Nutrient cycles: phosphorous, nitrogen, carbon

In a nutshell

Ions and nutrients are a limited resource, therefore they must be recycled in nutrient cycles. Microorganisms such as bacteria and fungi are critical in recycling chemical elements. This summary discusses examples of nutrient cycles and highlights their importance.

Recycling nutrients

Plants require inorganic ions to grow and build macromolecules. These will then be passed through the food chain to other organisms. Given that there is a limited ion availability, it is vital that they are recycled. Therefore, the flow of ions and nutrients in an ecosystem is circular. When the organisms die, the nutrients they contained are passed back into the soil by the action of various microorganisms.

Microorganisms

Microorganisms play an important role in recycling chemical elements.

Saprobionts

Saprobionts are a type of decomposer that feed on waste products and remains of dead animals and plants. They carry out extracellular digestion which means they secrete digestive enzymes and digest the organic molecules to produce inorganic ions.

Mycorrhizae

Mycorrhizae are a symbiotic relationship between fungi and plant roots. The fungi consist of long strands, called hyphae, which join to the plants roots and greatly increase the surface area of absorption of the plant. This facilitates a greater uptake of inorganic ions for the plant. In return, the fungi receives organic compounds, like glucose, from the plant.

Nitrogen cycle

Nitrogen is an important element required by plants and animals. They use the nitrogen to produce proteins and nucleic acids (DNA and RNA). Nitrogen makes up around $78\%$ of the Earth's atmosphere but it is too unreactive for plants and animals to use it directly from the air. It must first be converted into soluble ions. The nitrogen compounds will be passed along the food chain until it is returned back to the atmosphere during decay.

Biology; Energy transfers; KS5 Year 12; Nutrient cycles: phosphorous, nitrogen, carbon

There are four main processes that occur in the nitrogen cycle and these are carried out by various microorganisms.

	Process	Microorganism involved	Description
1.	Nitrogen fixation	None	Nitrogen gas ( $N_2$ ) makes up around $78\%$ of the Earth's atmosphere but it is too unreactive for plants and animals to use it directly from the air. It must first be converted into soluble ions. Lightning bolts contain enough energy to make nitrogen react with oxygen in the air to produce nitrate ions.
2.	Nitrogen fixation	Nitrogen-fixing bacteria Example Rhizobium are found in root nodules and they can turn nitrogen into ammonia. The ammonia will go on to form ammonium ions.	Nitrogen fixation can also occur in the soil by nitrogen-fixating bacteria. These are found free in the soil and in the root nodules of leguminous plants. These bacteria can convert nitrogen gas into nitrates.
3.	Ammonification	Decomposers Example Saprobionts	Decomposers convert nitrogen-containing compounds like proteins and urea into ammonia ( $NH_3$ ). Ammonia forms ammonium ions ( $NH_4\ ^+$ ) in the soil that plants can use.
4.	Nitrification	Nitrifying bacteria Example Nitrosomonas converts ammonium ions into nitrites and then Nitrobacter converts nitrites into nitrates.	Nitrifying bacteria turn ammonium ions in the soil into nitrites ( $NO_2\ ^-$ ). Then, different nitrifying bacteria will turn the nitrites into nitrates ( $NO_3\ ^-$ ).
5.	Dentrification	Denitrifying bacteria	Denitrifying bacteria turns nitrates in the soil back into $N_2$ gas during respiration. This happens in anaerobic conditions, like in waterlogged soils, where there is very little oxygen available.

Phosphorous cycle

Phosphorous is another important nutrient that must be recycled. Phosphorous is used to make biological molecules such as phospholipids which are a vital component of cell membranes. Phosphorous is also used to make nucleic acids such as DNA and ATP.

1.	Phosphorous is found in rocks.
2.	The weathering of rocks releases phosphorous into oceans, lakes and rivers in the form of phosphate ions ( $PO_4{^-}{^3}$ ).
3.	Phosphate ions will be taken up by algae or other aquatic producers.
4.	The phosphate ions will be passed to birds in the next trophic level.
5.	The waste of sea birds is called guano and it is rich in phosphate ions. The phosphate ions in guano will be returned to the soils. Note: Guano is often used as a natural fertiliser.
6.	Phosphate ions are taken up by plant roots. *Note:* Mycorrhizae increase the rate of the ion uptake.
7.	Phosphate ions are passed through the food chain.
8.	Phosphate ions are returned to the soil through animal excretion and through decomposition of plants and animals. Saprobionts are responsible for breaking down organic compounds to release the phosphate ions back into the soil.

Carbon cycle

Plants take in carbon from carbon dioxide. The carbon is then converted into complex compounds such as carbohydrates that make up living organisms. These will then be passed along food chains when animals eat plants and other animals. There is a finite amount of carbon available therefore it must be recycled. Waste products and dead organisms are broken down by decomposers so the carbon can be returned to the air.

Biology; Energy transfers; KS5 Year 12; Nutrient cycles: phosphorous, nitrogen, carbon

1	Carbon dioxide in the atmosphere.
2	Green plants use carbon from carbon dioxide during photosynthesis to make carbohydrates, fats and proteins.
3	The fossilisation of plants and animals.
4	Carbon dioxide is stored in fossil fuels (coal/oil).
5	When plants and animals die, they decompose and turn into useful products.
6	Decomposition is done by microorganisms, such as saprobionts, that respire and release carbon dioxide into the air.
7	Carbon is passed to animals when they eat plants.
8	When plants and animals respire they release carbon dioxide back into the air.
9	The burning of plant and animal products such as wood or fossil fuels released carbon dioxide back into the air.

Nutrient loss

An important part of the nutrient cycles is the return of the nutrients to the soil as the plants and animals decompose. However, crops are usually harvested and animals may be slaughtered which halts this cycle and means the nutrients they contain aren't returned to the soil.

This causes the nutrient concentration of the soil to decrease which limits the growth of the other plants. Farmers can take certain measures to increase the concentration of nutrients in their soil, whilst still harvesting their crops.

Fertilisers

Fertilisers can be applied to the soil to increase the nutrient concentration. There are two types of fertilisers; natural and artificial.

Natural fertilisers

Natural fertilisers are formed from organic matter.

Examples

Manure, household compost and sewage sludge.

Artificial fertilisers

Artificial fertilisers are inorganic and man-made. They are often powdered or pellets that contain pure nutrients and chemicals.

Example

Ammonium nitrate.

Eutrophication

Despite increasing plant growth and restoring the nutrient concentration of the soil, there are environmental issues surrounding the use of fertilisers. Occasionally, more fertiliser can be added to the soil than the plants need. This can lead to the fertilisers leaching into nearby rivers and lakes. The consequence of this is eutrophication.

Note: It is more likely that leaching will occur during rainy periods and the leaching of nitrates is more common than the leaching of phosphates because nitrates are more soluble in water.

The process of eutrophication is shown and explained below.

Biology; Energy transfers; KS5 Year 12; Nutrient cycles: phosphorous, nitrogen, carbon

1.	Fertilisers leach into waterways which releases excess mineral ions, such as nitrates and phosphates into the water.
2.	The excess mineral ions stimulate algae to grow rapidly.
3.	The overgrowth of algae causes an algal bloom (A) on the surface of the water. This prevents the light reaching the organisms below.
4.	The lack of light, caused by the algal bloom, means that the plants below can't photosynthesise. As a result, they die and are decomposed by the decomposers (B). The decomposers replicate and their population size increases as they feed on the decaying matter (C). The increased number of decomposers decreases the oxygen concentration of the water as they carry out aerobic respiration.
5.	The lack of oxygen causes the death of many aquatic organisms, such as fish, that require oxygen to survive.