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:
- Atmospheric nitrogen gas is converted into nitrogen-containing compounds such as ammonia, which forms ammonium ions (in solution) that can then be used by plants
- This biological nitrogen fixation is carried out by
- Nitrogen-fixing bacteria found ‘free living’ in soil and also in the root nodules of certain plants (peas, beans, clover – we call them leguminous plants) take N2 gas and change it into nitrates in the soil
- Lightning can ‘fix’ N2 gas, splitting the bond between the two atoms and turning them into nitrous oxides like N2O and NO2 that dissolve in rainwater and ‘leach’ into the soil
- Ammonification:
- Nitrogen compounds in waste products (e.g. urine and faeces) and dead organisms are converted into ammonia by saprobionts (a type of decomposer including some fungi and bacteria)
- This ammonia forms ammonium ions in the soil
- 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
- 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
