1) Decomposers (Putrefying Bacteria) These aerobic bacteria live in the soil. When an organism dies decomposers digest the proteins and DNA that are found in the cells of the organism and produce ammonium ions (NH4+) as a waste product. Decomposers will also break down the molecules in animal faeces and will decompose the urea in animal urine into ammonium ions.
2) Nitrifying Bacteria These are also aerobic bacteria that live in the soil. They get their energy by converting ammonium ions into nitrates (via an intermediate ion called a nitrite). Nitrifying bacteria are essential for the Nitrogen cycle because the nitrates they form are the ions that plants will absorb through their roots. Nitrates will be used by the plant to make amino acids, proteins and DNA and these can pass up food chains.
3) Nitrogen-fixing Bacteria The picture now gets a little more complicated. Nitrogen-fixing bacteria can convert atmospheric nitrogen found in air spaces in the soil into ammonium ions (see diagram above). These bacteria live in the soil and respire aerobically like all the bacteria mentioned so far. Some nitrogen-fixing bacteria have formed a symbiotic relationship with a family of plants that includes peas, beans and clover. These plants are called leguminous plants and have small nodules in their roots that contain the nitrogen-fixing bacteria. The relationship is a mutualistic one as both parties benefit. The plant benefits as these bacteria produce nitrate ions that the plant can absorb and use to make proteins and the bacteria benefit as they are protected from soil predators and have a stable environment in which to live.
4) Denitrifying bacteria
These are anaerobic bacteria that thrive in conditions where there is little oxygen in the soil. This often happens when the soil becomes water-logged so all the air spaces are flooded. Denitrifying bacteria are “bad news” for Nitrogen cycle as they get their energy by taking nitrates from the soil and converting them into nitrogen gas. This obviously reduces the nitrate available to the plants with roots in the soil and this is one of the reasons farmers like to keep soil well aerated for their crops.
I will post a quiz on Nitrogen cycle on Zondle later this evening and so if you want to test your understanding of this potentially tricky topic, I would suggest you have a go at my quiz. There are also plenty of past paper questions on Nitrogen Cycle in the red question booklet.