Some of the carbon atoms in your body may once have been part of a dinosaur, a fern in an ancient swamp, or the breath of someone who lived a thousand years ago. That is not science fiction. Apart from the occasional meteorite, Earth gains almost no new matter from space, so the atoms that make up living things have been recycled again and again for billions of years. Energy flows through ecosystems and is lost as heat, but matter goes around in cycles, moving between the biosphere, atmosphere, hydrosphere and lithosphere. In this lesson you will follow three of the most important: the water cycle, the carbon cycle and the nitrogen cycle.
Two words help describe any cycle. A reservoir (or store) is a place where matter is held for a while, such as the ocean, the atmosphere or a forest. A process (or flux) moves matter from one reservoir to another, such as evaporation or photosynthesis. When the amount moving into a reservoir is about the same as the amount moving out, the reservoir stays roughly steady even though matter is constantly moving. That balance is called dynamic equilibrium: "dynamic" because things are always moving, "equilibrium" because the overall amounts stay about the same. Think of a bathtub with the tap running and the drain open: if water flows in as fast as it drains out, the level stays the same.
The water cycle
- Evaporation: the Sun's energy turns liquid water from oceans, lakes and soil into water vapour.
- Transpiration: plants draw water up from their roots and release water vapour through the stomata in their leaves. A large tree can release hundreds of litres on a hot day.
- Condensation: as water vapour rises and cools, it turns back into tiny liquid droplets that form clouds.
- Precipitation: droplets join until they are heavy enough to fall as rain, snow, sleet or hail.
- Runoff and infiltration: water flows over the ground into streams, rivers and lakes (runoff), or soaks into the soil (infiltration) to become groundwater, which can feed wells and springs.
The carbon cycle
Carbon is the backbone of every living thing: sugars, fats, proteins and DNA are all built around carbon atoms. Carbon is stored in the atmosphere (mostly as carbon dioxide), in living things, in soil, in the ocean (dissolved carbon dioxide and the shells of sea creatures), in fossil fuels (coal, oil and natural gas, formed from ancient living things buried for millions of years), and in rocks like limestone. A reservoir that takes in more carbon than it releases is called a carbon sink; one that releases more than it takes in is a carbon source. Forests, oceans and peatlands are major sinks. Burning fossil fuels turns ancient stored carbon into a source.
| Process | What it does to atmospheric CO₂ | Natural or human? |
|---|---|---|
| Photosynthesis | Removes it: plants and algae build CO₂ into glucose | Natural |
| Cellular respiration | Adds it: organisms break down glucose and release CO₂ | Natural |
| Decomposition | Adds it: decomposers respire as they break down dead matter | Natural |
| Ocean exchange | Both: CO₂ dissolves into seawater, and seawater releases CO₂ | Natural (but the ocean now absorbs extra CO₂ from human emissions) |
| Volcanic eruptions | Adds it: CO₂ escapes from deep in Earth | Natural (small compared with human emissions) |
| Burning fossil fuels | Adds it: carbon stored for millions of years is released in moments | Human |
| Deforestation | Adds it: burning or rotting trees release CO₂, and fewer trees remain to absorb it | Human |
The nitrogen cycle
Living things need nitrogen to build proteins and DNA. The atmosphere is about 78% nitrogen gas (), but here is the catch: plants and animals cannot use nitrogen gas directly. The two nitrogen atoms in are joined by a very strong triple bond, and almost no organism can break it. Nitrogen must first be fixed, meaning changed into compounds such as ammonium () or nitrates () that plants can absorb. Bacteria do most of this work.
- Nitrogen fixation: certain bacteria convert nitrogen gas into ammonium. Some live freely in soil; others live in swellings called root nodules on legumes such as beans, peas, clover and alfalfa, trading fixed nitrogen for sugar from the plant. Lightning also fixes a small amount: it splits nitrogen molecules in the air, forming nitrogen oxides that rain carries into the soil as nitrates.
- Nitrification: other soil bacteria convert ammonium into nitrites and then into nitrates, the form most plants absorb most easily.
- Uptake (assimilation): plant roots absorb nitrates (and some ammonium) and build the nitrogen into proteins and DNA. Animals get nitrogen by eating plants or other animals.
- Decomposition (ammonification): when organisms die or produce waste, decomposers break down their proteins and release the nitrogen as ammonium, which can be used again.
- Denitrification: bacteria living in waterlogged, low-oxygen soils and sediments convert nitrates back into nitrogen gas, which returns to the atmosphere and closes the cycle.