Nitrous oxide (N2O) and nitric oxide (NO) are among the trace gases of concern because of their importance in global climate and atmospheric chemistry. Modeling techniques are needed for simulating the spatial and temporal dynamics of N2O and NO emissions from soils into the atmosphere. In this study, we modified the ecosystem model CENTURY to simulate changes in N2O and NO soil emissions through the process of converting tropical moist forests to pastures in the Atlantic Lowlands of Costa Rica. Measurements of water-filled pore space (WFPS) and fluxes of N2O and NO from a chronosequence of pastures were used for calibration and testing of the model. It was found that the N2O+NO-WFPS and N2O:NO-WFPS relationships as developed from primary forests could be generalized to the chronosequence of pastures and other land use systems in the region. Modeled net increases (compared to primary forests) in total N2O and NO production after conversion from forest to pasture were 514 kg N ha-1 during the first 15 years under normal field conditions. The nitrogen loss in the form of N2O and NO during the first 15 years could range from 401 to 548 kg N ha-1, depending on the amounts of forest residue remaining on pasture sites. N2O-N accounted for 90% of the gas fluxes, while NO-N accounted for 10%. Sensitivity analysis indicated that the impacts of forest-pasture conversion on N2O and NO emissions from soil into the atmosphere were complex, depending on the initial conditions of the forest-derived pastures, management practices, soil physical and chemical conditions and their changes over time, N availability, and climate. It is therefore important to incorporate the spatial and temporal heterogeneities of those controlling factors in estimating regional and global N2O and NO emissions from soils into the atmosphere. ¿ 1999 American Geophysical Union