Nitrification and denitrification
Once nitrogen is in the soil as ammonium compounds, bacteria carry it round the rest of the cycle.
Nitrification
The nitrifying bacteria Nitrosomonas first turn ammonium compounds into nitrite, and then Nitrobacter turns nitrite into nitrate.
Into living things
Plants absorb the nitrate and use it to make proteins. The nitrogen in the proteins of legumes and other plants passes to animals along food webs.
Ammonification
After death, the activity of micro-organisms turns the nitrogen in bodies into ammonium compounds, which rejoin the soil.
Denitrification
The denitrifying bacteria Pseudomonas and Thiobacillus turn nitrate back into atmospheric nitrogen.
Assignment 15.5
Make a creative display board showing the nitrogen cycle or the carbon cycle.
Why the bacteria names are worth sorting out
Four groups of bacteria appear in the cycle, and each does a different job. It helps to follow the nitrogen: N2 from the air is fixed by Rhizobium and Azotobacter; ammonium is nitrified by Nitrosomonas (to nitrite) and Nitrobacter (to nitrate); nitrate is denitrified back to N2 by Pseudomonas and Thiobacillus. Fixing and nitrifying help plants; denitrifying takes nitrogen away from the soil. Waterlogged, airless soil favours denitrifiers, which is one reason drained, well-aired soil is more fertile.
Nitrifying and denitrifying are opposites
Nitrifying bacteria make nitrate for plants; denitrifying bacteria destroy it, returning nitrogen gas to the air.
Ammonium → nitrite → nitrate → back to air
Nitrosomonas and Nitrobacter nitrify; plants absorb nitrate; decay ammonifies the dead; Pseudomonas and Thiobacillus denitrify.