The Nitrogen Cycle

• Why is it important that elements such as carbon, nitrogen, and phosphorus get recycled?

  There is limited availability of these nutrients in a usable form.
• What is the simple sequence that all nutrient cycles have?

  Nutrients are taken up by producers as simple, inorganic molecules, they make them into complex organic molecules, this is passed along several consumers, and when the producers and consumers die their complex molecules are broken down by saprobionts, which release the nutrients back into their simple form.
• What do living organisms require nitrogen for?

  To manufacture proteins, nucleic acids, and other nitrogen-containing compounds.
• What form of nitrogen in the soil do plants take up?

  Nitrate ions (NO3-).
• How do plants take up nitrate ions?

  By active transport in the roots.
• How do animals obtain nitrogen-containing compounds?


  By eating and digesting plants.
• How is the concentration of soil nitrate increased in agriculture?

  By the addition of fertilisers.
• What are the four main stages of the nitrogen cycle?

  Ammonification, nitrification, nitrogen fixation, and denitrification.
• What do all stages of the nitrogen cycle involve?

  Saprobiontic microorganisms.
• What is ammonification?


  The production of ammonia from organic nitrogen-containing compounds such as urea, proteins, nucleic acids, and vitamins found in feces and dead organisms.
• What do saprobionts do during ammonification?

  Feed on feces and dead organisms releasing ammonia which then forms ammonium ions in the soil.
• What is nitrification?


  The conversion of ammonium ions to nitrate ions by nitrifying bacteria.
• What kind of reaction is nitrification?

  An oxidation reaction.
• What does the nitrification reaction release?

  Energy.
• What is nitrification carried out by?


  Free-living microorganisms called nitrifying bacteria.
• What are the two stages of nitrification?

  Oxidation of ammonium ions to nitrite ions (NO2-), and the oxidation of nitrite ions to nitrate ions (NO3-).
• What do nitrifying bacteria require to carry out nitrification?

  Soil that has many air spaces for oxygen to diffuse through.
• What is nitrogen fixation?

  The process by which nitrogen gas is converted into nitrogen-containing compounds.
• How is nitrogen fixation carried out?


  Industrially and naturally when lightning passes through the atmosphere, and by microorganisms.
• What are the two types of microorganisms that carry out nitrogen fixation?


  Free-living nitrogen-fixing bacteria, and mutualistic nitrogen-fixing bacteria.
• How do free nitrogen-fixing bacteria carry out nitrogen fixation?

  They reduce nitrogen gas to ammonia which they then use to make amino acids, to then release these nitrogen-rich compounds when they die.
• How do mutualistic nitrogen-fixing bacteria carry out nitrification?

  They live in root nodules of plants, they obtain carbohydrates from those plants for the plants to then acquire amino acids from the bacteria.
• What happens when soils become waterlogged and have a low oxygen concentration?

  The microorganisms present change from aerobic nitrifying and nitrogen-fixating bacteria to anaerobic denitrifying bacteria.
• What do denitrifying bacteria do?

  Convert soil nitrates into nitrogen gas.
• Why is the soil on which crops grow kept aerated?


  To prevent the build-up of denitrifying bacteria as they reduce the availability of nitrogen-containing compounds for plants.