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

An updated version of the simulation model NCSOIL was calibrated with data from a long-term laboratory incubation (728 days at 28-degrees-C) of three cultivated soils amended with (KNO3)-N-15 and either [C-14]g lucose or [C-14]cellulose. The kinetics of tracer and non-tracer C and N (CO2-C, inorganic-N and microbial biomass-C) were measured in the t hree soils on various sampling dates. The new NCSOIL version considers four organic pools: residues, microbial biomass (pool I) with two com ponents (labile and resistant), humads (pool II) which correspond to t he active fraction of soil organic matter (SOM), and pool III which is the highly resistant fraction of SOM. To fit the long-term incubation experimental data, it was necessary to reduce all decomposition rates by 60-70% after 35-85 days of incubation, depending on the soil and t he treatment. The labile fraction of pool I in non-amended soils was a lso reduced to 0.20 from the previously-used value of 0.56; the later value, however, was adequate for the glucose and cellulose-enriched so ils. Simulations agreed well with experimental data and gave values of pool II which represented about 30% of the SOM, and which had a C:N r atio of 12-13. Results suggested that > 60% of the SOM was very resist ant.