The Nitrogen Cycle
- The nitrogen cycle shows how nitrogen is recycled in ecosystems
- Plants and animals require nitrogen in order to produce proteins and nucleic acids (DNA and RNA)
- About 78% of the atmosphere is actually nitrogen gas but plants and animals cannot access the nitrogen in this gaseous form
- Instead, they rely on certain bacteria to convert the nitrogen gas into nitrogen-containing compounds, which can be taken up by plants
- The nitrogen cycle shows this conversion, as well as how the nitrogen in the nitrogen-containing compounds is then passed between trophic levels or between living organisms and the non-living environment
The role of bacteria in the nitrogen cycle
- There are four key processes in the nitrogen cycle that are carried out by different types of bacteria
- Nitrogen fixation:
- Nitrogen fixing bacteria convert N2 gas into ammonium compounds, which can then be converted to usable nitrates
- Nitrogen fixing bacteria can be free-living in the soil or can live within the root nodules of some plants
- Lightning can also split the bond between the two N atoms, turning them into nitrous oxides like N2O and NO2 that dissolve in rainwater and leach into the soil
- Nitrogen fixing bacteria convert N2 gas into ammonium compounds, which can then be converted to usable nitrates
- Ammonification:
- Nitrogen compounds in waste products (e.g. urine and faeces) and dead organisms are converted into ammonia by saprobionts
- These are decomposers, e.g. fungi and bacteria
- This ammonia forms ammonium ions in the soil
- Nitrogen compounds in waste products (e.g. urine and faeces) and dead organisms are converted into ammonia by saprobionts
- Nitrification:
- The ammonium ions in the soil are converted by nitrifying bacteria into nitrogen compounds that can be used by plants, known as nitrates
- Initially, nitrifying bacteria convert ammonium ions into nitrites
- Different nitrifying bacteria then convert these nitrites into nitrates
- The ammonium ions in the soil are converted by nitrifying bacteria into nitrogen compounds that can be used by plants, known as nitrates
- Denitrification:
- Denitrifying bacteria use nitrates in the soil during respiration
- This process produces nitrogen gas, which returns to the atmosphere
- This process occurs in anaerobic conditions (when there is little or no oxygen available, such as in waterlogged soil)
The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification
