Tracing the Fate of Applied 15N Fertilizers in Douglas-fir Plantations Stephani Michelsen-Correa, Rob Harrison, and Betsy Vance University of Washington, School of Environmental and Forest Science Study Area: 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 • 10 sites along the Western Douglas-fir region of Oregon and Washington • Includes the range of parent material and latitudes typically seen in Douglas-fir plantations forests of the Pacific Northwest Pot Studies Field Studies ? Missing 57-88% 85-95% 12-43% Figure 1: Differences in the efficiency of Nitrogen fertilizer uptake between pot/greenhouse studies and actual field experiments1 2. Installation Sites 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) • Progress to-date: • All sampling for this project has been completed (Table 2) • Preliminary results are available for volatile losses, foliage uptake, forest floor, and mineral soil retention for 2011 • Processing of the remaining 2012 data continues as we wait to receive our 2011 δ15N analysis back from Virginia Tech. Treatment Plot 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. Table 1: Five treatment types used at each of the 10 installations. The fertilizers were enhanced with 15N (0.5 AP, ~370 0/0015N) Table 2: Summary of progress. Items marked with X indicate completion and – denotes work in progress. Results: • 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 • All four treatments show leaching of the applied fertilizers as indicated by the increasing δ15N value with depth • Urea + NBPT was substantially higher at the surface and showed the least amount of leaching • 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. • Preliminary results suggest greater uptake efficiency for the standard urea fertilizer 100m2 plot boundary 224 kg N ha-1 Target tree Litter and Soil Sample Aerial view 2012 100 km Figure 5: Changes in δ15N with mineral soil depth one year after fertilization. Figure 4: Changes in δ15N values in the 2011 foliage from 0-34 weeks after treatment. 2011 References: 1. 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. 2. 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.