Model simulation of changes in N2O and NO emissions with conversion of tropical rain forests to pastures in the Costa Rican Atlantic Zone

Nitrous oxide (N2O) and nitric oxide (NO) are among the trace gases of conc ern because of their importance in global climate and atmospheric chemistry . Modeling techniques are needed for simulating the spatial and temporal dy namics of N2O and NO emissions from soils into the atmosphere. In this stud y, we modified the ecosystem model CENTURY to simulate changes in N2O and N O soil emissions through the process of converting tropical moist forests t o pastures in the Atlantic Lowlands of Costa Rica. Measurements of water-fi lled pore space (WFPS) and fluxes of N2O and NO from a chronosequence of pa stures were used for calibration and testing of the model. It was found tha t the N2O + NO - WFPS and N2O:NO - WFPS relationships as developed from pri mary forests could be generalized to the chronosequence of pastures and oth er land use systems in the region. Modeled net increases (compared to prima ry 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 c onditions. 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 g as fluxes, while NO-N accounted for 10%. Sensitivity analysis indicated tha t the impacts of forest-pasture conversion on N2O and NO emissions from soi l into the atmosphere were complex, depending on the initial conditions of the forest-derived pastures, management practices, soil physical and chemic al conditions and their changes over time, N availability, and climate. It is therefore important to incorporate the spatial and temporal heterogeneit ies of those controlling factors in estimating regional and global N2O and NO emissions from soils into the atmosphere.