Nitrogen Fixation, nitrification and denitrification

Nitrogen fixation Impossible for plants to use nitrogen gas directly because it is very unreactive. Instead they need a supply of fixed nitrogen such as ammonium ions or nitrate ions. Can occur when lightning strikes or through the Haber process. BUT these processes only account for 10% of nitrogen fixation around the world. Nitrogen-fixing bacteria account for the rest. Nitrogen fixing bacteria account for the rest. Many of these live freely in the soil and fix nitrogen gas in the air. The bacteria have a mutualistic relationship with the plant: the bacteria provide the plant with fixed nitrogen and receive carbon compounds such as glucose in return.

Nitrification Nitrification happens when chemoautotrophic bacteria in the soil absorb ammonium ions. Ammonium ions are released by bacteria involved in putrefaction of proteins found in dead waste or organic matter. Chemoautotrophic bacteria obtain their energy by oxidising nitrites to nitrates. Oxidation requires oxygen, this can only happen in well aerated soils. Nitrated can be absorbed from the soil by plants and used to make nucleotide bases and amino acids.

Denitrification Other bacteria convert nitrates back to nitrogen gas. When the bacteria involved are growing under anaerobic conditions such as in waterlogged soils, they use nitrates as a source of oxygen for their respiration and produce nitrogen gas and nitrous oxide.