Nutrient Cycles


Nutrient Recycling (1)

Fungi and Bacteria have an important role in nutrient recycling 

A natural ecosystem is one that hasn't been changed by human activity. In natural ecosystems nutrients are recycled through the food webs, but human activity often disrupts the cycling of nutrients. 

Microorganisms, such as bacteria and fungi, are an important part of food webs. Many are saprobionts (a type of decomposer)- they feed on the remains of dead plants and animals and on their waste products (faeces, urine) breaking them down. This allows important chemical elements in the remains to be recycled. 

Saproionts secrete enzymes and digest their food externally, then absorb the nutrients that they need. This is known as extracellular digetsion. During this process, organic molecules are broken down into inorganic ions. Obtaining nutrients from dead organic matter using extracellular digestion is known as saprobiotic nutrition

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Nutrient Recycling (2)

Some fungi form symbiotic relationshoips with the roots of plants. These relationships are known as mycorrhizae. 

  • The fungi are made up of long, thin strands called hyphae, which connect to the plant's roots. 
  • The hyphae greatly increase the surface area of the plant's root system, helping the plant to absorb ions from the soil that are usually scarce (e.g. phosphorus). Hyphae also increase the uptake of water by the plant. 
  • In turn, the fungi obtain organic compounds, such as glucose, from the plant. 
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The Nitrogen Cycle

The nitrogen cycle shows how nitrogen is recycled in eco-systems 

Plants and animals need nitrogen to make proteins and nucleic acids (DNA and RNA). The atmosphere's made up of about 78% nitrogen gas, but plants and animals can't use it in that form- they need bacteria to convert it into nitrogen-containing compounds first. The nitrogen cycle shows how nitrogen is converted into a useable form and then passed on between different living organisms and the non-living environment. 

The nitrogen cycle includes food chains (nitrogen is passed on when organisms are eaten), and four different processes that involve bacteria - nitrogen fixation, ammonification, nitrification and dentrification. 

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Nitrogen Fixation

Nitrogen Fixation is when nitrogen gas in the atmosphere is turned into nitrogen-containing compounds. Biological nitrogen fixation is carried out by bacteria such as Rhizobium. They turn nitrogen into ammonia, which goes on to form ammonium ions in solution that can then be used by plants 

Rhizobium are found inside root nodules (growths on the roots) of leguminious plants (e.g. peas, beans and clover)

They form a mutualistic relationship with the plants- they provide the plant with nitrogen compounds and the plant provides them with carbohydrates. 

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Ammonification and Nitrification


Ammonification is when nitrogen compounds from dead organisms are turned into ammonia by saprobionts, which goes on to form ammonium ions. 

Animal waste (urine and faeces) also contains nitrogen compounds. These are turned into ammonia by saprobionts and go on to form ammonium ions. 


Nitrification is when ammonium ions in the soil are changed into nitrogen compounds that can then be used by plants (nitrates) 

First nitrifying bacteria called Nitrosomonas change ammonium ions into nitrites 

Then other nitrifying bacteria called Nitrobacter change nitrites into nitrates. 

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Denitrification is when nitrates in the soil are converted into nitrogen gas by dentrifying bacteria- they use nitrates in the soil to carry out respiration and produce nitrogen gas. 

This happens under anaerobic conditions (where there is no oxygen) e.g in waterlogged soils 

Other ways that nitrogen gets into the ecosystem are by: 

  • Lightning - which fixes atmospheric nitrogen 
  • Artificial fertilisers - they are produced from atmospheric nitrogen on an industrial scale in the Haber process
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Phosphorus Cycle (1)

Plants and animals need phosphorus to make biological molecules such as phospholipids (which make up cell membranes), DNA and ATP. Phosphorus is found in rocks and dissolved in the oceans in the form of phosphate ions. Phosphate ions dissolved in water in the soil can be assimilated (absorbed and then used to make more complex molecules) by plants and other producers. 

The phosphorus cycle shows how phosphorus is passed through an ecosystem. 

1.) Phosphate ions in rocks are released into the soil by weathering 

2.) Phosphate ions are taken into the plants through the roots. Mycorrhizae gently increase the rate at which phosphorus can be assimilated 

3.) Phosphate ions are transferred through the food chain as animals eat the plants and are in turn eaten by other animals. 

4.) Phosphate ions are lost from the animals in the waste products. 

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Phosphorus Cycle (2)

5.) When plants and animals die, saprobionts are involved in breaking down the organic compounds, releasing phosphate ions into the soil for assimilation by plants. These microorganisms also release the phsophate ions from urine and faeces. 

6.) Weathering of rocks also releases phosphate ions into seas, lakes and rivers. This is taken up by aquatic producers, such as algae. and passed along the food chain to birds. 

7.) The waste produced by sea birds is known as guano and contains a high proportion of phosphate ions. Guano returns a significant amount of phosphate ions to soils (particually in coastal areas). It is often used as a natural fertiliser. 

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Fertilisers and Eutrophication

Nutrients are lost when crops are harvested 

  • Crops take in minerals from the soil as they grow and use them to build their own tissues. 
  • When crops are harvested, they're removed from the field where they're grown rather than being allowed to die and decompose there. This means that the mineral ions that they contain (e.g. phosphates and nitrates) are not returned to the soil by decomposers in the nitrogen or phosphorus cycles. 
  • Phosphates and nitrates are also lost from the system where animals or animal products are removed from the land. Animals eat grass and other plants, taking in their nutrients. When they are taken elsewhere for slaughter or transferred to a different field, the nutrients aren't replaced through their remains or waste products. 
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Fertilisers are added to soils to replace lost nutrients 

Adding fertiliser replaces the lost minerals, so more energy from the ecosystem can be used for growth, increasing the efficiency of energy transfer. Fertilisers can be artificial or natural: 

Artificial fertilisers are inorganic - They contain pure chemicals (e.g. ammonium nitrate) as powders or pellets 

Natural fertilisers are organic matter- They include manure, composted vegetables, crop residues (the parts left over after the harvest) and sewage sludge. 

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Environmental issues with fertilisers

Sometimes more fertiiser is applied than the plants need. This can lead to the fertilisers leaching into waterways 

Leaching is when water soluble compounds in the soil are washed away, e.g. by rain or irrigation systems. They're often washed into nearby ponds and rivers. Leaching is more likely to occur if the fertiliser is applied just before heavy rainfall 

This can lead to eutrophication 

Inorganic ions in chemical fertilisers are relatively soluble. This means that excess minerals that are not used immediately are more likely to leach into waterways. In natural fertilisers, the nitrogen and phosphorus are still contained in organic molecules that ned to be decomposed by microorganisms before they can be absorbed by plants. This means that their release into the soil for uptake by plants is more controlled, and leaching is less likely. 

This leachng of phosphates is less likely than the leaching of nitrates because phosphates are less soluble in water. Using fertilisers also changes the balance of nutrients in the soil - too much of a partcular nutrient can cause crops and other plants to die 


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Eutrophication is caused by excess nutrients 

This is the process of eutrophication: 

1.) Mineral ions leached from fertilised fields stimulate the rapid growth of algae in ponds and rivers 

2.) Large amounts of algae (algal bloom) block light from reaching the plants below

3.) Eventually the plants die because they're unable to photosynthesise enough 

4.) Bacteria feed on the dead plant matter. The increased numbers of bacteria reduce the oxygen concentration in the water by carrying out aerobic respiration. 

5.) Fish and other aqauatic organisms die because there isn't enough dissolved oxygen. 

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