Modeling to discern nitrogen fertilization impacts on carbon sequestration in a Pacific Northwest Douglas-fir forest in the first-postfertilization year

摘要

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 similar to 83%, from 302 +/- 19 to 552 +/- 36 g m-2 yr-1, which resulted primarily from an increase in annual gross primary productivity of similar to 8%, from 1938 +/- 22 to 2095 +/- 29 g m-2 yr-1 concurrent with a decrease in annual ecosystem respiration (R-e) of similar to 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 R-e (similar to 93 g m-2 yr-1) principally resulted from the decrease in soil respiration R-s (similar to 62 g m-2 yr-1).