This study investigated how nitrogen (N) fertilization with 200 kg N ha−1 of urea affected ecosystem carbon (C) sequestration in the first-postfertilization year in a Pacific Northwest Douglas-fir (Pseudotsuga menziesii) stand on the basis of multiyear eddy-covariance (EC) and soil-chamber measurements before and after fertilization in combination with ecosystem modeling. The approach uses a data-model fusion technique which encompasses both model parameter optimization and data assimilation and minimizes the effects of interannual climatic perturbations and focuses on the biotic and abiotic factors controlling seasonal C fluxes using a prefertilization 9-year-long time series of EC data (1998–2006). A process-based ecosystem model was optimized using the half-hourly data measured during 1998–2005, and the optimized model was validated using measurements made in 2006 and further applied to predict C fluxes for 2007 assuming the stand was not fertilized. The N fertilization effects on C sequestration were then obtained as differences between modeled (unfertilized stand) and EC or soil-chamber measured (fertilized stand) C component fluxes. Results indicate that annual net ecosystem productivity in the first-post-N fertilization year increased by∼83%, from 302 ± 19 to 552 ± 36 g m−2 yr−1, which resulted primarily from an increase in annual gross primary productivity of∼8%, from 1938 ± 22 to 2095 ± 29 g m−2 yr−1 concurrent with a decrease in annual ecosystem respiration (Re) of∼5.7%, from 1636 ± 17 to 1543 ± 31 g m−2 yr−1. Moreover, with respect to respiration, model results showed that the fertilizer-induced reduction in Re (∼93 g m−2 yr−1) principally resulted from the decrease in soil respiration Rs (∼62 g m−2 yr−1).