Nitrogen fixation is the conversion of nitrogen gas (N2) from the atmosphere into ammonia (NH3), which forms ammonium ions (NH4+) in solution. This makes nitrogen available to living organisms.Breaking the strong triple bond in N2 needs a lot of energy. Nitrogen-fixing bacteria do it using the enzyme nitrogenase, with ATP from respiration.There are two main groups of nitrogen-fixing bacteria:• Free-living nitrogen-fixing bacteria in the soil (for example Azotobacter). They use the ammonia to make their own amino acids. When they die, saprobionts release the nitrogen into the soil as ammonium ions.
• Mutualistic nitrogen-fixing bacteria (for example Rhizobium) that live inside root nodules of leguminous plants such as peas, beans and clover. The bacteria give the plant nitrogen compounds, such as amino acids, and the plant gives the bacteria carbohydrates from photosynthesis. Both benefit, so the relationship is mutualistic.Nitrogenase is inactivated by oxygen. Root nodules contain leghaemoglobin, a pigment that binds oxygen and keeps its concentration low around the enzyme.Nitrogen is also fixed by lightning, and industrially by the Haber process, which makes ammonia for fertilisers.
Example: In crop rotation, a farmer grows clover one year and ploughs it into the soil. When saprobionts decompose it, the fixed nitrogen is released, raising soil nitrate levels for the next crop, such as wheat, without artificial fertiliser.
• Mutualistic nitrogen-fixing bacteria (for example Rhizobium) that live inside root nodules of leguminous plants such as peas, beans and clover. The bacteria give the plant nitrogen compounds, such as amino acids, and the plant gives the bacteria carbohydrates from photosynthesis. Both benefit, so the relationship is mutualistic.Nitrogenase is inactivated by oxygen. Root nodules contain leghaemoglobin, a pigment that binds oxygen and keeps its concentration low around the enzyme.Nitrogen is also fixed by lightning, and industrially by the Haber process, which makes ammonia for fertilisers.
Example: In crop rotation, a farmer grows clover one year and ploughs it into the soil. When saprobionts decompose it, the fixed nitrogen is released, raising soil nitrate levels for the next crop, such as wheat, without artificial fertiliser.