C AND N FLUXES BETWEEN POOLS OF SOIL ORGANIC-MATTER - MODEL CALIBRATION WITH LONG-TERM FIELD EXPERIMENTAL-DATA

NCSOIL, an updated simulation model of C and N behaviour in soil, was calibrated with data from published field experiments in which various authors studied the decomposition of C- or N-labelled plant materials under a wide range of climatic and soil conditions. Four organic pools were included in the new version of NCSOIL viz. plant residues (decay rate constant = 0.3 d-1); microbial biomass with a labile (0.33 d-1) and a resistant (0.04 d-1) component; humads (0.006 d-1); and stable OM (5.5 10(-5) d-1). Experimental data were fitted with a reduction factor (REDI), which applied to all decomposition rates and for the whole of the simulated period. This parameter took into account the effect of climatic and soil conditions on microbial decomposition rates. Optimization of this reduction factor gave values which ranged from 0.06 to 1.0 and which were related well to the climatic and edaphic conditions given by the authors. Optimization of the size of the humads pool showed that the active soil organic fraction represented 13-35% of the total soil organic matter.