Biogeochemical Cycles Diagram Generator

What is a biogeochemical cycles diagram?

A biogeochemical cycles diagram is a picture of how a chemical element or compound moves between living things and the non-living Earth. OpenStax Biology defines a biogeochemical cycle as the recycling of inorganic matter between living organisms and their environment, and the word itself breaks into three parts: “bio” for the organisms, “geo” for the rocks, soil, water, and air, and “chemical” for the element being followed. Matter on Earth is not used up and replaced — the same carbon, nitrogen, and phosphorus atoms have been passed around for billions of years — so every one of these diagrams is a loop.

Every diagram of a biogeochemical cycle is built from two kinds of thing. Reservoirs are where the element is stored: the atmosphere, the oceans, fresh water, rock and sediment, soil, and living organisms. Processes are what move it from one reservoir to another, drawn as labeled arrows: evaporation and precipitation for water, photosynthesis and respiration for carbon, fixation and denitrification for nitrogen, weathering and uplift for phosphorus. A biogeochemical diagram that names the reservoirs, labels each arrow with its process, and points each arrow the right way already does most of what a teacher or an exam asks for.

Biogeochemical cycles diagram: what each cycle shows

• Water cycle: evaporation from oceans and lakes, transpiration from leaves, condensation into clouds, precipitation, runoff into rivers, infiltration into the ground, and groundwater flow back to the sea. The ocean is the main reservoir; OpenStax notes that 97.5 percent of Earth’s water is salt water.
• Carbon cycle: photosynthesis pulls CO₂ out of the air into plants; respiration by plants, animals, and decomposers puts it back; burning fossil fuels and wood adds more; the ocean exchanges CO₂ with the air and holds most of its carbon as bicarbonate; limestone, sediments, and fossil fuels store carbon for millions of years.
• Nitrogen cycle: N₂ makes up about 78 percent of the air but cannot be used directly by plants or animals. Bacteria and lightning fix it into ammonium, nitrifying bacteria turn ammonium into nitrite and then nitrate, plants take up nitrate and ammonium, decomposers return ammonium from dead matter, and denitrifying bacteria send nitrate back to the air as N₂.
• Phosphorus cycle: weathering releases phosphate from rock into soil and water, plants take it up through their roots, animals eat the plants, decomposers return it to the soil, runoff carries it to the ocean, it settles into sediments, and uplift returns it to land as rock over millions of years — with no gas phase at any point.
• Sulfur cycle, often added as a fifth: SO₂ from volcanoes, decomposition, and fossil fuels enters the air, returns in rain, and is taken up by plants as sulfate for building amino acids such as cysteine.

The phosphorus cycle, step by step

The phosphorus cycle starts in rock. Phosphate minerals such as apatite are locked in rocks on land, and as rain, frost, and roots break the rock down — weathering — phosphate ions (PO₄³⁻) are released into the soil and into the water running through it. This is the only way new phosphorus enters the part of the cycle that living things can reach, which is why weathering is the first arrow on every phosphorus cycle diagram.

From the soil, phosphate enters the food web through plants. Roots absorb dissolved phosphate from the soil water, and most plants get help from mycorrhizal fungi, whose fine threads reach far further into the soil than root hairs can. Animals obtain phosphorus by eating plants or by eating other animals. When plants and animals excrete wastes, shed leaves, or die, decomposers — bacteria and fungi — break down the organic matter and release the phosphate back into the soil, a step called mineralisation. On land, this short loop of soil, plants, animals, and decomposers can turn over many times.

Some phosphate escapes the land loop. Runoff and leaching carry it into streams, rivers, and lakes and eventually to the ocean, where it is taken up by phytoplankton and passed along the marine food web. When marine organisms die, their remains, and the phosphate in them, sink and settle on the sea floor as sediment. Over millions of years those sediments are compacted into sedimentary rock and raised above sea level by geological uplift, and weathering starts the cycle again. OpenStax gives the average time phosphate spends dissolved in the ocean before it reaches the sediments as 20,000 to 100,000 years.

How to draw a simple diagram of the phosphorus cycle

If the task is to draw a simple diagram of the phosphorus cycle, six boxes and one side branch are enough. Draw the boxes in a loop and join them with arrows in this order: rocks, soil (phosphate), plants, animals, decomposers, and back to soil. Label the arrows weathering (rocks to soil), absorbed by roots (soil to plants), eating (plants to animals), and wastes and dead bodies (plants and animals to decomposers). Then add the long, slow branch: runoff from the soil to the ocean, ocean to sediments, and an arrow from the sediments back to rocks labeled uplift, over millions of years.

Two details make a simple phosphorus cycle look like it was drawn by someone who understands it. First, leave the sky empty — no arrow should carry phosphorus up into the air or down in the rain, because phosphorus has no gas phase. Second, write the time scale on the uplift arrow, because the contrast between the fast loop through living things and the very slow return through rock is the most important idea in the whole cycle. For a younger class, use pictures instead of words in the boxes — a rock, a patch of soil, a plant, a rabbit, a mushroom, and a wave.

Why the phosphorus cycle has no gas phase, and why it is the slowest

Water, carbon, nitrogen, and sulfur all form gases at the temperatures and pressures found at the Earth’s surface — water vapour, CO₂ and methane, N₂, SO₂ and hydrogen sulfide — so each of their cycles includes the atmosphere as a reservoir, and atoms can travel around the planet in a matter of days or weeks. Phosphorus does not form a stable gas under those conditions. It stays in rock, soil, water, and living things, mostly as phosphate. Windblown dust, volcanic ash, and sea-spray aerosols can carry small amounts through the air, and OpenStax notes these can be significant phosphate sources in remote regions, but that is particles drifting, not a gas phase, and it is left off standard diagrams.

Without an atmospheric shortcut, phosphorus that reaches the ocean has only one way home: through the rock cycle. It must settle into sediments, be compressed into rock, and be lifted above sea level by tectonic forces before weathering can release it again. That is why the phosphorus cycle is the slowest of the major biogeochemical cycles, and why phosphorus is so often the limiting nutrient in ecosystems — at any given moment, very little of it is dissolved where plants and algae can use it.

Biogeochemical cycles easy drawing: one scene, four colours

A biogeochemical cycles easy drawing does not need four separate diagrams. Draw one simple landscape — a sun, a cloud, a tree, an animal, a strip of soil, a river, and the ocean — and then draw each cycle on top of it as its own colour of arrows. Blue for water: evaporation from the ocean up to the cloud, and rain falling back down. Grey for carbon: photosynthesis from the air into the tree, respiration from the tree and the animal back into the air. Green for nitrogen: bacteria fixing N₂ near the tree roots, and the roots taking up nitrate. Brown for phosphorus: a rock weathering into the soil, and the roots taking up phosphate.

Two or three labels per cycle keeps the drawing easy to copy, and a small colour key in the corner keeps the cycles apart. The one rule worth repeating to students is that every brown arrow stays at ground level or below. If the drawing is for an elementary or middle-school class, ask for large text and everyday words; for high school, add the chemical forms, such as CO₂, N₂, and NO₃⁻.

If the drawing is going to be marked, it is worth sketching it in pencil first in this order: the landscape, then the blue water arrows, then grey, green, and brown. Drawing the water cycle first fixes where the cloud, the river, and the ocean sit, and the other three cycles then hang off the same tree, animal, and soil. Finish by writing the colour key, so a reader can tell the four cycles apart without reading a single arrow label.

Biogeochemical cycle drawing: the names to label

• Water: ocean, cloud, groundwater; evaporation, transpiration, condensation, precipitation, runoff, infiltration.
• Carbon: CO₂ in the air, plants, animals, decomposers, fossil fuels, ocean, limestone; photosynthesis, respiration, decomposition, combustion, diffusion between air and ocean.
• Nitrogen: N₂ in the air, nitrogen-fixing bacteria, root nodules, ammonium (NH₄⁺), nitrite (NO₂⁻), nitrate (NO₃⁻), plants, animals, decomposers; nitrogen fixation, ammonification, nitrification, assimilation, denitrification.
• Phosphorus: phosphate rock, phosphate (PO₄³⁻) in soil and water, plants, animals, decomposers, ocean sediments; weathering, uptake, consumption, decomposition, runoff, sedimentation, uplift.
• Sulfur, if included: volcanoes, SO₂ in the air, sulfate in soil and water, plants, decomposers, hydrogen sulfide; deposition, uptake, decomposition, acid rain.
• For every cycle: an arrow for each process, pointing from where the element comes from to where it goes.

Phosphorus, nitrogen, and eutrophication: how fertiliser makes dead zones

Farming moves phosphorus faster than any natural process. Phosphate rock is mined and spread on fields as fertiliser, moving phosphorus out of a store that would normally release it over millions of years. Crops do not take up all of it. The excess, together with nitrogen from fertiliser, manure, and sewage, washes off the land in runoff and ends up in lakes, rivers, and coastal seas.

Because phosphorus and nitrogen are usually the nutrients in shortest supply in water, adding them causes eutrophication: algae grow explosively into a bloom that turns the water green. When the algae die, they sink, and the bacteria that decompose them use up the dissolved oxygen. Water with too little oxygen for most animals is called hypoxic, and large hypoxic areas are called dead zones; fish and other mobile animals leave and the rest die. NOAA identifies nutrient pollution as the primary cause of human-made dead zones. A diagram of this process is a good companion to a phosphorus cycle diagram, because it shows what happens when the slowest cycle on Earth is suddenly sped up.

The sulfur cycle in brief

Sulfur is the fifth cycle most courses mention. Unlike phosphorus, it has an atmospheric phase: sulfur dioxide (SO₂) enters the air from volcanoes and geothermal vents, from the decomposition of organic matter, and from burning coal and oil. It returns to land and water dissolved in rain, as fallout, and by the weathering of sulfur-containing rock. Plants take up sulfate from the soil and build it into the amino acid cysteine, whose disulfide bonds help proteins hold their shape. Decomposers can release sulfur back to the air as hydrogen sulfide (H₂S). Burning fossil fuels has raised atmospheric SO₂, which forms acid rain that lowers the pH of lakes and damages buildings, and that is the human impact a sulfur cycle diagram usually shows.

How the cycles connect through living things

The cycles are drawn separately, but in real ecosystems they are linked, because every organism needs all of these elements at once. A single molecule of DNA contains carbon, nitrogen, and phosphorus; every protein needs carbon and nitrogen, and many need sulfur; every membrane needs phosphorus. When a plant cannot get enough nitrogen or phosphorus, its photosynthesis slows and it takes less CO₂ from the air, so a shortage in one cycle slows another. Decomposers sit at the centre of all four diagrams, returning carbon as CO₂, nitrogen as ammonium, phosphorus as phosphate, and sulfur as sulfate or hydrogen sulfide from the same dead leaf. For a deeper diagram of one cycle, the carbon, nitrogen, and water cycle generators on this site each give that cycle a full figure of its own.

Biogeochemical cycles drawing: what to check before you hand it in

• Follow every arrow from start to finish: each one should run from the source reservoir to the destination.
• Look at the sky: there should be no phosphorus arrows in it.
• Check the carbon cycle for plant respiration, not just animal respiration.
• Check the nitrogen cycle: N₂ is fixed by bacteria or lightning, never absorbed by roots, and denitrification starts from nitrate.
• Check the water cycle for transpiration from leaves.
• Check the labels: each process name sits on its own arrow and each cycle keeps its own colour.
• Check the time scale, if one is shown: phosphorus is the slowest cycle, and water the fastest.