A MODEL FOR CALCULATING NITROGEN FLUXES IN SOIL USING N-15 TRACING

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.