There is limited availability of usable ions in the environment. Therefore, important elements such as carbon, oxygen and phosphorus need to be recycled. This happens in a nutrient cycle.
A nutrient cycle can be defined as the movement and exchange of organic and inorganic material back into the production of living matter. Inorganic material will be used by the producers and converted into organic matter that can be ingested by the primary consumers.
What are the main types of nutrient cycles?
The main nutrient cycles include water, carbon, nitrogen, oxygen, and phosphorus cycles. Primary producers, such as plants in the ecosystem, take up inorganic nutrients from the non-living environment. These inorganic nutrients can be transformed into biomass. The nutrient cycles work in balance with each other. They are responsible for replenishing the ecosystem with nutrients and getting rid of waste.
Biomass is defined as organic matter. Energy travels between organisms through biomass.
The water cycle
The water cycle can also be referred to as the hydrological cycle . The hydrological cycle is important in determining the weather and climate. Water is needed by producers (such as plants) to grow and therefore, it is critical for all life.
- Evaporation - most water evaporates from the oceans. Some evaporation will also occur from lakes, rivers, and streams.
- Transpiration - water loss from the plant stomata.
- Condensation - water vapor is converted back into liquid. Condensation will form clouds.
- Precipitation - water is returned to the Earth by rain, snow and ice. Most of the runoff will return to lakes and rivers, but some will sink into the ground by the infiltration process.
- Melting and freezing - water can be locked up in ice by freezing, in bodies such as glaciers. Water can be returned by melting these glaciers.
Stomata are small pores found on the surface of plant leaves and stems by which substances, such as water and oxygen, can flow through.
The infiltration process describes the flow of water into soil and rock.
Fig. 1 - The water cycle
The oxygen cycle
Oxygen plays an important role in respiration in all living organisms and is recycled between living organisms and the air. Breathing and respiration are processes that result in the removal of oxygen from the environment. Meanwhile, photosynthesis, which is complementary to respiration, results in the addition of oxygen.
The carbon cycle
This is a series of processes by which carbon compounds are interchanged in the environment. This involves the incorporation of carbon dioxide into living tissue by photosynthesis, its return to the atmosphere through respiration, the decay of dead organisms, and the burning of fossil fuels.
Carbon moves via a series of steps:
- Combustion - the burning of fossil fuels releases carbon back into the atmosphere as carbon dioxide.
- Photosynthesis - carbon dioxide is taken up by plants and converted into organic molecules, such as glucose.
- Cellular respiration - organic molecules, such as glucose, are broken down during cellular respiration to release energy, water and carbon dioxide.
- Precipitation - carbon dioxide can precipitate from the atmosphere as carbonate in the ocean sediment.
- Decomposition - carbon can be released back into the atmosphere as carbon dioxide when an organism dies and enters the decay process.
Fig. 2 - The carbon cycle
The nitrogen cycle
Nitrogen is the most abundant gas in the atmosphere. It is used in the production of amino acids, proteins and nucleic acids (DNA and RNA). Although abundant, nitrogen is tricky, in that the gaseous form present in the air is not available to all organisms. Hence, it has to be converted to more readily available forms. Only a few single-celled organisms, such as bacteria, are able to take up nitrogen in its gaseous form.
Plants can take up nitrogen in other forms such as nitrates and ammonia. These substances are produced during nitrogen fixation which describes the conversion of atmospheric nitrogen into ammonium ions, carried out by nitrogen-fixing bacteria. You can learn more about this process in our Nitrogen Cycle article.
Phosphorus is an important component of ATP, phospholipids, nucleic acids and other substances. Unlike the other nutrients we have talked about earlier in this article, phosphorous lacks the gaseous phase in the atmosphere. Phosphorus reservoirs mainly lay in mineral form - as a phosphate ions () in sedimentary rock deposits.
The main steps in the phosphorus cycle include:
- Weathering - rocks erode, due to rain and other weather events. Phosphorus is washed into the soil.
- Absorption by plants and animals - microorganisms are able to take up phosphorous from the soil. Plants can also absorb phosphorous directly from the soil and when they are consumed, animals also gain the nutrient. Animals can also gain phosphorous from drinking water.
- Decomposition - decay results in the release of phosphorus back into the environment, and the cycle of being absorbed by plants and animals can be repeated.
Microorganisms in nutrient cycles
Microorganisms, such as fungi and bacteria, can form mutual symbiotic associations with plants.
You might have noticed that sometimes viruses are included in microorganisms. However, viruses should be referred to as microbes rather than microorganisms, because they are non-living!
Fig. 4 - The classification of microorganisms
Microorganisms and plant symbiosis
Mutual symbiotic relationships refer to an instance where two organisms live in close physical proximity and they both benefit from the association. The relationship between nitrogen-fixing bacteria and plants is one example.
In mutualistic relationships between plants and microorganisms, microorganisms receive organic compounds, such as sugars and amino acids, from the plants. This helps them perform metabolic processes, such as photosynthesis and protein synthesis. In return, microorganisms increase the total surface area of the plant for water and mineral absorption.
If plants lost their associations, they would become more susceptible to droughts and fewer nutrients would be available for the plants to take up. Less susectibility to draughts comes from the fungal ability to temporarily absorb water and store it for the future that plant can utilize when needed.
Examples of plant and microorganism associations
- Legumes and Rhizobium - Rhizobium colonize the legume plant's root cells, where they fix nitrogen. In return, they receive sugars.
- Endophytic fungi and plants - endophytic fungi colonize plant tissue without damaging it and release toxins to repel herbivores. They can also increase resistance to infections from other microorganisms.
- Azotobacter - Aztobacter are a group of free-living nitrogen-fixing bacteria. They do not directly form a relationship with the plant. However, aztobacter will increase the soil fertility by fixing nitrogen.
Fig. 5 - The symbiosis between a plant and fungi
Human impact on the nutrient cycles
What impact do we have on nutrient cycles? We will cover a few examples below.
Agriculture
When crops are harvested, nutrients are removed from the soil. Human population growth and increased agriculture have caused significant changes to the nutrient cycles.
Agriculture can also accelerate land erosion, leading to more nutrients draining off. Nutrient runoff will enter rivers and oceans and will lead to eutrophication, which describes increased algae growth due to nutrient excess. Uncontrolled algae growth can form dead zones, ie, a lack of oxygen for other organisms.
The addition of fertilisers will increase levels of macronutrients and cause imbalances. This can cause a decrease in biodiversity and can have other impacts.
Deforestation
Similar to agriculture, felling trees removes stored nutrients vital for plant growth. Trees are major carbon dioxide consumers. Less carbon dioxide will be taken up from the environment and will lead to an increase in carbon dioxide concentration in the atmosphere. The build-up of carbon dioxide could lead to global warming, as it is the main greenhouse gas.
You have most definitely heard about global warming mentioned before. This is for a good reason! Most of what you may have heard is about raising awareness on this issue b ecause we are the main cause of it. By burning fossil fuels, there has been an increase in greenhosue gasses, ie gasses that trap heat in our atmosphere. This includes carbon dioxide and others such as nitrous oxide.
The heat trapped in the atmosphere raises our Earth's temperature which affects not only us but species diversity. In the areas where the temperatures has risen more than a species can tolerate, consequences such as suitable habitat loss and even death follow. A good example is bleaching in the coral reefs due to temperature rises.
Burning of fossil fuels
This affects the amount of nitrogen in the atmosphere. Fossil fuel releases nitrogen oxides due to combustion and consequently, they can combine with other elements, such as sulfur oxides. This leads to acid rain and smog. Excess nutrients will also wash into nearby water bodies such as rivers, causing eutrophication.
Nutrient Cycles - Key takeaways
- The nutrient cycle is the movement and exchange of organic and inorganic material back into the production of living matter. Energy and matter are transferred between living organisms and the non-living environment.
- The main nutrient cycles include the water cycle, oxygen cycle, carbon cycle, nitrogen cycle and phosphorus cycle.
- Plants and fungi form symbiotic associations, known as mycorrhizae . Fungi will absorb water and minerals, allowing plants to resist drought better and absorb nutrients more readily. Meanwhile, fungi receive organic compounds such as sugars and amino acids from the plants.
- Human activity, such as the combustion of fossil fuels, deforestation and agriculture, alter nutrient cycles.
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