Published methods for calculating gross N rates differ in their assump
tions 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 im
portance of the various assumptions. It combines a numerical model for
solving the mass balance equations and a non linear fitting program f
or optimizing the N rate parameters. It can be applied to a single ''p
aired'' treatment(s) of an experiment in a bare soil. The model has be
en evaluated in two experiments made in the laboratory with wheat stra
w, 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 validate
d the compartmental model and enabled calculation of six N fluxes: min
eralisation (m), ammonium immobilisation (i(a)), nitrate immobilisatio
n (i(n)), nitrification (n), volatilisation (v) or denitrification (d)
and remineralisation of recently immobilised N (r), Sensitivity analy
sis indicated that the classical assumptions of exclusive ammonium imm
obilisation (i(n) = 0) and absence of N remineralisation (r = 0) had t
o be rejected. NH4+ immobilisation appeared to be dominant when ammoni
um and nitrate were both present, but was not exclusive: a Langmuir-ty
pe relationship could be established between the immobilisation ratio
i(a)(i(a) + i(n)) and the molar ratio of soil N concentrations NH4+/(N
H4+ + NO3-). Remineralisation of N occurred simultaneously with immobi
lisation during wheat straw decomposition and represented 7-18% of gro
ss immobilisation. Taking into account small gaseous losses, volatilis
ation or denitrification, allowed a better fit to be obtained between
observed and simulated N and N-15 pools. Nitrification was better desc
ribed by first order than by zero order kinetics. The eventuality of d
irect assimilation of organic N by microbial biomass or N humification
could not be determined but had no significant influence on the calcu
lation of other fluxes. When FLUAZ was applied to a single treatment (
NH4+ labelled), it also gave a good fit but only m, i (= i(a) + i(n)),
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 disappeare
d when r was fixed at the value obtained with the paired treatments. T
he ''apparent'' immobilisation rates (i - r) were then similar. The mo
del is very useful to test the consistency of measurements, estimate s
everal N rates simultaneously and quantify the importance of various a
ssumptions. (C) 1998 Elsevier Science Ltd. Ail rights reserved.