Nutrient cycling is the movement of nutrients from the environment to living organisms and back. The carbon, oxygen, nitrogen, and phosphorus cycles are important. Human activities have impacted these cycles, causing negative effects. Photosynthesis is the beginning of the carbon and oxygen cycles, while nitrogen is essential for life but difficult to use. Phosphorus is found in rocks and enters water and soil through erosion and weathering. Fertilizers are used to replace lost nutrients in cultivated areas, but excess can lead to algae growth.
In biology, nutrient cycling is a concept that describes how nutrients move from the physical environment to living organisms and are subsequently recycled back into the physical environment. This circular movement of nutrients is essential for any given ecosystem and must be balanced and stable for the system to be maintained. In many cases, human activities have had a major impact on these processes, resulting in negative effects. There are many different nutrient cycles, each with its own particular pathways, but perhaps the most important are those involving the elements carbon, oxygen, nitrogen and phosphorus.
The carbon cycle
This nutrient cycle begins with photosynthesis, the process by which plants, algae and some bacteria use the energy of sunlight to combine carbon dioxide (CO2) from the atmosphere and water to form sugars, starch, fats , proteins and other compounds they use to build cells or store as food. In this way, plants remove carbon from the atmosphere and store it, making it available to herbivores that eat the plants. Herbivores use some of the carbon they consume to build and repair cells, so it is stored in their bodies. The rest is used to provide energy: it combines with oxygen in the air to form CO2, which is then exhaled, returning the carbon directly to the atmosphere.
Carbon stored in the body of a herbivore, such as a deer, can be recycled when the animal dies. Alternatively, the animal can be killed and eaten by a carnivore, such as a wolf, in which case the recycling will occur when the carnivore dies. Dead plant and animal matter is broken down by other organisms, such as fungi and bacteria. This process releases carbon, in the form of carbon dioxide, into the atmosphere.
There are a number of complications within this overall process. For example, dead organic matter can sometimes be buried under sediment, making carbon unavailable to living organisms. This buried material has formed deposits of coal and oil, which humans are now exploiting as fossil fuels. Combustion of these compounds forms carbon dioxide, which is released into the atmosphere. There is broad consensus among scientists that rising CO2 levels from burning fossil fuels are changing the Earth’s climate on a global scale.
Carbon can also be locked up in rocks when carbon dioxide dissolves in water. Some types of marine organisms can combine dissolved carbon dioxide with calcium to build shells made up of calcium carbonate. When these organisms die, the shells accumulate as sediment, eventually forming limestone rocks. Over large time scales, limestone can be raised to the surface by geological processes, where acidic water can react with it to release CO2 into the atmosphere.
The oxygen cycle
This cycle is closely related to the carbon cycle and begins in the same place: photosynthesis, which releases oxygen into the air. This, in turn, is taken up by oxygen-breathing organisms, which combine it with carbon and release carbon dioxide into the atmosphere. The CO2 is then used in photosynthesis to release oxygen again. Carbon dioxide from other sources, such as the decomposition of dead organic material and the burning of fossil fuels, is also used in photosynthesis, producing oxygen.
The nitrogen cycle
Nitrogen is an essential element for all known life forms and is required to form amino acids, proteins and DNA. Although 78% of the earth’s atmosphere is made up of this element, it cannot be used directly by plants in this form. The molecules of the gas are made up of two atoms held together by a very strong triple bond, which makes it very difficult to react with other elements. However, nitrogen has its own nutrient cycle.
There are two main ways this element can become available to living organisms. Normally, a lot of energy is required to break the bonds between the atoms in a nitrogen molecule. This energy can come from a lightning strike, which causes some of the nitrogen to combine with oxygen, forming nitrogen oxides. These can dissolve in rainwater to form very dilute nitric acid, which reacts with minerals in the soil to form nitrates. Nitrates are water soluble and can be easily absorbed by plants.
Most of the nitrogen in living organisms comes from a process known as nitrogen fixation. This involves the conversion of atmospheric nitrogen in soils into ammonia by various types of bacteria and some algae. One such group of bacteria, called Rhizobium, forms nodules in the roots of peas and beans. For this reason these plants are often grown as crops by farmers when the soil needs to be enriched with this element.
The ammonia generated in this way is then converted by other types of bacteria into nitrates, which are taken up by plants. Another process, called denitrification, returns nitrogen gas to the atmosphere. Again, this is done by bacteria, which reduce the nitrates in the soil to nitrogen.
Humans have had a significant impact on the nitrogen cycle. Because nitrates are very soluble in water, they can be quickly removed from the soil by rain. Where crops are grown intensively, lost nitrates often need to be replaced by nitrate fertilisers. These compounds are produced industrially by processes that first combine atmospheric nitrogen with hydrogen to form ammonia, then combine it with oxygen to form nitric acid, which is used to make fertilizers.
The phosphorus cycle
Like nitrogen, this element is an essential part of DNA. It is also needed for the production of adenosine triphosphate (ATP), a compound that cells use for energy. The main natural source of phosphorus comes from rocks. The element enters water and soil in the form of phosphate through erosion and weathering and is taken up by plants. It then progresses through the food chain via herbivores and carnivores, returning to the soil when these organisms die.
Phosphates can be washed out of the soil by rainwater, accumulating in lakes and rivers, where some of it is used by aquatic plants and other organisms. Some phosphate, however, undergoes chemical reactions that form insoluble compounds that settle as sediment. These eventually form rock and, in this way, phosphorus can be locked up for very long periods, perhaps tens or hundreds of millions of years. Eventually, geological processes can uplift this rock, allowing erosion and weathering to return it to living organisms.
In cultivated areas, as with nitrogen, phosphorus lost from the soil often has to be replaced with phosphate fertilizers in order for agriculture to continue to be profitable. These fertilizers consist mainly of phosphate rocks such as apatite. The use of animal manure in crop fields is another example of humans adding phosphorus to the soil. In some cases, the excess phosphate is washed into rivers and lakes. From here, it can settle in the sediments, but some can remain dissolved, leading to excessive algae growth.
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