Nitrogen is essential for the formation of amino acids in proteins or for use in necessary biomolecules such a nucleic acids.
- 78% of the air is nitrogen.
- Nitrogen is unreactive so it cannot be used directly by plants to make protein.
- Only nitrates are useful to plants, so other processes are needed to convert free nitrogen (N2) to useable forms.
- Plants must secure their nitrogen in “fixed” form, i.e., incorporated in compounds such as:
- nitrate ions (NO3–)/ammonium (NH4+) are used directly
- ammonia (NH3)/nitrite ions (NO2–) are converted to the above
Four biological processes participate in the cycling of nitrogen through the biosphere:
- Nitrogen fixation
- Turning nitrogen into nitrates.
- Decomposition
- Converting organic material into ammonia and ammonium.
- Nitrification
- Turning ammonia into nitrites and nitrates.
- Denitrification
- Converting nitrates into nitrogen.
Four types of bacteria participate in the cycling of nitrogen through the biosphere
- Saprophytic bacteria
- (Also fungi). Undertake ammonification by converting organic material into ammonia/ammonium.
- Nitrifying
- Live in the soil and turn ammonia/ammonium into nitrates.
- Nitrogen-fixing
- Either free-living in the soil, or in root nodules. Convert nitrogen gas into nitrates.
- Denitrifying
- Undertake denitrification by converting nitrates into nitrogen. Usually in waterlogged soils.
Nitrogen Cycle Steps
1a. Nitrogen Fixation
Lightning converts nitrogen gas in the air (N2) to nitrates (NO3–) by bonding it with oxygen and then dissolving in rain.
1b. Nitrogen Fixation
Nitrogen gas (N2) is also converted to ammonia (NH3) or ammonium (NH4+) by nitrogen-fixing bacteria in soil or root nodules of legumes or clover plants.
2. Assimilation
Plants absorb nitrates (NO3–) from the soil and use these to build up proteins. The plant may be eaten by an animal, and its biomass used to produce animal protein.
3. Ammonification / Nitrification
Urea and egested material is broken down by decomposers. This results in nitrogen being returned to the soil as ammonia (NH3) or ammonium (NH4+).
Ammonia (NH3) and ammonium (NH4+) are converted to nitrites (NO2–) and then nitrates (NO3–) by nitrifying bacteria in the soil.
4. Ammonification / Nitrification
Decomposers also break down the bodies of dead organisms resulting in nitrogen being returned to the soil as ammonia (NH3) and ammonium (NH4+).
Ammonia (NH3) and ammonium (NH4+) are converted to nitrites (NO2–) and then to nitrates (NO3–) by nitrifying bacteria.
5. Denitrification
In some conditions denitrifying bacteria in the soil break down nitrates (NO3–) by liberating oxygen and returning nitrogen to the air (N2). This is usually in waterlogged soil.
In 1999, it was also found that nitrites (NO2–), ammonia (NH3) and ammonium (NH4+) can also be converted back into nitrogen by a newly discovered bacteria (Brocadia anammoxidans – the anammox reaction).
Nitrogen compounds in detritus are broken down by bacteria to ammonium ions (NH₄⁺). Describe how ammonium ions are converted into a form that can be readily absorbed by producers.
Explain the roles of decomposers and nitrifying bacteria in converting nitrogen in organic compounds in sewage into a soluble, inorganic form.
Describe the part played by nitrogen-fixing bacteria in the nitrogen cycle.
Explain what is meant by nitrification.
Explain what is meant by denitrification.
Wet moorland soils often contain low concentrations of nitrogen compounds, as a result of denitrification. Sundew is a plant which lives in wet moorlands. Its leaves have sticky hairs which can trap small insects that are then digested. Explain how digestion of insects helps the sundew to obtain additional nitrogen compounds.