US Geological Survey's 2026 summary on phosphate rock estimates world reserves at about 73 billion tonnes, of which Morocco holds 50 billion tonnes and India 31 million tonnes, and states that "there are no substitutes for phosphorus in agriculture". Elser and Bennett called this a broken biogeochemical cycle in Nature.
Worked example: phosphate reserves
Morocco's share: 50 / 73 = 0.68, so about 68 per cent of world reserves.
India's share: 31 million tonnes is 0.031 billion tonnes. 0.031 / 73 = 0.0004, about 0.04 per cent of world reserves.
Unit check: the USGS table is in thousand metric tons, so its entry of 31,000 for India means 31 million tonnes. Reading the units first stops you being off by a factor of a thousand.
Common mistakes
Giving phosphorus a gaseous phase. It cycles through rock, soil, water and living things.
Mixing carbon and CO2 units. Always convert with 44/12.
Thinking all emitted CO2 stays in the air. In 2023, ocean and land took up nearly half.
Calling a ppm value a mass. ppm is a concentration; convert with about 2.1 GtC per ppm.
A practice routine
Draw both cycles from memory and label each arrow with its process.
Redo the carbon example with your own numbers: if emissions were 12 GtC and the atmosphere grew by 6.5 GtC, what fraction stayed in the air?
Write three differences between gaseous and sedimentary cycles.
Link this to energy flow in our ecology questions guide.
If a diagram of moving stores is easier to follow when it moves, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Tonight, convert 40.6 billion tonnes of CO2 back into carbon and check that you get about 11.1.
