Page 3 - 2. Lesson notes- Bio- geo chemical cycles-1

transform the unusable form of nitrogen into other compounds that are more readily usable.

These nitrogen compounds get fixed in the soil by these microbes.

(I) Nitrification

In this process, the ammonia is converted into nitrate by the presence of bacteria in the soil,
with the help of Nitrosomonas bacteria. Later, the produced nitrites are converted into nitrates

by Nitrobacter.

(II) Assimilation

Primary producers – plants take in the nitrogen compounds from the soil with the help of their

roots, which are available in the form of ammonia, nitrite ions, nitrate ions or ammonium ions
and are used in the formation of the plant and animal proteins. This way, it enters the food chain

when the primary consumers eat the plants.

(III) Ammonification

When plants or animal die, the nitrogen present in the organic matter is released back into the
soil. The decomposers, namely bacteria or fungi present in the soil, convert the organic matter

back into ammonium. This process of decomposition produces ammonia.

(IV) Denitrification
Denitrification is the process in which the nitrogen compounds makes its way back into the

atmosphere by converting nitrate (NO3-) into gaseous nitrogen (N). This process of the nitrogen

cycle is the final stage and occurs in the absence of oxygen.

Denitrification is carried out by the denitrifying bacterial species- Clostridium and

Pseudomonas.

Nitrogen cycle:-