0610 · 19.3Nutrient cycles

Matter, energy and nutrient cycles

0:00A rabbit eats a leaf, and some of the material in that leaf becomes part of the rabbit. Later, the rabbit produces waste, and eventually it dies. So why does the natural world not simply run out of the materials needed to build living things? The answer is recycling. By the end of this video, you will understand how carbon and nitrogen move between organisms and their environment, and why microorganisms are essential to those journeys.

0:24Start with a distinction between matter and energy. Organisms need both, but they do not move through ecosystems in the same way. Energy flows through an ecosystem and is eventually transferred to the surroundings as heat. Chemical elements, however, can be reused. A carbon atom in a leaf can later become part of an animal, return to the atmosphere, and enter another plant. You need just a little background knowledge.

0:47Carbon is found in carbohydrates, fats and proteins, while nitrogen is needed to make amino acids, the building blocks of proteins. Plants build organic molecules using materials from their surroundings, and animals obtain organic molecules by feeding. A nutrient cycle follows these materials as they move between living organisms and non-living parts of the environment.

Carbon transfers and returns

1:08Let us begin with carbon dioxide in the atmosphere. During photosynthesis, plants take in carbon dioxide and use light energy to make carbohydrates, such as glucose. The carbon atoms from carbon dioxide become part of these organic molecules. This is the main route by which atmospheric carbon enters food chains. The carbon has not disappeared from the air without a trace, as it has changed its location and chemical form.

1:31Now imagine the rabbit eating the plant. Feeding transfers carbon-containing molecules from the plant to the animal. Some of that material can be used to build the rabbit’s tissues, while some is used in respiration. If another animal eats the rabbit, carbon moves along the food chain again. Feeding transfers carbon between organisms rather than directly returning it to the atmosphere. Respiration provides a route back to atmospheric carbon dioxide.

1:56During aerobic respiration, cells break down organic molecules, such as glucose, releasing energy and producing carbon dioxide. Animals respire, but so do plants and microorganisms. This means a plant has arrows going both ways in a carbon-cycle diagram. Photosynthesis takes carbon dioxide in, while respiration releases carbon dioxide. What happens to carbon in dead organisms and waste material? Decomposers, including bacteria and fungi, break down this organic material. They obtain nutrients from it and use some of its carbon-containing molecules in respiration.

2:29Their respiration releases carbon dioxide into the atmosphere. So decomposition helps return carbon from dead material, but the release of carbon dioxide happens through the respiration of the decomposers. Not all dead material decomposes completely. Under certain conditions, some remains become buried, and their carbon can stay out of the atmosphere for a very long time. Over millions of years, buried organic material can be transformed into fossil fuels, including coal, oil and natural gas.

2:57This formation of fossil fuels is a slow route in the carbon cycle, unlike the much faster movement through feeding and respiration. Combustion means burning. When fossil fuels burn in oxygen, their stored carbon is released as carbon dioxide, returning it to the atmosphere. Burning wood also releases carbon dioxide from carbon-containing material, without that wood first becoming a fossil fuel. The important relationship is that photosynthesis removes atmospheric carbon dioxide, while respiration and combustion return it.

3:27Fossil fuel formation stores some carbon for a much longer time.

Worked carbon cycle pathway

3:30Let us work through a carbon example. Suppose you are asked to describe how carbon in atmospheric carbon dioxide could enter a rabbit and later return to the atmosphere. First, identify how carbon enters an organism from the air, by noting that a plant takes in carbon dioxide during photosynthesis. Next, connect the plant to the rabbit, by noting that feeding transfers carbon-containing molecules when the rabbit eats the plant.

3:54Finally, respiration in the rabbit releases carbon dioxide into the atmosphere. There is also another possible ending to that journey. Carbon could remain in the rabbit’s tissues until the rabbit dies. Decomposers then break down the remains and release carbon dioxide through their respiration. Both routes are scientifically valid, but they involve different processes. When describing a cycle, follow the carbon from one location to the next rather than simply listing every process you remember.

Nitrogen fixation and protein synthesis

4:21Now for nitrogen, where the central puzzle is different. The atmosphere contains a large supply of nitrogen gas, but plants cannot use that gas directly to make proteins. Nitrogen gas is relatively unreactive, so its nitrogen must first be converted into usable compounds. For this cycle, think carefully about chemical forms. Atmospheric nitrogen gas, soil ammonium ions and soil nitrate ions all contain nitrogen, but they are not interchangeable. Nitrogen fixation converts atmospheric nitrogen into nitrogen compounds that organisms can use.

4:53Nitrogen-fixing bacteria can carry out this conversion, producing compounds such as ammonium ions. Some exist freely in the soil, while others are found in root nodules of leguminous plants, such as peas and beans. A root nodule is a small swelling containing bacteria. It is the bacteria, not the plant itself, that fix atmospheric nitrogen. Lightning provides another route for nitrogen fixation. Its energy allows nitrogen and oxygen in the atmosphere to react, leading to the formation of nitrogen compounds.

5:22These can dissolve in rainwater and enter the soil as nitrates. Both lightning and nitrogen-fixing bacteria bring atmospheric nitrogen into forms that can enter the living part of the cycle. They do not simply move unchanged nitrogen gas into plant proteins. Plants absorb nitrate ions from the soil through their roots. They use nitrogen from these ions, together with materials made through photosynthesis, to produce amino acids. Amino acids are then joined together to make proteins.

5:50This explains why a plant needs more than carbon dioxide and water to grow. Carbohydrates contain no nitrogen, so they cannot supply all the materials needed for proteins. When an animal eats a plant, nitrogen in plant proteins enters the animal’s digestive system. Digestion breaks those proteins down into amino acids, which can be absorbed. The animal then uses amino acids to make its own proteins. So the plant’s proteins are not simply fitted unchanged into the animal.

6:16Their building blocks are released and then assembled into proteins the animal needs. Animals cannot store excess amino acids in the way they can store some other nutrients. In humans, excess amino acids are deaminated in the liver. Deamination is the removal of the nitrogen-containing part of an amino acid, and this nitrogen is converted into urea. Urea is excreted in urine, transferring nitrogen from the animal to its surroundings.

6:41We only need that connection here, rather than the full details of excretion.

Nitrogen recycling in soil

6:45Dead plants, dead animals and waste materials all provide routes for nitrogen to return to the soil. Decomposer microorganisms break down plant and animal proteins, ultimately producing ammonium ions. Nitrogen-containing waste, including urea, can also be broken down to ammonium ions. Notice the important endpoint, which is that this decomposition produces ammonium ions, not directly nitrogen gas. That distinction tells us which process must happen next. Nitrification converts ammonium ions into nitrate ions through the action of nitrifying bacteria.

7:17These bacteria require oxygen, so nitrification is favoured by well-aerated soil. The nitrate ions produced can then be absorbed by plant roots, linking decomposition back to the production of new plant proteins. There is also a route from soil nitrates back to atmospheric nitrogen gas. Denitrifying bacteria convert nitrate ions into nitrogen gas through denitrification. This occurs in conditions with little or no oxygen, such as waterlogged soil. Waterlogging fills soil spaces with water and reduces the oxygen available.

7:49Denitrification therefore returns nitrogen to the atmosphere while reducing the nitrate supply available for plant uptake. The bacterial processes are easiest to distinguish by asking where the nitrogen starts and ends. Nitrogen fixation starts with atmospheric nitrogen and produces usable nitrogen compounds. Nitrification starts with ammonium ions and produces nitrate ions. Denitrification starts with nitrate ions and produces atmospheric nitrogen gas. Decomposition is different again, as it returns nitrogen from organic material to the soil as ammonium ions.

Nitrogen cycle exam practice

8:21Here is a worked example that brings those connections together. A dead leaf contains proteins, and a nearby seedling later uses some of its nitrogen to make new proteins. First, the nitrogen must leave the dead leaf’s organic material, so decomposer microorganisms break down its proteins to ammonium ions. Next, nitrifying bacteria convert those ammonium ions into nitrate ions. The seedling absorbs the nitrate ions through its roots and uses the nitrogen to make amino acids and proteins.

8:50Now change the conditions, and suppose the soil becomes waterlogged. To reason through the effect, ask which processes depend on oxygen availability. Nitrification may decrease because nitrifying bacteria require oxygen. Denitrification is favoured by the low-oxygen conditions, so denitrifying bacteria convert more nitrate ions into nitrogen gas. Both changes can reduce the nitrate ions available for the seedling, but they do so by different routes. Watch for three common misunderstandings.

9:20Plants do not absorb proteins from the soil as their normal source of nitrogen in this cycle. They absorb nitrate ions and manufacture their own proteins. Nitrification and nitrogen fixation are not the same, because only fixation brings atmospheric nitrogen into usable compounds. And decomposers do not make nutrients vanish, because they change organic material into forms that can re-enter the cycles. In an examination, you might be asked to identify a labelled arrow, describe a pathway, or state the role of microorganisms.

9:49For example, consider this question. Describe how nitrate ions return to the atmosphere in waterlogged soil. Start by identifying the initial form, nitrate ions, and the final form, nitrogen gas. That tells you the process is denitrification, rather than nitrification or nitrogen fixation. A precise answer would be as follows. Denitrifying bacteria convert nitrate ions into nitrogen gas. This process is called denitrification. It occurs in conditions with little or no oxygen, and the nitrogen gas returns to the atmosphere.

10:21Each sentence contributes a clear point, and naming the starting and finishing forms makes the direction of the process unmistakable. Let us finish with the big picture. In the carbon cycle, photosynthesis brings atmospheric carbon into organic molecules, and feeding transfers it between organisms. Respiration returns carbon dioxide to the atmosphere, including respiration by decomposers breaking down dead material. Some carbon becomes stored in fossil fuels, and combustion releases it again.

10:50Carbon cycles because its atoms can be reused in different molecules. In the nitrogen cycle, fixation brings atmospheric nitrogen into usable compounds, while decomposition produces ammonium ions from organic material. Nitrification produces nitrate ions, which plants absorb to make amino acids and proteins. Feeding, digestion and deamination move nitrogen through animals and into waste, while denitrification returns nitrogen to the atmosphere. If you can follow the nitrogen’s form, location and direction of movement, the cycle becomes a connected story rather than a collection of names.