In this lesson, you'll learn about
A nutrient cycle refers to the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by the food web pathways which decompose organic matter into inorganic nutrients. Nutrient cycles occur within ecosystems.
Nutrient cycles in nature are called biogeochemical cycles because the elements move cyclically from the environment to living organisms and back to the environment.
Ecosystems illustrate closed-loop recycling where the demand for nutrients adding to the growth of the biomass exceeds the supply in that system. There exist regional and spatial differences in the growth and exchange rates of materials, where some ecosystems can be in nutrient debt (sinks) and others will have an extra supply (sources). These differences are brought about by geological history and topography.
In a food web, a loop or a cycle is defined as a directed sequence of one or more links that starts from and ends at the same species. For example, in the ocean, bacteria are exploited by protozoa such as heterotrophic microflagellates which are then exploited by the ciliates. This grazing activity is followed by the excretion of substances that are then used by bacteria so that the operation of the system is a closed circuit.
The enzymatic digestion of cellulose is an example of ecological recycling. Cellulose, which is among the most abundant organic compounds on earth, is the main polysaccharide in plants where it forms the cell walls. Enzymes that degrade cellulose participate in the natural plant materials’ ecological recycling. Different ecosystems may have different rates of recycling litter.
The chemical elements are constantly recycled after they are used as the following:
Every element has its nutrient cycle and every cycle has a unique pathway that includes reservoirs, exchange pools, and resident times.
Reservoir – A region where the element is in its highest concentration and is held and stored for some time. For example, coal or fossil fuels are reservoirs for carbon.
Exchange pools – When elements are held for short periods of time. For example, plants and animals use these elements in their systems temporarily and release them back into the environment.
Resident time – The amount of time that an element is held in a place.
Energy flows directionally through Earth’s ecosystems, typically entering in the form of sunlight and exiting in the form of heat. However, the chemical components that make up living organisms are different: they get recycled.
Carbon dioxide and methane are examples of carbon compounds that circulate in the atmosphere and influence global climates. Through the processes of photosynthesis and respiration, carbon is also circulated between living organisms and nonliving components of the ecosystem.
The 'fast' carbon cycle is the movement of carbon through biotic components in the environment. Plants and other organisms that are capable of photosynthesis, obtain carbon dioxide from their environment and use it to build biological substances. Plants, animals, and decomposers such as bacteria and fungi, return carbon dioxide to the atmosphere by respiration.
The movement of carbon through the abiotic elements in the environment such as rocks, soil, and oceans forms the slow carbon cycle. The moving of carbon through these abiotic elements can take as long as 200 million years.
As organisms like nitrogen-fixing bacteria use nitrogen to synthesize the biological molecules needed for survival, atmospheric nitrogen has to be first converted to ammonia by nitrogen-fixing bacteria in aquatic and soil environments. Ammonia is then converted to nitrite and nitrate by the bacteria. Plants obtain nitrogen from the soil by absorbing ammonium (NH4-) and nitrate through their roots. Nitrate and ammonium are then used to produce organic compounds. Animals then consume plants and thus attain the nitrogen in the organic compounds. The nitrogen in organic form is then passed down the food chain when other animals eat these animals. Decomposers then return ammonia into the soil by decomposing solid waste and dead or decaying matter. Nitrifying bacteria convert ammonia to nitrite and nitrate. Denitrifying bacteria then convert nitrite and nitrate to nitrogen, releasing nitrogen back into the atmosphere.
Phosphorus is an essential nutrient needed for plant growth and animals as well. It has a vital role in cell development and is a key component of molecules that store energy such as Adenosine Triphosphate (ATP), Deoxyribonucleic Acid (DNA), and lipids.
Rocks when in contact with rainwater release phosphate ions and other minerals over time. This inorganic phosphate is then distributed in soils and water. Plants then take up inorganic phosphate from the soil, and these plants may then be consumed by animals. The phosphate is then incorporated into organic molecules such as DNA, and when plants or animals die and decay, the organic phosphate is returned to the soil. Bacteria in the soil then break down the organic matter into forms of phosphate that are absorbable by plants. It is also a process called mineralization. Phosphorus in the soil can then end up in waterways and oceans and can be incorporated into sediments over time.
Sulfur is a solid in its natural form and in this form; it is restricted to the sedimentary cycle. It can be transported by physical processes such as wind, erosion by water, and geological events like volcanic eruptions. It can also be transported by the ocean and to the atmosphere, land, and back to the oceans through its compounds such as sulfur dioxide, sulfuric acid, salts of sulfate, or organic sulfur by rainfall and rivers.
Plants and animals both play a role in cycling oxygen through the atmosphere. As you know, oxygen is crucial for many animals, including humans. We breathe in oxygen, and our bodies use it to make energy during a process called cellular respiration. This process releases carbon dioxide as a waste product, which is what we breathe out. Plants take in carbon dioxide during photosynthesis, in which they make food and oxygen. The oxygen is released, and the cycle starts again.
The most important criterion for life is water. Like the carbon cycle, the water cycle is the process of moving water between living things, the Earth, and the atmosphere. Water evaporates from bodies of water on Earth, like lakes, rivers, and oceans. The water vapor condenses in the clouds and forms precipitation that returns water to Earth. On Earth, some of the water returns to the lakes and oceans it originated from, and some soak into the ground, forming groundwater. Living organisms, like plants and animals, consume water. The water evaporates again, continuing the cycle.
Some scientists argue that the ecosystem is capable of complete recycling. Complete recycling has the meaning that 100% of the waste material is capable of being reconstituted indefinitely. Other scientists dispute this idea, claiming that complete recycling is not possible for technological waste.
Ecological recycling is very common in organic farming. Organic farms that conduct ecosystem recycling support more species, therefore, having a different food web structure. The ecological recycling agriculture model sticks to the principles below:
1. Transformation of matter from one form to another – Nutrient cycles allow the transformation of matter to different specific forms that enable the utilization of that element in different organisms.
2. Transfer of elements from one location to another – Nutrient cycles allow the transfer of elements from one location to another. Some elements are highly concentrated in areas that are inaccessible to most living organisms, such as nitrogen in the atmosphere. Nutrient cycles allow these elements to be transferred to more accessible locations such as the soil.
3. Functioning of ecosystems – Nutrient cycles assist the functioning of ecosystems. The ecosystem, which requires the state of equilibrium to function properly, restores to the equilibrium state through the nutrient cycles.
4. Storage of elements – Nutrient cycles facilitate the storage of elements. Elements that are carried through the nutrient cycles are stored in their natural reservoirs and are released to organisms in small amounts that are consumable.
5. Link organisms, living and non-living – Nutrient cycles link living organisms with living organisms, living organisms with the non-living organisms and non-living organisms with non-living organisms. This is essential because all organisms depend on one another and are vital for the survival of living organisms. These organisms are linked by the flow of nutrients which is engineered by the nutrient cycles.
6. Regulate the flow of substances – Nutrient cycles regulate the flow of substances. As the nutrient cycles pass through different spheres, the flow of elements is regulated as each sphere has a particular medium and rate at which the flow of elements is determined by the viscosity and density of the medium. Therefore, the elements in the nutrient cycles flow at different rates within the cycle and this regulates the flow of elements in those cycles.
