Mineralisation / Immobilisation
The mineralisation/immobilisation of nitrogen is dependent on the CN ratio in the microbial biomass. This means that the nitrogen flux between the microbes and the soil ammonium pool is calculated as:
If the carbon content in the source pools is high compared to the nitrogen content, the CN ratio becomes large by definition. Studying equations (6.8) and (6.17) one finds that if the CN ratio becomes large enough compared to the parameter cnm, the flux to the soil ammonium pool becomes negative. This means that instead of having net mineralisation there is a net immobilisation of nitrogen, i.e. the flux of nitrogen is from the soil ammonium pool to the litter, faeces (microbes implicit) or microbial (microbes explicit) pools respectively.
If a net immobilisation takes place the flow from the soil ammonium pool, NNH4, is:
where iN is a parameter (note that the term NLitter→NH4 is negative). The same formula is used to calculate the flow to the faeces and microbial pools by exchanging the flux-term to the right to the appropriate flux.