The Nitrogen Cycle and Eutrophication

• Plants and animals need nitrogen to make proteins and nucleic acids (DNA and RNA)
• Atmosphere is made up of about 78% nitrogen
• Plants unable to use it in that form so need bacteria to convert it into nitrogen compounds
• Shows how nitrogen is converted into a useable form and then passed on between different living organisms and the non-living environment
• Includes food chains (nitrogen passed on when organisms are eaten) and four different processes that involve bacteria - nitrogen fixation, ammonification, nitrification and denitrification
• Parts can be carried out artificially and on an industrial scale
• Haber process produces ammonia from atmospheric nitrogen - used to make fertilisers

• When nitrogen gas in the atmosphere is turned into ammonia by bacteria called Rhizobium - can then be used by plants
• Bacteria found inside root nodules (growths on the roots) of leguminous plants
• Form mutualistic relationship with plants - provide plants with nitrogen compounds and plants provide them with carbohydrates

• When nitrogen compounds from dead organisms turned into ammonium compounds by decomposers
• Animal waste also contains nitrogen compounds - turn into ammonium compounds by decomposers

• When ammonium compounds in the soil changed into nitrogen compounds which can then be used by plants
• First nitrifying bacteria (e.g. Nitrosomonas) changed ammonium compounds into nitrites
• Other nitrifying bacteria (Nitrobacter) change nitrites into nitrates

• When nitrites in the soil are converted into nitrogen gas by denitrifying bacteria - use nitrites in soil to carry out respiration and produce nitrogen gas
• Happens under anaerobic conditions (no oxygen) e.g. waterlogged soils

Leaching: when water-soluble compounds in the soil are washed away, e.g. by rain or irrigation systems into nearby ponds and rivers. - if nitrogen fertiliser is leached into waterways it can cause eutrophication

• Nitrates leached from fertilised fields stimulate the growth of algae in ponds and rivers
• Large amounts of algae block light from reaching plants below
• Eventually plants die because they are unable to photosynthesise enough
• Bacteria feed on the dead plant matter
• Increased numbers of bacteria reduce oxygen concentration by carrying out aerobic respiration
• Fish and other organisms die as there isn't enough dissolved oxygen