Nitrogen cycle

Importance

• 78% of the atmosphere is nitrogen, however animals and plants cannot obtain nitrogen through gas exchange  

• Nitrogen (N2) contains a triple bond 

• Microorganisms are needed to convert nitrogen gas into nitrogen containing substances that plants and animals can absorb/assimilate  

• Proteins/amino acids, ATP, DNA and RNA contain nitrogen 

Nitrogen fixation

Nitrogen fixation: where nitrogen gas is converted into another nitrogen containing compound, by bacteria in the soil with nitrogen fixing abilities. They reduce nitrogen gas to ammonia, which subsequently dissolves to form ammonium ions. This process can be done directly in leguminous plants through bacteria in the root nodules. The relationship between nitrogen fixing bacteria and plants is known as mutualistic or symbiotic, as it benefits both organisms (the bacteria provide the plants with nitrogen-containing compounds, and the plant provides the bacteria with organic compounds such as carbohydrates) 

Nitrification

Nitrification: it is preferrable for ammonia to be nitrified before it is able to be absorbed and assimilated by plants. Nitrifying bacteria converts ammonia to nitrate ions in an oxidation reaction (where nitrite ions act as an intermediate). Most plants can then actively uptake nitrate ions through their roots, and assimilate these ions into their cells – used to create amino acids, ATP and DNA. 

Ammonification

Ammonification: is related to decomposition, represents the completion of nitrogen cycling within ecosystems. Animals/plants will produce waste/die, and microbes/decomposers (saprobionts) are able to break down this organic matter (including urine and faeces) into ammonia. Firstly, proteins are broken down into amino acids with the use of extracellular protease enzymes. These are then subsequently broken down further to remove amino acid groups with the use of deaminase enzymes. Saprobionts use the products of decomposition for respiration 

Denitrification

Denitrification: where denitrifying bacteria in the soil convert nitrate ions back into nitrogen gas in the atmosphere. This process is wasteful in agriculture, and the loss of nitrogen can be prevented from occurring by soil being well drained and aerated (ploughing). This is because denitrifying bacteria are anaerobic 

Effects of waterlogging

Waterlogging reduces the oxygen. This means that nitrifying bacteria cannot oxidise ammonia (into nitrite and nitrate), thus reducing the inflow of nitrate production. However, simultaneously, a lack of oxygen will have no impact upon the denitrifying bacteria, as they can work in anaerobic conditions. This means that more nitrates are converted back into nitrogen gas in the atmosphere - therefore less nitrates overall

Describe the role of saprobionts in the nitrogen cycle [2]

1. They use enzymes to decompose proteins/DNA/RNA/urea

2. Producing/releasing ammonia/ammonium compounds/ammonium ions