Long-term impact of nitrogen fertilization on carbon and water fluxes in a Douglas-fir stand in the Pacific Northwest

Douglas-fir (Pseudotsuga menziesii) forests in the Pacific Northwest are the most productive managed forests in North America. Nitrogen (N) fertilizers are generally applied in this region to increase the rate of tree growth and consequently carbon (C) sequestration. However, the long-term effects of N fertilization on C and water exchanges of Douglas-fir forests are not clear. This study presents 15 years of eddy-covariance (EC) measurements of C and water fluxes above a pole-sapling-stage Douglas-fir stand on Vancouver Island, British Columbia, Canada, and determines how N fertilization with 200 kg ha(-1) of urea-N affected these fluxes over the 10-year period after fertilization. A process-based forest growth model, 3-PG, was calibrated using EC-measured C and water fluxes during the 5 pre-fertilization years, and then used to predict these fluxes for the next 10 years assuming the stand was not fertilized. The modelled C and water fluxes for the pre-fertilization years agreed well with the measurements (with R-2 values of 0.90, 0.89, and 0.89 for gross primary production (GPP), ecosystem respiration (R-e) and evapotranspiration (ET), respectively) and only slightly underestimated GPP, R-e, and ET by 4%, 1%, and 3%, respectively. The effects of N fertilization on C sequestration and water use (ET) were then obtained as differences between EC-measured (fertilized stand) and modelled (unfertilized stand) fluxes. Application of N fertilizer to this stand led to a short-term (first two years) increase in GPP followed by little change over the long term. R-e increased over the short-term (first year), while it was appreciably suppressed over the long term. N fertilization resulted in an average increase in net ecosystem productivity by 170 g m(-2) year(-1) of C during 2007-2016 with interannual variation depending on annual weather conditions. N fertilization led to an average increase in annual water use of 15% (or 53 mm year(-1)).