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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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