Impact of nitrogen fertilization on carbon and water balances in a chronosequence of three Douglas-fir stands in the Pacific Northwest

This study investigated the 2-year response to nitrogen (N) fertilization of three different-aged Douglas-fir stands on the west coast of Canada on eddy covariance (EC)-measured net ecosystem productivity (NEP), gross primary productivity (GPP), ecosystem respiration (R) and evapotranspiration (E). Fertilizer application took place in January 2007, aerially at 200kgurea-Nha⁻¹ in 19- and 59-year-old stands (hereafter referred to HDF88 and DF49, respectively), and manually at 80g urea per tree (60kgNha⁻¹) along the tree drip line in a 7-year-old stand (HDF00). N fertilization substantially increased foliar N content as well as needle mass in all the three stands leading to N-sufficiency compared to severe N-deficiency in similar trees in the adjacent unfertilized areas of the respective stands. In the absence of a second EC flux tower in similar unfertilized stands, we used simple empirical models fitted to multi-year pre-fertilization monthly fluxes to calculate C and water fluxes for 2007 and 2008 assuming the stands were not fertilized and compared these with measured fluxes to determine the effects of fertilization. Our modelling analysis suggested that fertilization increased GPP and R in all the three stands with increases in GPP being greater than R in DF49 and HDF88 but smaller than R in HDF00. Fertilizer-induced increase in net C sequestration or NEP was the highest (182 and 179gCm⁻² y⁻¹ in 2007 and 2008, respectively) in HDF88 followed by DF49 (168 and 78gCm⁻² y⁻¹ in 2007 and 2008, respectively) and small decreases (−3 and −82gCm⁻² y⁻¹ in 2007 and 2008, respectively) in HDF00. Fertilization resulted in an insignificant increase in E in all the three stands, thereby resulting in substantial increase in water use efficiency (GPP/E) in the younger (HDF88 and HDF00) stands but little effect in the intermediate-aged (DF49) stand. Tree ring analysis for DF49 showed that fertilization increased bole volume above that in an adjacent unfertilized control area by about 5.0 and 8.0m³ ha⁻¹ in 2007 and 2008, respectively.