Fate of Applied 15N Fertilizers in Douglas-Fir Plantations of the Pacific Northwest.

As the predominant forest plantation species in the Pacific Northwest, Douglas-fir (Pseudotsuga menziesii) and the productivity of its underlying soil are important to the sustainability of the region’s timber economy and forest health.   Nitrogen (N) is known to be a limiting nutrient in Douglas-fir ecosystems.  Previous studies have found that N fertilization can increase tree growth when applied to low productivity sites.  However, of the total N fertilizer applied, less than 30% is taken up by the target trees.  A better understanding of the fate of the remaining N is necessary to mitigate any potential environmental impacts and increase the financial return of costly fertilizer applications.  This poster presents the preliminary results of our study to deterimne the fate of the applied N fertilizer that is currently unaccounted for in the above ground biomass.  Target trees were selected and treated with 0.5% ¹⁵N enriched urea fertilizer applied at a rate of 224kgN/ha.  Four types of urea fertilizer were tested and compared to an unfertilized control plot.  The industry standard urea as well as three enhanced efficiency fertilizers (CuF, ESN, and NBPT) were compared.  Forest floor, mineral soil to a depth of 60cm, bark, bole, foliage, and understory vegetation were sampled before application and 1 year following treatment.