Nitrogen Cycle

What happens in natural ecosystems

• In natural ecosystems nutrients are recycled through food webs. If decomposers were not recycling these nutrients, dead plant and animal material would remain undecomposed for long periods of time and ecosystems would have much lower productivity 

What are an important part of food webs?

Microorganisms, such as bacteria and fungi, are an important part of food webs

An example of a microorganism that is a decomposer

• Saprobionts (mainly fungi and bacteria)

  • A decomposer

How do saprobionts work?

• Secrete enzymes on dead organic remains of organisms. They digest this material and absorb some of the nutrients which they need. During this process, organic molecules are broken down into inorganic ions and some of the nutrients remain in the surroundings and may be absorbed by other organisms. 

• Some of the nutrients they release are mineral ions such as phosphate and nitrate, which are very important in ecosystems

Whch fungi forms a symbiotic relationship

• Some fungi form symbiotic relationships (any close and long-term biological interaction between two different species) with the roots of plants. These relationships are known as mycorrhizae

How do mycorrhizae work and how do they benefit the plant

• Mycorrhizae are fungi that are associated with the roots of most plants. They grow in and around the plants and are often specific to the plant.

• Mycorrhizae, like most other fungi, are made up of tiny threads called hyphae, which increase the surface area of the roots of the plant for taking up water and nutrients from the soil

• The mycorrhizal fungi secrete enzymes that digest dead organic matter in the soil, then they absorb the nutrients released and transport them back into the plant roots

  • In return, the fungi obtain carbohydrates from the plant. This is called a mutualistic relationship, since both the plant and the mycorrhizae benefit

Why is the nitrogen cycle important

• The nitrogen cycle is vital for recycling nitrogen. Without this cycle, ecosystems would run out of usable nitrogen, leading to a decline in vital biological molecules needed by organisms, which include:

  • Proteins

  • DNA

  • RNA

What do plants and animals need to use the nitrogen from the air?

• The atmosphere is made up of about 78% nitrogen gas, but plants and animals can’t use it in that form - they need bacteria to convert it into nitrogen-containing compounds first

What are the four process of the nitrogen cycle

• The nitrogen cycle includes food chains (nitrogen is passed on when organisms are eaten), and four different processes that involve bacteria - nitrogen fixation, ammonification, nitrification and denitrification

Stage 1 of the nitrogen cycle (nitrogen fixation)

• Nitrogen fixation is when nitrogen gas in the atmosphere is turned into nitrogen-containing compounds

• Nitrogen fixating bacteria fixate the nitrogen and convert it to ammonium compounds

Stage 2 of the nitrogen cycle (ammonification)

• Ammonification is when nitrogen compounds from dead organisms are turned into ammonia by saprobionts which goes on to form ammonium ions

• Animal waste also contains nitrogen compounds. These are also turned into ammonia by saprobionts and go on to form ammonium ions

Stage 3 of the nitrogen cycle (nitrification)

• Nitrification is when ammonium ions in the soil are changed into nitrogen compounds that can then be used by plants

• Nitrifying bacteria convert ammonium compounds to nitrites which then gets converted to nitrates

Stage 3 of the nitrogen cycle (denitrification)

• Denitrification is when nitrates in the soil are converted into nitrogen gas by denitrifying bacteria

• This happens under anaerobic conditions e.g. waterlogged soil