Nitrogen cycle

• All living organisms require a source of nitrogen from which to manufacture proteins, nucleic acids and other nitrogen containing compounds
• Although 78% of the atmosphere is nitrogen, very few organisms can use nitrogen gas directly
• Plants take up most of the nitrogen they need in the form of nitrate ions (NO3-) from the soil
• Nitrate ions are absorbed using active transport by root hair cells
• Animals obtain nitrogen containing compounds by eating and digesting plants
• Nitrate ions are very soluble and easily leach through the soil away from the reach of plant roots
• In natural ecosystems, nitrate levels are restored through the recycling of nitrogen containing compounds
• In agricultural ecosystems, the soil levels of nitrate are increased with the addition of fertilisers
• when plants and animals die, decomposition helps replenish the nitrate levels in the soil
• There are four main stages in the nitrogen cycle: ammonification, nitrification, nitrogen fixation and denitrification
• Every stage in the nitrogen cycle requires saprobiotic microorganisms
• In nature, ammonium containing compounds include urea (from the breakdown of excess amino acids) and proteins, nucleic acids and vitamins (found in faeces and dead organisms)
• Saprobiotic organisms (mainly fungi and bacteria) feed on ammonium containing compounds and release ammonia, which forms ammonium ions in the soil
• Free living soil microorganisms called nitrifying bacteria convert ammonium ions to nitrate ions
• The conversion of ammonium ions to nitrate ions is an oxidation reaction and so releases energy
• The conversion of ammonium ions to nitrate ions occurs in two stages:

  • oxidation of ammonium ions to nitrite ions (NO2-)
  • oxidation of nitrite ions to nitrate ions (NO3-)
• Nitrifying bacteria require oxygen to carry out ammonium conversions and so need soil that has many air spaces
• Good drainage prevents air spaces from being filled with water to allow the nitrifying bacteria to work
• Nitrogen fixation can be carried out industrially but also occurs naturally when lightning passes through the atmosphere
• nitrogen cycle: the process by which nitrogen is recycled

  
• The most important forms of nitrogen fixation is carried out by microorganisms
• Free living nitrogen fixing bacteria reduce gaseous nitrogen to ammonia which they then use to manufacture amino acids. Nitrogen rich compounds are released when they die and decay
• Mutualistic nitrogen fixing bacteria live in nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria
• When soils become waterlogged, the type of microorganisms present changes. Fewer aerobic nitrifying and nitrogen-fixing bacteria are found, and there is an increase in anaerobic denitrifying bacteria
• Denitrifying bacteria convert soil nitrates into gaseous nitrogen, reducing the availability of nitrogen containing compounds for plants
• Nitrate leaching occurs where excess nitrate is washed away with rainwater
• For land to be productive, the soils on which crops grow must be kept aerated to prevent the build up of denitrifying bacteria
• Nitrogen rich soils favour the growth of grasses, nettles and other rapidly growing species. These can outcompete other species. Species rich areas only exist when nitrogen levels are low enough to allow other species to compete
• Denitrification is the reduction of NO3- to N2
• Denitrification is the reduction of nitrate ions to nitrogen gas
• Nitrogen fixation is the reduction of nitrogen gas to ammonia
• In nitrogen-fixing bacteria, the conversion of nitrogen into ammonia is catalysed by an enzyme called nitrogenase
• Nitrogenase does not function in the presence of oxygen. Thickening of tissue in a root nodule increases diffusion distance and so decreases diffusion into the nodule, promoting the action of nitrogen fixing bacteria
• Legumes promote nitrogen fixing bacteria, so farmers often plant a field with legumes after several harvests of wheat
• Nitrogen gas is converted to ammonium ions, then to nitrite, then to nitrate
• The majority of plants obtain nitrogen in the form of nitrate
• The process of converting nitrogen gas to ammonia is known as nitrogen fixation
• Free living bacteria in the soil convert nitrogen gas in the air to ammonia. This ammonia dissolves in water in the soil to form ammonium ions
• Bacteria living in the roots of certain plants convert nitrogen gas to ammonia. This ammonia is used immediately by the plant
• Nitrogen fixing bacteria in the soil are called free living nitrogen fixing bacteria and nitrogen fixing bacteria in roots are called mutualistic nitrogen fixing bacteria