Model Structure

Mineral nitrogen exists in the soil as relatively immobile ammonium, N_NH4, and mobile nitrate, N_NO3. In agricultural soils, nitrate is normally essentially more abundant than ammonium whereas in forest soils the abundance of ammonium is often higher than that of nitrate due to low pH reducing nitrification. This section describes the fluxes of mineral nitrogen between these two mineral nitrogen pools i.e. nitrification, the plant uptake of mineral nitrogen, denitrification, leaching and vertical redistribution of mineral N.

The initial content of nitrifying microbials (if explicitly simulated) in the soil is given as a parameter, i_nitrmicr, i.e. a total value for the whole soil profile (see switch Initial Nitrifier). Either the nitrogen will be distributed evenly in the soil profile (“Constant”), or it will decrease linearly from the top layer (“Linear”), or finally it will decrease exponentially from the top layer (“Exponential”). The lowest depth is determined by the parameter i_ndepth and the distribution of remaining microbial biomass below i_ndepth is determined by the parameter i_nexpfrac.

Denitrification can optionally be simulated (see switch Denitrification). If denitrifying microbials are simulated explicitly simulated, the initial biomass of the denitrifiers is calculated as:

                                                                                       (6.26)

where i_denitmicr is the initial biomass of denitrifiers for the whole soil profile and  d_dist is a distribution coefficient that varies with depth. This variation is determined by the switch Denit depth distribution Either the denitrification rate can be given as a table (“Table”), it can be distributed evenly in the soil profile (“Constant”), it can decrease linearly from the top layer (“Linear”) or finally it can decrease exponentially from the top layer (“Exponential”). The lowest depth is determined by the parameter d_z and the distribution of remaining microbial biomass below d_z is determined by the parameter d_exp. The initial denitrifier activity is equal to zero.

Note that when nitrifiers and denitrifiers are considered explicitly, their nitrogen content is not included in the total nitrogen budget of the soil, and no flows of nitrogen to and from the microbial pools are calculated. The reason for this is the small nitrogen content of the microbes in comparison to the total nitrogen content of the soil.