Biology 9.15 - Nitrogen cycle and farming

Nitrogen and nitrate uptake

Plants need nitrogen to make amino acids and proteins for new cells and growth. Although the atmosphere contains a large store of nitrogen gas, most plants cannot use nitrogen in the form $\ce{N2}$. They obtain nitrogen mainly by absorbing dissolved nitrate ions, $\ce{NO3^-}$, from soil water through their roots.

Making nitrate available therefore requires conversions between different nitrogen compounds. Microorganisms carry out the key conversions.

Following the nitrogen cycle

Nitrogen fixation moves nitrogen from the abiotic atmosphere into compounds that can enter living biomass. Plants use absorbed nitrogen compounds to make amino acids and proteins. Feeding then moves those compounds into animals.

Waste and dead biomass still contain nitrogen. Decomposer microorganisms return that organic nitrogen to ammonium compounds in soil, and nitrifying bacteria convert ammonium into nitrate. Roots can absorb the nitrate again. Denitrifying bacteria complete another return route by converting soil nitrate to atmospheric nitrogen gas.

The cycle is not one bacterium doing every job. Different bacterial groups carry out different chemical conversions, and plants do not usually absorb nitrogen gas directly from the air.
[DIAGRAM: asset_slug: biof_nitrogen_cycle; description: Nitrogen fixation by free-living and root-nodule bacteria, ammonium, nitrification, nitrate uptake and feeding, decomposition, denitrification and nitrate fertiliser.]

Fertilisers and crop rotation

Harvesting removes nitrogen in plant biomass from a field. Farmers can restore plant-available nitrogen in two assigned ways:

• Fertilisers add nitrogen-containing compounds. Nitrate fertilisers supply nitrate ions directly; ammonium fertilisers supply ammonium ions that nitrifying bacteria can convert into nitrates. The ions dissolve in soil water and can be taken up by roots.
• Crop rotation alternates crops. A legume such as peas, beans or clover is grown in one part of the rotation. Nitrogen-fixing bacteria in its root nodules convert atmospheric nitrogen into nitrogen compounds used by the legume. When roots and other residues remain and decompose, ammonium compounds are released; nitrifying bacteria then make nitrates available to the next crop.