Nitrogen Cycle

What is the importance of the nitrogen cycle?

• 78% of the air contains Nitrogen, however plants and animals can’t use Nitrogen in this form as it’s inert

• Plants must secure their Nitrogen in ‘fixed’ form, i.e. incorporated in compounds such as: nitrate ions (NO₃^-), ammonia (NH₃₎, ammonium (NH₄⁺)

• Animals secure their Nitrogen compounds from feeding on plants that contain those compounds

Name the 4 biological processes that are involved in the cycle

• Nitrogen fixation

• Ammonification

• Nitrification

• Denitrification

Nitrogen Fixation

• N₂ quite inert

• Nitrogen-fixing bacteria (found in root nodules of legumes) contain an enzyme called nitrogenase that converts nitrogen and hydrogen to ammonia

• The ammonia is used to make amino acids which the legume can use to make proteins in return for supplying the bacteria with carbohydrates → symbiotic relationship

• NH₄⁺ is also formed from NH₃ in the soil

Ammonification

• Nitrogen-containing compounds eg. proteins/ urea from dead organisms are decomposed

• Saprobionts decompose those compounds by secreting enzymes for extracellular digestion

• These compounds are then converted to NH₃, which forms NH₄⁺ in the soil

Nitrification

• NH₄⁺ in soil converted into nitrites (NO₂^-) then nitrates (NO₃^-), via a two-step oxidation reaction

○ For uptake by plant root hair cells by active transport

• By nitrifying bacteria in aerobic conditions (oxygen); this reaction releases heat energy - soil needs to be aerated by ploughing

Denitrification

• Nitrates in soil converted into nitrogen gas (reduction)

• By denitrifying anaerobic bacteria - waterlogged or poorly aerated soil results in more of them being present in the soil and less aerobic nitrifying bacteria