biology unit 6(nitrogen cyle) new The Nitrogen Cycle
Higher Tier Only
Nitrogen is present as N2 gas in the atmosphere and within biological molecules, e.g. proteins, in the tissues of living organisms
Nitrogen is cycled through ecosystems by the processes of the nitrogen cycle
Uptake of nitrogen by living organisms
N2 in the atmosphere is made available to living organisms by the process of nitrogen fixation
Nitrogen fixation is carried out by nitrogen-fixing bacteria which convert N2 gas into ammonium compounds; these compounds are converted into nitrates in the soil
Nitrogen-fixing bacteria can be free-living in the soil or they can live within root nodules of legume plants, e.g. peas, beans and clover
Nitrogen gas can also be fixed by lightning when it strikes the earth, or during the production of chemical fertilisers
After nitrogen fixation has occurred plants absorb the nitrates in the soil and use the nitrogen to build plant proteins
Transfer of nitrogen between living organisms
Animals feed on plants and digest the proteins in the plant tissues, providing nitrogen to build animal proteins
Nitrogen may then be passed from one consumer to another up the food chain in the same way
Release of nitrogen from living tissues
Nitrogen from living organisms is returned to the soil in the form of ammonia by the action of decomposers such as bacteria and fungi
When animals and plants die the proteins inside their tissues are broken down by the action of decomposers and returned to the soil in the form of ammonia
Waste, i.e. urine and faeces, from animals contains urea, which is converted into ammonia by the action of the bacterial enzyme urease
The plants can’t absorb ammonia so nitrifying bacteria convert the ammonia to nitrates which can then be taken up again by plants
The conversion of ammonium compounds to nitrates is known as nitrification
Returning nitrogen to the atmosphere
Nitrates in the soil can be converted back into nitrogen gas (N2) by the action of denitrifying bacteria
This process is known as denitrification
Denitrifying bacteria are active in anaerobic conditions, e.g. in waterlogged or compacted soil
Farmers can decrease the activity of denitrifying bacteria by ploughing the soil to increase aeration
Nitrogen cycle diagram
nitrogen-cycle-gcse
The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification
Factors affecting the nitrogen cycle
Because so many processes within the nitrogen cycle are carried out by microorganisms the cycle can be affected by factors that affect microorganism activity, e.g.
Temperature
This affects the rate at which enzyme-controlled reactions can occur
Oxygen availability
Aerobic bacteria rely on oxygen for respiration
Low oxygen availability may lead to an increase in the activity of anaerobic bacteria, e.g. denitrifying bacteria
pH
This affects the rate of enzyme-controlled reactions as extreme pH levels can cause denaturation
Water
Water is needed by living organisms, so the rate of microbial activity increases in soil where moisture is present
The presence of heavy metals in the soil
Heavy metals, e.g. mercury and lead, can be toxic to the metabolism of microorganisms
These factors are known to influence the rate at which decomposition occurs in compost heaps and landfill sites
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