Cycling of Matter: Water, Carbon, Nitrogen and Phosphorus

1Recycling on a planetary scale

The atoms in a person's body are not new. The carbon in a muscle was once part of the carbon dioxide in the air, and before that it may have been in a leaf, a fossil, or a shell on the seafloor. The nitrogen in each protein was recently in the soil, and the phosphorus in each bone was recently in a rock. Earth receives a steady supply of energy from the Sun, but the matter that living things need has to be reused over and over.

The curriculum names this as the second part of its Big Idea: the biosphere, geosphere, hydrosphere and atmosphere are interconnected, as matter cycles and energy flows through them. The examples specified are the water, nitrogen, carbon and phosphorus cycles.

2Reservoirs, sources and sinks

A reservoir is a place where matter is stored, such as the atmosphere, the ocean, soil, living things or rock. Matter moves between reservoirs through processes such as photosynthesis, evaporation or weathering. Two words from the curriculum describe the direction of flow.

• A source releases a substance into a reservoir (for example, a volcano or a car releasing carbon dioxide into the air).
• A sink takes a substance out of a reservoir and stores it (for example, plants absorbing carbon dioxide from the air).

The same thing can be a source or a sink at different times. OpenStax uses the word reservoir rather than “sink”, and notes that reservoirs hold matter for very different lengths of time: from about a week in living organisms to tens of thousands of years in the deep oceans and ice. The next lesson explores how people change sources and sinks.

3The carbon cycle

Carbon is the backbone of the molecules of life: sugars, fats, proteins and DNA all contain it. Its movement links air, water, rock and living things.

1. Photosynthesis removes carbon dioxide. Photosynthetic organisms use carbon dioxide from the air (or dissolved carbon in water) to build energy-rich compounds, and release oxygen.
2. Feeding passes carbon along. Heterotrophs eat these compounds. The carbon becomes part of their bodies.
3. Respiration returns carbon dioxide. Organisms use aerobic respiration, which consumes oxygen and releases carbon dioxide to the air.
4. Decomposition. Decomposers return carbon from dead organisms to the soil, and eventually to the air as carbon dioxide.
5. The ocean. Carbon dioxide dissolves in water and forms carbonic acid, which breaks down into bicarbonate and carbonate ions. Over 90% of the carbon in the oceans is bicarbonate. Marine organisms combine it with calcium to build calcium carbonate shells. Over time, shells form limestone, the largest carbon reservoir on Earth.
6. Fossil fuels. Plant remains that decompose without oxygen over millions of years become fossil fuels. They form far more slowly than they are used, so they are non-renewable.
7. Geologic return. Volcanoes and hydrothermal vents release carbon dioxide. Subduction of the ocean floor carries carbon deep into Earth, where it can eventually return.

For a link to the biological side of this cycle, see How photosynthesis works.

4The nitrogen cycle

Nitrogen is essential for proteins and DNA. It makes up about 78% of the atmosphere as nitrogen gas (N₂), but plants and animals cannot use this form directly. Bacteria do the vital conversions.

1. Nitrogen fixation. Certain bacteria convert nitrogen gas into forms that living things can use. They include cyanobacteria, Rhizobium bacteria that live in nodules on the roots of legumes (such as peas and beans), and free-living soil bacteria.
2. Assimilation. Plants take up fixed nitrogen from the soil and build it into proteins. Animals get nitrogen by eating plants or other animals. Nitrogen availability often limits growth.
3. Ammonification. Bacteria and fungi convert nitrogen-containing wastes and dead organisms into ammonium (NH₄⁺).
4. Nitrification. Soil bacteria convert ammonium into nitrite and then into nitrate, a form that plants readily take up.
5. Denitrification. Other bacteria convert nitrate back into nitrogen gas, which returns to the atmosphere.

5The phosphorus cycle

Phosphorus is a key part of DNA and other nucleic acids, of cell membranes (phospholipids) and of bone (as calcium phosphate). In nature, it is found as the phosphate ion (PO₄³⁻). Phosphorus often limits growth in aquatic ecosystems.

The phosphorus cycle differs from the carbon and nitrogen cycles in an important way: it has no major gas phase. It moves mostly through rock, soil, water and living things (although dust and volcanic ash can carry small amounts through the air).

1. Weathering. Phosphate in rock (much of it from ancient ocean sediments) is slowly released into soil and water as the rock weathers.
2. Uptake. Plants absorb phosphate. Animals obtain it from food. Decomposers return it to the soil when organisms die.
3. Runoff. Surface runoff, groundwater and rivers carry phosphate to lakes and the ocean, where it enters food webs or settles as sediment.
4. Uplift. Over geologic time, sediments are uplifted and become land again.

Phosphate moves very slowly. The average phosphate ion stays in the ocean for 20 000 to 100 000 years. This explains why a human-caused change in phosphorus can last a long time.

The water cycle in the context of matter cycles

The water cycle was described in the lesson on solar radiation. In the context of matter cycles, it has a second role: it is the transport system for other cycles. Rain and runoff carry carbon, nitrogen, phosphorus and sulfur from land to water. Water's residence times differ among reservoirs: about 1.5 weeks in the atmosphere, about 2 weeks in rivers and about 4000 years in the oceans, as shown in the chart.

6Comparing the cycles

Decomposers: the recycling crew

None of the cycles would work without decomposers, the bacteria and fungi that break down dead organisms and wastes. In the carbon cycle, decomposers release carbon dioxide from dead material. In the nitrogen cycle, they carry out ammonification, converting nitrogen-rich wastes and remains into ammonium. In the phosphorus cycle, they return phosphate from dead organisms to the soil. Without decomposers, nutrients would stay locked in dead material and producers would run short.

The role of decomposers is a good reason to think of an ecosystem as a loop and not as a line. Producers build living matter from simple substances; consumers pass it along; decomposers turn it back into the simple substances that producers can use again.

Energy flow and matter cycling compared

How the cycles work together: a crop field

A single field of plants depends on all four cycles at once, and the cycles depend on one another.

Reading cycle diagrams

Cycle diagrams can look confusing. A few habits make them easier to read.

1. Find the reservoirs first. These are the boxes: places where matter is stored.
2. Read the arrows as processes. Each arrow is labelled with a process such as photosynthesis, runoff or nitrification.
3. Trace one atom. Follow a single atom through a full loop and note which arrows it takes.
4. Look for the fast and slow paths. In the carbon cycle, the loop through living things takes a short time, while the loop through limestone and fossil fuels takes millions of years.
5. Find where people interfere. Burning fossil fuels, adding fertilizer and clearing forests all change a flow.

Practice question: In the nitrogen diagram, which arrow would stop if all denitrifying bacteria were removed, and what would the result be over time? The arrow from nitrate back to nitrogen gas would stop, so nitrogen would stay in the soil and water as nitrate instead of returning to the air, and the reservoir in the atmosphere would slowly receive less nitrogen.

7Why cycles matter

• Cycles keep nutrients available for living things, without needing new matter to arrive from space.
• Different cycles are linked: water carries nutrients, nitrogen and phosphorus limit plant growth, and carbon links living things to the air and climate.
• Cycles run at different speeds. When people add matter to a fast cycle, the effects can be quick. When people add matter to a slow cycle (like phosphorus), the effects can last for a very long time.

Next, the effects of human activities on these cycles: climate change, deforestation, agriculture, and the build-up of pollutants in food chains.

?Quick check

Try each question first, then reveal the answer.