Biology · Introductory biology · Worked example
Trace a nitrogen atom through the cycle
A nitrogen atom starts in N₂ in the air above a meadow and ends up back in the air. Trace its path through soil bacteria, a grass plant, a rabbit and the soil, naming each process and who carries it out.
Fixation
Nitrogen-fixing bacteria in the soil reduce N₂ to ammonia, which becomes ammonium, NH₄⁺. Only some bacteria and archaea can break N₂’s triple bond.
Nitrification
Nitrifying bacteria oxidize the ammonium to nitrite, NO₂⁻, and other nitrifying bacteria oxidize the nitrite to nitrate, NO₃⁻. Together the two steps use oxygen, which is why nitrification stops in waterlogged soil.
Count the electrons
Nitrification is an oxidation. Nitrogen’s oxidation number climbs from −3 in ammonium to +3 in nitrite and +5 in nitrate, so each nitrogen atom gives up 8 electrons on the way, and nitrifying bacteria live on the energy this releases.
Assimilation and eating
The grass absorbs the nitrate through its roots and builds the nitrogen into amino acids and proteins. A rabbit eats the grass, digests its proteins and builds the amino acids into its own.
Ammonification
When the rabbit dies, or in its wastes, decomposers break down the proteins and release the nitrogen as ammonium again. Nitrifying bacteria can turn it into nitrate once more.
Denitrification
In waterlogged, low-oxygen soil, denitrifying bacteria use nitrate in place of oxygen and reduce it to N₂, which returns to the air.
Result
N₂ becomes ammonium by fixation (nitrogen-fixing bacteria), then nitrite and nitrate by nitrification (nitrifying bacteria), grass protein by assimilation and rabbit protein by eating; decomposers release it as ammonium by ammonification, it is nitrified again, and denitrifying bacteria return it to the air as N₂.
Your turn
A farmer’s field stays flooded for weeks, and the oxygen in its soil runs low. Which process speeds up, and what happens to the soil’s nitrate?
Show the answer and explanation
Denitrification speeds up, and nitrate is lost to the air as N₂.
Denitrifying bacteria use nitrate in place of oxygen when oxygen is scarce, so waterlogged soil loses its nitrate as nitrogen gas. Nitrification, which needs oxygen, slows down.
Keep exploring
The nitrogen cycle opens in Biology with every flux named; step through the fluxes and match each to a step of the atom’s path.
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