The Nitrogen Cycle

Nitrogen is needed to make amino acids (and so proteins) and nucleotides (DNA, RNA and ATP). The air is about 78% nitrogen gas (N₂), but plants and animals cannot use it directly. Plants absorb nitrogen from the soil, mainly as nitrate ions (NO₃⁻), by active transport. Animals obtain nitrogen by eating plants or other animals.Four processes, all carried out by bacteria, move nitrogen around the cycle:1. Nitrogen fixation: nitrogen gas is converted into ammonia by nitrogen-fixing bacteria, some free-living in the soil and some in the root nodules of legumes.
2. Ammonification: saprobionts convert nitrogen-containing compounds in dead organisms, faeces and urine into ammonia, which forms ammonium ions in the soil.
3. Nitrification: nitrifying bacteria oxidise ammonium ions to nitrite ions and then to nitrate ions. This needs oxygen.
4. Denitrification: in anaerobic conditions, denitrifying bacteria convert nitrates back into nitrogen gas.Example: N₂ in the air → fixed to ammonia in a clover root nodule → amino acid in the clover → protein in a rabbit that eats it → urea in the rabbit's urine → ammonium ions (ammonification) → nitrate ions (nitrification) → absorbed by grass roots, or turned back into N₂ (denitrification) if the soil becomes waterlogged.Farmers increase the nitrate available to crops by ploughing and draining soils (keeping them aerobic), growing legumes and adding fertilisers.