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Summary Alevel AQA biology unit 5 - nutrient cycles
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3.5.4 Nutrient cyclesThe nitrogen cycle
There are 4 main stages in the nitrogen cycle: Ammonification, nitrification, nitrogen fixation and
denitrification
Ammonification
This is the production of ammonia from organic nitrogen containing compounds. These compounds
include: urea, protein, nucleic acids, and vitamins (found in faeces and dead organisms). Saprobiontic
microorganisms such as fungi and bacteria feel on the faeces and dead organisms which release
ammonia. This then forms ammonium ions in the soil.
Nitrification
Ammonium ions in the soil are converted by nitrifying bacteria into nitrogen compounds that can be
used by plants known as nitrates. Nitrifying bacteria such as Nitrosomonas convert ammonium ions
into nitrates. Different nitrifying bacteria such as nitrobacterthen convert these nitrites into nitrates.
Nitrifying bacteria need oxygen for these conversions, so they need soil that have many air spaces.
Nitrogen fixation
When nitrogen gas is converted into nitrogen-containing compounds, this is carried out by free living
nitrogen-fixing bacteria and mutualistic nitrogen-fixing bacteria. Free-living bacteria reduce gaseous
nitrogen to ammonia so they can use it to manufacture amino acids. Nitrogen rich compounds are
released from them when they die and decay. Mutualistic nitrogen-fixing bacteria live in root nodules
of leguminous plants, and they obtain carbohydrates from plant and the plant gets amino acids from
the bacteria. The bacteria have a symbiotic relationship with these plants as the bacteria provide the
plants with nitrogen containing compounds and the plants give the bacteria organic compounds.
Denitrification
When the soil has little or no oxygen the microorganisms present chance. Fewer aerobic nitrifying
and nitrogen-fixing bacteria is found but there’s an increase in anaerobic denitrifying bacteria. Soil
nitrates are converted into nitrogen gas which is returned to the atmosphere. This reduces the
availability of nitrogen-containing compounds in plants so for land to be productive the soils must be
kept aerate to prevent denitrifying bacteria.
Phosphorus cycle
This shows how phosphate ions are transferred through an ecosystem. Phosphate ions are released
as the sedimentary rocks erode which releases the ions into the soil, sea, lakes, and rivers. Plants
absorb these ions through their roots. Land and aquatic plants absorb these ions with the help of
mycorrhizae. This increases the rate of absorption. The plants use phosphate ions to make
phospholipids and ATP. Consumers eat these plants and obtain the phosphate materials. Organic
matter is broken down by Saprobionic bacteria and the decomposition of dead plants, animals, and
waste releases phosphate ions back into the soil.
The role of mycorrhizae – they improve the uptake of phosphate ions
They fungi act like extensions of roots and increase the surface area for water and nutrients in the
soil. They act like a sponge, so they soak and hold in the water. The benefit of this is that the plants
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