Biogeochemical Cycles Explained: Carbon, Nitrogen, Phosphorus and Water Cycles (Animated)

Energy flows through ecosystems and is lost as heat, but the chemical elements that make up living things are used again and again. The carbon atoms in your body may once have been part of a dinosaur, a tree or the atmosphere of an ancient Earth. The pathways by which elements move between living organisms (the “bio”), rocks, soil and water (the “geo”) and chemical reactions (the “chemical”) are called biogeochemical cycles.

What you will learn

The difference between pools and fluxes, and between gaseous and sedimentary cycles
How the carbon, nitrogen, phosphorus, sulphur and water cycles work
The role of microbes in nutrient cycling
How human activities have altered each cycle

Pools, fluxes and cycle types

A pool (or reservoir) is where an element is stored, such as the atmosphere, the ocean, soils, living things or rocks.
A flux is the rate at which an element moves between pools, for example the carbon taken up by plants each year.
Residence time is how long, on average, an atom stays in a pool: days for water in the atmosphere, thousands of years for carbon in the deep ocean, and millions of years for carbon in limestone.

Cycles are often grouped into:

Gaseous cycles, with a large pool in the atmosphere: carbon, nitrogen and oxygen. These cycle relatively quickly.
Sedimentary cycles, with their main pool in rocks: phosphorus and, largely, sulphur. These cycle slowly.

The carbon cycle

Carbon is the backbone of all organic molecules. It moves through two linked cycles.

The fast (biological) cycle. Plants and algae take carbon dioxide from the air or water during photosynthesis and build it into sugars. Plants, animals, fungi and microbes release it again through respiration and decomposition. Every year land plants take up roughly 120 billion tonnes of carbon, and almost as much returns to the atmosphere. The oceans exchange a similar amount with the air.

The slow (geological) cycle. Over millions of years, carbon is locked into limestone, from the shells of marine organisms, and into fossil fuels, from buried plants and plankton. Volcanoes and weathering slowly return it.

The human effect. By burning fossil fuels and clearing forests, people move carbon from the slow cycle into the fast cycle far quicker than natural processes can remove it, currently about 10 billion tonnes of carbon a year. Roughly half of this stays in the atmosphere; the rest is absorbed by the oceans and by land plants. As a result, atmospheric carbon dioxide has risen from about 280 parts per million before the Industrial Revolution to over 420 ppm today. The carbon absorbed by the ocean makes seawater more acidic, a process called ocean acidification, which makes it harder for corals, shellfish and some plankton to build their skeletons.

The nitrogen cycle

Nitrogen makes up about 78 percent of the atmosphere, but most organisms cannot use nitrogen gas (N₂) directly. It must first be converted into forms such as ammonium or nitrate. Almost every step is carried out by microbes:

Nitrogen fixation: bacteria convert N₂ into ammonia. Some live freely in soil and water, and others, such as Rhizobium, live in nodules on the roots of legumes like beans, peas and acacias. Lightning also fixes a small amount.
Nitrification: other bacteria convert ammonium into nitrite and then nitrate, which plants absorb easily.
Assimilation: plants take up ammonium and nitrate to make proteins and DNA; animals obtain nitrogen by eating plants.
Ammonification: decomposers convert the nitrogen in dead organisms and waste back into ammonium.
Denitrification: in waterlogged, oxygen-poor soils and sediments, bacteria convert nitrate back into N₂ gas, completing the cycle.

The human effect. The Haber–Bosch process, developed in the early twentieth century, makes ammonia fertiliser from atmospheric nitrogen. Humans now fix roughly as much nitrogen as all natural land processes combined. Fertiliser feeds billions of people, but run-off causes algal blooms and “dead zones” such as the one in the Gulf of Mexico, and nitrous oxide from soils is a powerful greenhouse gas.

The phosphorus cycle

Phosphorus is essential for DNA, cell membranes and ATP, the energy currency of cells. Unlike carbon and nitrogen, it has no significant gas phase. Phosphorus is released slowly when rocks weather, taken up by plants, passed through food webs and returned to soil by decomposition. Much eventually washes into the ocean and settles as sediment, where it can stay for millions of years until geological uplift exposes it again.

Because phosphorus cycles so slowly, it is often the limiting nutrient in freshwater ecosystems. Seabirds and migrating salmon play an unusual role by carrying phosphorus from the sea back to land, in guano and in the bodies of fish dragged into forests by bears.

The human effect. Phosphorus fertiliser, mined from a small number of phosphate rock deposits, has greatly increased the flow of phosphorus into rivers and lakes, causing eutrophication: excessive algal growth, oxygen loss and fish kills.

The sulphur cycle

Sulphur is part of some amino acids and proteins. Most is stored in rocks and ocean sediments. It enters the atmosphere from volcanoes, from the breakdown of organic matter and from marine plankton, which release a gas called dimethyl sulphide that helps clouds form. Burning coal and oil released large amounts of sulphur dioxide in the twentieth century, causing acid rain that damaged forests and lakes in Europe and North America. Laws to reduce sulphur emissions have been one of the great environmental success stories.

The water cycle

Water moves between oceans, atmosphere, land, ice and living things through evaporation, transpiration from plants, condensation, precipitation, infiltration into soil and groundwater, and runoff into rivers and back to the sea. About 97 percent of Earth’s water is salt water in the oceans, and most fresh water is locked in ice caps and glaciers or underground.

Living things strongly influence the water cycle. The Amazon rainforest recycles so much water through transpiration that it creates “flying rivers” of moisture that bring rain to southern Brazil and beyond. Deforestation weakens this recycling and can reduce rainfall far away.

Case study: salmon feed the forest

Every year, millions of Pacific salmon swim upstream from the ocean to spawn and die in the rivers where they hatched. Bears catch salmon and carry them into the forest, eating only the richest parts and leaving the rest. Studies in British Columbia and Alaska found that trees near salmon streams contain nitrogen from the ocean, detectable by its chemical signature, and can grow faster than trees far from streams. Salmon link the ocean’s nutrient cycles to the forests’ in a way no one expected.

Try it yourself

Trace the journey of a single carbon atom through five pools over a thousand years. Start in the air, enter a tree, then continue however you like: a caterpillar, a bird, a fallen log, the soil, the ocean, a shell, limestone. Which steps are fast and which are slow?

Common misconceptions

“Plants get their mass from the soil.” Most of a plant’s dry mass comes from carbon dioxide in the air.
“Nitrogen is scarce.” It is abundant in the air, but only in a form most organisms cannot use.
“The carbon cycle is balanced, so emissions don’t matter.” Natural fluxes were roughly balanced; human emissions have tipped the balance, adding carbon faster than it is removed.

Key terms

Pool (reservoir) and flux: where an element is stored and how fast it moves.
Nitrogen fixation: converting N₂ gas into usable forms.
Denitrification: converting nitrate back to N₂ gas.
Eutrophication: nutrient enrichment of water leading to algal blooms and oxygen loss.
Ocean acidification: the fall in seawater pH caused by absorbing carbon dioxide.

Quick quiz

Test what you have learned. Scoring 60 percent or more marks this module as complete.

Biogeochemical Cycles: Carbon, Nitrogen, Phosphorus and Water