The use of various soil and site variables for estimating growth response of Douglas-fir to multiple applications of urea and determining potential long-term effects on soil properties

Estimating the growth response of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands after nitrogen (N) fertilization is difficult because of the high site variability present in the Pacific Northwest. Our objective was to determine as urea once every 4 years. The how site and soil variables relate to stand response to repeat applications of 224 kg N.ha(-1) unstandardized residuals of two dependent variables (total cumulative Volume and 4-year periodic annual increment, or PAI) were regressed against site and soil variables using stepwise regression. Data were stratified by three different stand density treatments: unaltered stand density (SD), one-half SD (SD/2), and one-quarter SD (SD/4). Both total cumulative volume and 4-year PAI after the second application of urea was significantly higher in the fertilized plots (p = 0.008; 0.009), whereas only total cumulative volume was significant after the third fertilizer application (p = 0.021). Thinning effects were highly significant (p < 0.001) for all three fertilizer applications. The strongest related stand, site, or soil variable to fertilization response existed between percent N at the 30-50 cm depth and total cumulative volume (R-2 = 0.833) for the SD/2 stand density management regime. Regression analysis showed that C, N. NH4+, and NO3- concentration data explained the most variation, while stand and site variables contributing the least. The results demonstrate that multiple applications of urea provide significant increases in total volume, but effects of successive applications diminish over time.