The effects of urea fertilization on carbon sequestration in Douglas-fir plantations of the coastal Pacific Northwest

If long-term carbon (C) sequestration can be quantifiably attributed to forest plantation nitrogen (N) fertilization, the net C credits could be used to offset the rising cost of fertilization and C released during the production, transportation, and application of N fertilizer and the effect of NOx volatilized after application. The purpose of our study was to determine the net change in C sequestration following N fertilization of second-growth Douglas-fir (Psuedotsuga menziesii [Mirb.] Franco) plantations in the Pacific Northwest. The C content of the trees, understory vegetation, forest floor, and mineral soil was quantified at age 26-33 at five sites, each with a fertilized plot that received a total of 896-1120 kg N ha−1 as urea over 16 years paired with an unfertilized control plot. Tree biomass was estimated using biometric equations and by subtracting the difference between treatment and control at the year of site establishment from the difference between treatment and control final measurement. Understory vegetation on the fertilized plots contained significantly more C than on the control plots (0.2 Mg C ha−1, S.D. 0.2). Nitrogen fertilization significantly increased C sequestered per tree by 2.2 Mg C ha−1 (S.D. 1.8), but there was no significant increase in C sequestered in trees per plot. No significant change was found in forest floor, A horizon, and subsoil C contents due to fertilization. These results indicate that, while there is a greater amount of C stored per tree after fertilization, there was more difficulty in accessing C sequestration in forest plantations due to tree mortality and assumed soil variability between plots.