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Nutrient cycles are processes that move essential nutrients within ecosystems. These cycles include the carbon cycle, nitrogen cycle, and phosphorus cycle. In each cycle, nutrients are taken up by living organisms, released into the environment through waste or death, and eventually recycled back into the ecosystem.
Human activities can disrupt these cycles, leading to environmental problems such as pollution and soil degradation. Understanding nutrient cycles is important for maintaining healthy ecosystems and sustainable agriculture.
The carbon cycle
The carbon cycle describes the movement of carbon through all four spheres. The carbon cycle occurs in many stages.
Table below summarises the forms of carbon in each of the chemical spheres. The carbon cycle can be summarised in seven processes, which either increase or decrease carbon dioxide in the atmosphere.
The main stages and processes in the carbon cycle. The numbers in this figure relate to the text that follows.
1. Photosynthesis
Carbon dioxide in Earth’s atmosphere is absorbed by plants during photosynthesis.
The carbon dioxide reacts with water taken in from the soil to make glucose, a carbon-containing compound.
The following equations summarise the process of photosynthesis for plants:
carbon dioxide + water → glucose + oxygen
6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O (full)
6CO2 + 6H2O → C6H12O6 + 6O2 (net)
2. Transfer of carbon through the food chain
Animals obtain their carbon by eating plants and other animals. When animals and plants are eaten, their carbon content is transferred through the food chain.
3. Respiration
Plants and animals break down glucose using oxygen to form carbon dioxide, water and ATP (a form of energy that the body uses). This process is called respiration. The carbon dioxide is added to the atmosphere when organisms breathe out.
During respiration, organisms release carbon dioxide gas into the atmosphere.
The following equation summarises respiration:
glucose + oxygen → carbon dioxide + water + ATP
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP
4. Excretion, egestion, death and decomposition
Some of the carbon content of animals and plants enters the soil as waste such as urine, faeces and leaf litter.
Dead animals and plants (organic matter) also contribute to the carbon content of the soil.
Waste material and dead organic matter is broken down by decomposers, which respire, releasing carbon dioxide back into the atmosphere.
Excretion of waste materials, such as these wombat faeces, increases the carbon content of the soil.
5. Formation of fossil fuels
Fossil fuels like coal, oil and gas are composed of the carbon from plants and animals that died millions of years ago. Carbon is stored as fossil fuels for millions of years before re-entering the carbon cycle.
Figure 8.17 Coal is a fossil fuel formed from the carbon content of dead plants that have been buried and compressed for millions of years.
6. Combustion
When fuel is burned in oxygen (combustion), carbon dioxide and water are formed. Carbon dioxide is released back into the atmosphere.
The following equation shows the combustion of octane, a component of oil:
octane + oxygen → carbon dioxide + water
2C8H18 + 25O2 → 16CO2 + 18H2O
7. Formation of limestone
Carbon dioxide gas in the atmosphere can dissolve in the oceans, forming carbonic acid.
The carbon dioxide also provides the carbon that combines with calcium in mineral deposits to make calcium carbonate (CaCO3), which is the major component of shells and the exoskeletons of marine microorganisms.
These dead organisms sink to the bottom of the ocean. Over and over millions of years, become compacted forming limestone. Limestone is a sedimentary rock and an important building material.
Fossilised shells in limestone
The nitrogen cycle
Nitrogen in its elemental form has the formula N2. Nitrogen is important for all living things as it is the basis of DNA and amino acids, the subunits of proteins. The majority of gas found in the atmosphere is N2 (78%). However, a large proportion of organisms cannot access this form of nitrogen.
The nitrogen cycle can be summarised in six processes, each of which can result in either a gain or loss of nitrogen in the form of nitrates from the soil.
Table below shows the forms of nitrogen in each of the chemical spheres.
The main stages and processes in the nitrogen cycle
Nitrogen-fixing bacteria can be trapped in root nodules of certain plants.
What are the key nutrient cycles that occur in ecosystems?
How do the nutrient cycles contribute to the functioning and sustainability of ecosystems?
What human activities can disrupt nutrient cycles and have negative impacts on ecosystems?
How can we manage nutrient cycles to promote healthy and sustainable ecosystems?
What are the potential consequences of nutrient cycle disruptions on human societies and economies?
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