Nitrogen and phosphorus cycles: every step and every microorganism

The nitrogen cycle in four processes

• Nitrogen fixation: Rhizobium in root nodules of legumes (and free-living bacteria) reduce N₂ gas to ammonia.
• Ammonification: saprobionts decompose proteins, urea and nucleic acids in dead matter and waste, releasing ammonium ions.
• Nitrification: nitrifying bacteria oxidise ammonium to nitrites, then nitrites to nitrates, in aerobic conditions.
• Denitrification: anaerobic denitrifying bacteria convert nitrates back to N₂ gas — common in waterlogged soils.

The phosphorus cycle

Phosphorus has no atmospheric gas phase. Phosphate ions are released slowly by weathering of rocks, absorbed by plants for DNA, ATP and phospholipids, passed along food chains, returned by saprobionts, and eventually locked into sediments that form new rock. Guano from seabirds transfers phosphate from sea to land.

Mycorrhizae

Mycorrhizae are symbiotic associations between fungi and plant roots. The fungal hyphae greatly increase surface area for absorbing water and mineral ions such as phosphate; in return the fungus receives carbohydrates from the plant.

Fertilisers and eutrophication

• Nitrate leaches into rivers and lakes, causing rapid algal growth (algal bloom).
• Algae block light, so plants below die.
• Saprobiotic bacteria decomposing the dead plants respire aerobically, reducing dissolved oxygen.
• Fish and other aerobic organisms die.