Biogeochemical cycles

The first principle

The Earth receives a steady stream of energy from the sun but almost no new matter, so every atom of carbon, nitrogen, phosphorus and sulphur in a living body has been used before and will be used again. A biogeochemical cycle is the path such an element takes between living things (bio), rocks, soil, water and air (geo), driven by chemical change (chemical). Each element sits mostly in a large, slow store and moves through living things in a small, fast loop, and human beings now move some elements faster than nature does, which is the root of climate change, acid rain and dead lakes.

The core

Two kinds of cycle

• Gaseous and sedimentary. In a gaseous cycle the main store is the air or the ocean, so the element spreads quickly and local shortages correct themselves, as with carbon and nitrogen. In a sedimentary cycle the store is rock, so the element returns only over geological time and can be exhausted locally, as with phosphorus; sulphur has stores in both rock and air.

The carbon cycle

• The fast loop. Plants and algae take carbon dioxide (CO₂) from the air and fix it by photosynthesis, while respiration, decomposition and fire release it again.
• The ocean and the rocks. CO₂ dissolves in seawater to form carbonic acid and then bicarbonate, and marine organisms lock carbon into shells of calcium carbonate that settle as sediment and, over millions of years, become limestone. Fossil fuels are carbon buried in the same slow way, and burning them returns in decades what took millions of years to store.
• How far we have pushed it. The Global Carbon Budget 2025 put fossil CO₂ emissions at about 38.1 billion tonnes in 2025, and total human emissions, including land use change, at about 42.2 billion tonnes. Land and ocean sinks together remove only about half of this, so the concentration of CO₂ in the air rose to just above 425 parts per million, against about 280 before the industrial age.
• Ocean acidification. The CO₂ the ocean absorbs makes seawater more acidic, and the pH of the surface ocean has fallen by about 0.1 since pre industrial times, which is roughly a 30 per cent rise in acidity and makes it harder for corals and shellfish to build skeletons.

The nitrogen cycle

• Locked in the air. Nitrogen is about 78 per cent of the atmosphere, but the triple bond in the N₂ molecule makes it unusable by most organisms, so life depends on fixation, which converts it into ammonia.
• Who fixes it. Rhizobium bacteria fix nitrogen in the root nodules of legumes, free living Azotobacter does so in soil, and cyanobacteria such as Anabaena and Nostoc do so in water and paddy fields, often inside the water fern Azolla. The enzyme nitrogenase works only without oxygen, which is why nodules contain leghaemoglobin to mop it up. Lightning fixes a little, and the industrial Haber–Bosch process, developed in the early twentieth century, now fixes about as much as all natural processes on land.
• The microbial relay. Decomposers release ammonia from dead matter, which is ammonification; Nitrosomonas oxidises ammonium to nitrite and Nitrobacter oxidises nitrite to nitrate, which is nitrification; and in waterlogged, oxygen poor soils bacteria such as Pseudomonas reduce nitrate back to N₂, which is denitrification and closes the loop.

Phosphorus, sulphur and water

• Phosphorus. Phosphorus is released only by the weathering of rock, it has no significant gaseous phase, and much of what reaches the sea sinks into sediments and is lost to living things for millions of years. It is therefore often the scarcest nutrient in fresh water, and mycorrhizal fungi on roots help plants take up the little that soil holds.
• Sulphur. Sulphur enters the air as sulphur dioxide (SO₂) from volcanoes and from burning coal and oil, and as dimethyl sulphide given off by marine phytoplankton, whose sulphate particles help clouds form. In the air SO₂ turns into sulphuric acid and falls as acid rain, while in oxygen free mud bacteria reduce sulphate to hydrogen sulphide, the gas that gives polluted drains their smell.
• Water. Solar energy drives evaporation from the oceans, which hold about 97 per cent of the Earth's water, and transpiration from plants returns a large share of rainfall on land to the air. A warmer atmosphere holds about 7 per cent more water vapour for each degree Celsius, which is why warming brings heavier downpours; rainfall patterns are covered in GS1 Geography and groundwater in the water resources note.

Dimensions

• Planetary. The planetary boundaries framework, proposed by Johan Rockström and colleagues in 2009, sets safe limits for nine Earth processes, and the Planetary Health Check of September 2025 found seven of them crossed, including the flows of nitrogen and phosphorus and, for the first time, ocean acidification.
• Climate. Nitrous oxide (N₂O) from fertilised soils has a global warming potential of about 273 times that of CO₂ over a hundred years in the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report, so farm nitrogen is a climate issue as well as a water issue.
• Health. Nitrate from fertiliser seeps into groundwater and is unsafe for infants to drink, ammonia from fields forms fine particles in the air, and SO₂ from coal plants damages lungs.
• Economic and strategic. Phosphate rock is mined in only a few countries, so a country that imports most of its phosphate fertiliser, as India does, carries a supply risk for its food security.

In practice

• India's nitrogen problem. The Indian Nitrogen Assessment of 2017, prepared by the Indian Nitrogen Group of the Society for Conservation of Nature, found that only about 33 per cent of the fertiliser nitrogen applied to rice and wheat is taken up by the crop, while the rest is lost to air, soil and water. It identified agriculture as the main source of nitrogen pollution in India and cereals as the largest within it, and urea, whose price has stayed controlled while other fertilisers were decontrolled after 1991, encourages overuse.
• Making urea work harder. The government made all domestically produced urea neem coated in 2015, because the neem oil slows the conversion of urea and so cuts losses and diversion to industry. The Soil Health Card scheme, also launched in 2015, gives farmers a test based recommendation for each nutrient, and nano urea, introduced by the Indian Farmers Fertiliser Cooperative (IFFCO) in 2021, is sprayed on leaves to reduce the bulk applied to soil.
• Rewarding States that use less. The PM Programme for Restoration, Awareness, Generation, Nourishment and Amelioration of Mother Earth (PM-PRANAM), approved in June 2023, passes part of the subsidy saved to States that reduce the use of chemical fertilisers.
• India at the global table. India piloted the resolution on sustainable nitrogen management at the fourth United Nations Environment Assembly in 2019, and the Colombo Declaration of the same year, backed by the United Nations Environment Programme (UNEP), set the aim of halving nitrogen waste by 2030.
• Cutting sulphur. Bharat Stage VI fuel, in force across India from 1 April 2020, carries no more than 10 parts per million of sulphur. The Environment (Protection) Amendment Rules of December 2015 set limits on SO₂ from coal power plants that require flue gas desulphurisation units, though the deadlines for fitting them have been extended several times.

Issues and way forward

• Cheap urea skews fertiliser use. Because urea is far cheaper than phosphate and potash, farmers apply too much nitrogen and too little of the other nutrients, which lowers yields per bag and raises losses; bringing nitrogen into a nutrient based subsidy and paying for soil tested, balanced use are the reforms most often recommended.
• Losses are invisible to the farmer. Nitrate in a well or N₂O in the air costs the farmer nothing, so nitrogen use efficiency should be tracked as a national indicator, and practices such as split doses, deep placement of urea, legumes in rotation and biofertilisers should be extended at scale.
• Sinks are not keeping pace. The land and ocean sinks have stopped growing in step with emissions, and forest fires, drought and warming seas may weaken them further, so protecting forests, wetlands and soil carbon matters alongside cutting fossil fuel use.
• Phosphorus is mined once and lost. Most phosphorus ends up in rivers and the sea, so recovering it from sewage, manure and food waste and reducing runoff from fields would ease both import dependence and eutrophication.

Common confusions

• Nitrification and denitrification. Nitrification turns ammonium into nitrate in well aerated soil and keeps nitrogen in the soil, while denitrification turns nitrate into N₂ gas in waterlogged soil and returns it to the air.
• Nitrogen fixation and nitrogen assimilation. Fixation turns N₂ gas into ammonia and is done by microbes, lightning or industry, while assimilation is the uptake of ammonium or nitrate by plants to build proteins.
• Ocean acidification and ocean warming. Acidification is a chemical change caused by dissolved CO₂, while warming is a physical change caused by trapped heat; both harm corals, but they would continue separately even if the other stopped.
• Gaseous and sedimentary cycles. A gaseous cycle has its main store in the air or ocean and recovers quickly, while a sedimentary cycle has its store in rock and recovers only over geological time.

For the exam

• Keyword bank: gaseous and sedimentary cycles, carbon sink, ocean acidification, biological nitrogen fixation, nitrification, denitrification, reactive nitrogen, nitrogen use efficiency, neem coated urea, balanced fertilisation, phosphorus runoff, eutrophication, acid rain, flue gas desulphurisation, planetary boundaries.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: Only about a third of the nitrogen fertiliser applied to rice and wheat in India is taken up by the crop, and the rest pollutes air, soil and water (Indian Nitrogen Assessment, 2017).
  • Introduction with a report: The Planetary Health Check of 2025 found seven of nine planetary boundaries crossed, including the flows of nitrogen and phosphorus that sustain agriculture.
  • Conclusion: Matter on Earth is finite and circulates, so every element we add faster than the cycle can absorb, whether carbon, nitrogen or sulphur, returns as a pollutant somewhere else.
  • Conclusion: Balanced fertilisation, cleaner fuels and protected carbon sinks together would let India grow food and energy without overloading the cycles that sustain them.