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This section contains three questions:

1. What are the major processes in the nitrogen cycle?
2. What are the organisms involved in nitrification and what are the soil conditions where it occurs?
3. What organisms are involved in denitrification and what are the soil conditions where it occurs?

Question 1: What are the major processes in the nitrogen cycle?

The nitrogen cycle is a way to describe how the different forms of nitrogen in the air, soil, water and living organisms are interconnected. It is described as a cycle because the nitrogen is never lost completely, it just changes form and is held in different places. There are 5 major processes involved:

1. Ammonification

Ammonification is the conversion of organic forms of nitrogen (for example, nitrogen in proteins in dead plants and soil animals) to ammonium. Ammonium is an inorganic form of nitrogen that has the symbol NH4+ . The ammonification process is carried out by a wide range of soil organisms. Many different types of bacteria and fungi are involved.

2. Nitrification

Nitrification is the conversion of ammonium to nitrate (another inorganic form of nitrogen) by a specific group of bacteria. Nitrate is shown by the symbol NO3-.

3. Denitrification

Denitrification is the conversion of different forms of nitrogen in soil to forms of gaseous nitrogen. One example is nitrous oxide and another is nitrogen gas, which is the form of nitrogen most common in the atmosphere.

4. Nitrogen fixation

Nitrogen fixation is the conversion of nitrogen gas (N2) to ammonium - either by free living bacteria in soil or water, or by bacteria in symbiotic association with plants (eg legume symbiosis).

5. Nitrogen immobilization

This is the process whereby nitrogen is taken up by soil organisms and retained in the 'microbial pool' of nitrogen.

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Question 2: What are the organisms involved in nitrification and what are the soil conditions where it occurs?

Nitrification is the process where ammonium is converted to nitrite and then to nitrate. The organisms involved in nitrification are a relatively small number of species in soil and they are autotrophic. One group of organisms converts ammonium to nitrite and another group of organisms converts nitrite to nitrate.

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Question 3: What organisms are involved in denitrification and what are the soil conditions where it occurs?

Denitification is the process where oxides of nitrogen (nitrate and nitrite) are converted into gaseous nitrogen and are removed from the soil system. Many species are capable of denitrification of soil and it occurs mostly when there is little or no oxygen in the soil such as when the soil is waterlogged.

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