ANALYSIS AND FIELD-EVALUATION OF THE CERES MODELS SOIL COMPONENTS - NITROGEN TRANSFER AND TRANSFORMATIONS

For estimating N losses in soil-crop systems with the simple and funct ional CERES models, we evaluated their N modules and compared them wit h the more complex SLIM (for solute transport) and NCSOIL (for N miner alization) models. SLIM is based on the concept of immobile and mobile water regions in soil, and CERES on a piston-dow hypothesis. In NCSOI L, the soil organic matter (SOM) mineralizes through two active pools, whereas in CERES the SOM decomposes as a whole with a gross decay rat e. We used data on mineral N dynamics under bare soils (silt loam, loa m, and sandy loam) from 1-yr-long experiments at three locations in Fr ance, including measurements of weekly NO3 leaching fluxes. The origin al CERES mineralization submodel did not correctly simulate N supply f rom potentially degradable SOM. When using NCSOIL instead, the simulat ions improved and CERES predicted NO3 leaching reasonably well, with a root mean square error of 6 to 21 kg N ha(-1), representing 5% of the yearly flux. The SLIM model performed as well, and better simulated t he intense NO3 percolation regime that occurred in wintertime. Its imm obile water fraction parameter had to be calibrated, however, or the y early leaching flux was underestimated. When linked to NCSOIL, the CER ES model showed a good potential for estimating N dynamics in soil, ev en if its piston-how type of NO3 transfer was not always relevant. In such case, SLIM was a more appropriate approach, although it required a site-specific calibration.