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Tracing the Fate of Applied 15 N Fertilizers in Douglas-fir Plantations Stephani Michelsen-Correa, Betsy Vance, and Rob Harrison University of Washington, School of Environmental and Forest Science. Background:

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Tracing the Fate of Applied 15N Fertilizers in Douglas-fir Plantations

Stephani Michelsen-Correa, Betsy Vance, and Rob Harrison

University of Washington, School of Environmental and Forest Science

  • Background:
  • Nitrogen (N) is known to be a limiting nutrient in Pacific Northwest forests. Fertilization is commonly used to maintain the quantity of N needed to support high growth rates in Douglas-fir plantations.
  • Research on the growth response to fertilization has produced variable results. One explanation for the poor response is that of the applied fertilizer, only 12-43% is actually being taken up by the trees1 2. The fate of the remaining 57-88% is currently unknown.
  • Objectives:
  • Use 15N labeled urea fertilizers to trace the fate of nitrogen in the ecosystem following application.
  • Compare the uptake efficiency and losses of four commonly used fertilizers

% of Applied Nitrogen retained by target trees

Pot Studies

Field Studies






Figure 1: Differences in the efficiency of Nitrogen fertilizer uptake between pot/greenhouse studies and actual field experiments1 2.

Study Area:

  • Methods:
  • 10 sites (Figure 2) were installed over a 2 year period (2011 and 2012)
  • -Sampled for baseline 15N values
  • Each installation consists of a randomized block design with five treatment plots (Figure 3)
  • The four fertilizers used have all been enhanced with 15N, a stable isotope of N that is of relatively low abundance in the environment compared to 14N (Table 1)
  • Ecosystem components were sampled again one year after fertilization and analyzed for 15N recovery (Figure 3)

Installation Sites


100 km

  • Figure 3: Treatment plot showing ecosystem components sampled for 15N recovery. Each of the 10 sites contains five of these plots, one for each treatment.


Figure 2: 10 sites along the Western Douglas-fir region of Oregon and Washington

  • Table 1: Five treatment types used at each of the 10 installations. The fertilizers were enhanced with 15N (0.5 AP, ~370 0/0015N)

Treatment Plot

(Aerial view)

100m2 plot boundary

-224 kg N ha-1


  • Projected Deliverables:
  • Estimates of N losses due to leaching, volatilization, and uptake by competing understory vegetation
  • Determine the relative efficiency of the four fertilizer treatments
  • Produce data that can be incorporated into a model useful for land managers wanting to predict stand response to fertilizer applications in the PNW

Target tree

Litter and Soil Sample

Mineral Soil

(1 Year)




Urea + NBPT

Urea + CUF


Figure 4: Concentrations of the fertilizer label in the 2011 foliage from 0-34 weeks after treatment. Urea, NBPT, and CUF follow a similar trajectory through week 10. However, by week 16 urea had the highest concentrations followed by NBPT and CUF respectively.

Figure 5: Concentrations of 15N with mineral soil depth one year after fertilization. This is preliminary data as it represents the concentrations from only one of our study sites.

  • References:
  • Amponsah, I., Lieffers, J., Comeau, P., Landhausser, S. (2004). Nitrogen-15 uptake by Pinuscontorta seedlings in relation to phonological stage and season. Scandinavian Journal of Forest Resources. 19:329-338.
  • Salifu, K. and Trimmer, V. 2003. Nutrient retranslocation response of Piceamariana seedlings to nitrogen-15 supply. Soil Science Society of America Journal 67:905-913.