Modelling of soil nitrogen forms after organic amendments under controlled conditions

This N model is derived from the C model (MOMOS) published by Sallih and Pansu (1993). Both models were fitted to experimental data obtained from an incubation experiment of 14C- and 15N-labelled plant material in two soils with contrasting characteristics over 2 y under controlled laboratory conditions. The N model uses the same structure as the C model with five organic compartments: labile plant material (VL); stable plant material (VR); microbial biomass (B); labile humified material (A); and stable humified material (H). Two additional compartments are included: exchangeable NH4N and NO3N. The transfers of N between the organic compartments are described according to first-order kinetics. Nitrogen transferred to the NH4 compartment results from the balance between the output and input of all the organic compartments. Ammonium N output is split between nitrification and immobilisation into B (77%) and H (23%). Nitrification is controlled by a microbial growth law. The N model uses the parameters defined in the C model, with a constant multiplying factor for the N kinetic constants (fn = 1.4 and 1.9 in soil 1 and 2, respectively). The additional parameters defined for inorganic pools are comparable in both soils. The predicted 14C-to-15N ratios of each organic compartment agree with the experimental data, showing a rapid incorporation of 15N into microbial biomass and a gradual build-up into stable humified compounds. The model was adjusted to 15N experimental data from five time series (each series containing from five to eight sampling occasions), and was validated using five series of corresponding total N data. MOMOS-C and -N formulation is relatively simple, combining mechanistic first-order kinetic models and growth models. The predictions are in agreement with 16 data sets including different forms of organic 14C (three series), total C (three series), organic and inorganic 15N and total N.