Bacteria make atmospheric nitrogen biologically available through nitrogen fixation and return nitrogen to the atmosphere through denitrification. These transformations are essential flows in the nitrogen cycle because most organisms cannot use atmospheric nitrogen directly.
This is HL-only content in IB ESS subtopic 2.3 Biogeochemical cycles.
The Bacterial Processes
| Process | Role of bacteria | Environmental significance |
|---|---|---|
| Nitrogen fixation | Nitrogen-fixing bacteria convert atmospheric nitrogen, , into ammonia, which can form ammonium ions, , in soil. | This introduces usable nitrogen into ecosystems, supporting plant growth and productivity. |
| Denitrification | Denitrifying bacteria convert nitrates, , into gaseous nitrogen, mainly , under anaerobic conditions. | This returns nitrogen to the atmosphere and reduces the amount of plant-available nitrogen in soil or water. |
Some nitrogen-fixing bacteria are free-living, while others form mutualistic associations with plants. For example, Rhizobium bacteria inhabit root nodules of legumes such as peas and clover. The bacteria receive carbohydrates from the plant, while the plant receives nitrogen compounds needed to produce proteins and nucleic acids.
Denitrification commonly occurs in waterlogged soils, wetlands and oxygen-depleted sediments. Because oxygen is limited, denitrifying bacteria use nitrate during respiration. This process can naturally remove excess nitrate from water, reducing the risk of eutrophication, although it can also reduce soil fertility.
A common misconception is that plants fix atmospheric nitrogen themselves. Most plants cannot use directly; they absorb nitrogen mainly as nitrate or ammonium after microorganisms have transformed it.
Exam Technique
For an explain question, identify the nitrogen compound before and after each transformation, state the bacterial role, and link the process to ecosystem productivity or nitrate removal. Do not confuse denitrification with nitrogen fixation: their overall flows operate in opposite directions.
