Nitrogen cycling (C:N >20)

Residue breakdown

In this example, assume the residue has C:N = 80:1. As in the high N scenario, for an exact nutritional balance of C and N, the microorganisms require a substrate with C:N of about 20.

Immobilisation

In this case, the residue contains less N than is required by the microorganisms. Their nutritional balance is maintained by taking up N from the inorganic N pool. This is termed immobilisation and involves inorganic N being converted to organic N (organic compounds within the microorganisms). Plants may suffer N deficiencies due to lack of available N.

Mineralisation

However, that is not the end of the process. As the first generations of microorganisms die, they form the substrate (humus) for subsequent generations of microorganisms. Because the dead microorganisms have C:N = 20, mineralization occurs.

Summary graph

Plant available N decreases during the initial immobilisation phase (which lasts weeks to months), but then starts to increase as mineralization takes over.