A model for calculating nitrogen fluxes in soil using 15N tracing

Published methods for calculating gross N rates differ in their assumptions and the method of calculation (algebrical equations or numerical methods). The calculation model presented here called FLUAZ considers the major N processes occurring in soil and enables testing of the importance of the various assumptions. It combines a numerical model for solving the mass balance equations and a non linear fitting program for optimizing the N rate parameters. It can be applied to a single or “paired” treatment(s) of an experiment in a bare soil. The model has been evaluated in two experiments made in the laboratory with wheat straw, each experiment involving two “paired” treatments. When FLUAZ was applied to the “paired” treatments, a good fit was obtained between the simulated and measured values of 10 variables (amount of NH4+ and NO3−, isotopic excess of NH4+, NO3− and organic N). This fit validated the compartmental model and enabled calculation of six N fluxes: mineralisation (m), ammonium immobilisation (ia), nitrate immobilisation (in), nitrification (n), volatilisation (v) or denitrification (d) and remineralisation of recently immobilised N (r). Sensitivity analysis indicated that the classical assumptions of exclusive ammonium immobilisation (in=0) and absence of N remineralisation (r=0) had to be rejected. NH4+ immobilisation appeared to be dominant when ammonium and nitrate were both present, but was not exclusive: a Langmuir-type relationship could be established between the immobilisation ratio ia/(ia+in) and the molar ratio of soil N concentrations NH4+/(NH4++NO3−). Remineralisation of N occurred simultaneously with immobilisation during wheat straw decomposition and represented 7–18% of gross immobilisation. Taking into account small gaseous losses, volatilisation or denitrification, allowed a better fit to be obtained between observed and simulated N and 15N pools. Nitrification was better described by first order than by zero order kinetics. The eventuality of direct assimilation of organic N by microbial biomass or N humification could not be determined but had no significant influence on the calculation of other fluxes. When FLUAZ was applied to a single treatment (NH4+ labelled), it also gave a good fit but only m, i (=ia+in), n, v or d could be determined. The mineralisation and immobilisation rates were slightly lower than those found with the paired treatments: this difference was mainly due to the hypothesis r=0 and disappeared when r was fixed at the value obtained with the paired treatments. The “apparent” immobilisation rates (i−r) were then similar. The model is very useful to test the consistency of measurements, estimate several N rates simultaneously and quantify the importance of various assumptions.