• Living organisms require?nutrients?from their?environments?for?growth?and other processes (e.g. reproduction)
  • These nutrients are then?returned?to the environment when organisms produce?waste?or?die?and?decompose
  • This is due to the waste products and dead organisms being?digested?(decomposed) by?microorganisms
  • The?products?of this decomposition are available to?plants?as?nutrients?in the?soil
  • These plants can then sustain organisms in higher trophic levels (consumers)

   

• In?stable?communities, the processes that?remove?nutrients (e.g. plant growth) are?balanced?by the processes that?return?these nutrients (e.g. decomposition of dead plants and animals)
  • This means these nutrients are constantly being?cycled?in ecosystems

   

• Two examples of these nutrient cycles are:
  • The?nitrogen?cycle
  • The?phosphorous?cycle

   

The nitrogen cycle

• The nitrogen cycle shows how nitrogen is recycled in ecosystems
• Plants and animals require nitrogen in order to produce?proteins?and?nucleic acids?(DNA and RNA)
• About 78% of the atmosphere is actually?nitrogen gas?but plants and animals?cannot?access the nitrogen in this gaseous form
  • Instead, they rely on certain?bacteria?to convert the nitrogen gas into?nitrogen-containing compounds, which can be taken up by plants

   

• The nitrogen cycle shows this conversion, as well as how the nitrogen in the nitrogen-containing compounds is then passed between trophic levels or between living organisms and the non-living environment

The role of bacteria in the nitrogen cycle

• There are?four?key processes in the nitrogen cycle that are carried out by?different types of bacteria
• Nitrogen?fixation:
  • Atmospheric nitrogen gas is converted into nitrogen-containing compounds
  • This biological nitrogen fixation is carried out by?nitrogen-fixing bacteria?such as?Rhizobium
  • The bacteria convert nitrogen into?ammonia,?which forms?ammonium ions?(in solution) that can then be used by?plants
  • These nitrogen-fixing bacteria are found inside the?root nodules?(small growths on the roots) of?leguminous?plants such as peas, beans and clover
  • The bacteria have a?symbiotic?(mutually beneficial) relationship with these plants - the bacteria provide the plants with nitrogen-containing compounds and the plants provide the bacteria with organic compounds such as carbohydrates

   

• Ammonification:
  • Nitrogen compounds in?waste products?(e.g. urine and faeces) and?dead organisms?are converted into?ammonia?by?saprobionts?(a type of decomposer including some fungi and bacteria)
  • This ammonia forms?ammonium ions?in the soil

   

• Nitrification:
  • The ammonium ions in the soil are converted by?nitrifying bacteria?into nitrogen compounds that can be used by plants, known as?nitrates
  • Initially, nitrifying bacteria such as?Nitrosomonas?convert ammonium ions into?nitrites
  • Different nitrifying bacteria such as?Nitrobacter?then convert these nitrites into?nitrates

   

• Denitrification:
  • Denitrifying bacteria use?nitrates?in the soil during?respiration
  • This process produces?nitrogen gas, which returns to the?atmosphere
  • This process occurs in?anaerobic?conditions (when there is little or no oxygen available, such as in waterlogged soil)

   

The nitrogen cycle

The phosphorus cycle

• The phosphorus cycle shows how phosphorus is recycled in ecosystems
• Plants and animals require phosphorus in order to produce certain biological molecules such as?phospholipids?(for cell membranes),?nucleic acids?(DNA and RNA) and?ATP
• The phosphorus cycle includes the following processes:
  • Phosphorus?in?rocks?is slowly released into the soil and into water sources in the form of?phosphate ions?(PO?3?) by the process of?weathering?(the slow breaking down and erosion of rocks over time)
  • Phosphate ions are taken up from the soil by?plants?through their?roots?or absorbed from water by?algae
  • Phosphate ions are transferred to?consumers?during?feeding
  • Phosphate ions in?waste products?and?dead organisms?are released into the soil or water during?decomposition?by?saprobionts
  • The phosphate ions can now be taken up and used once again by producers or may be trapped in?sediments?that, over very long geological time periods may turn into phosphorus-containing?rock?once again

   

The phosphorus cycle