Uptake Efficiency and Transport of Applied Nitrogen Fertilizers within Douglas-Fir Forests of the Pacific Northwest.

Nitrogen (N) is known to be a limiting nutrient in Pacific Northwest forests, particularly in Douglas-fir ecosystems.  Theoretically, N fertilization should ameliorate these deficiencies and increase tree growth.  However, previous studies have shown that of the total N applied, the surrounding trees absorb only between 12-43%.  This study is composed of 10 sites along the western Douglas-fir region of Oregon and Washington.  These sites were selected to include the range of parent materials and latitudes of plantation forests in the region.  A complete randomized block design was used where each installation is a replicate that includes five treatment plots.   The plots were treated with ¹⁵N-enriched Urea, Agrium ESN, Urea + NBPT, or Urea + CUF type fertilizers.  The remaining plot was an unfertilized control.  The fertilizers were enriched to 0.5AP ¹⁵N (~370 ⁰/₀₀ ¹⁵N).  Using a fertilizer enriched with ¹⁵N, the nitrogen is traced as it moves between pools within the study area.