Quantifying the Fate of Applied N Fertilizer Over a One-Year Period in Douglas-Fir Plantations Using Stable Isotope Technology.

Nitrogen is understood to be a commonly limiting nutrient in Douglas-fir ecosystems in the Pacific Northwest. In managing these ecosystems, application of fertilizer Nitrogen is routinely used as a means to mitigate nitrogen deficiencies and to increase the overall production. However, of the total amount applied less than 30 percent is actually taken up by the trees. Achieving a better understanding of the fate of applied fertilizer nitrogen will allow stand managers to improve returns on their fertilizer investment as well as reduce any potentially negative environmental impacts. Stable isotopes are commonly used in ecological research and past studies have shown ¹⁵N to be particularly effective in measuring fertilizer uptake in forested ecosystems.

Four different fertilizers are being used in this study, each labeled with 0.5 atom% ¹⁵N. The fertilizers were applied at a rate of 224kg/ha at 5 different Douglas-fir stands in 2011 with a control established on each site as well. These sites are located throughout the Pacific Northwest and represent a wide range of climate and parent materials.

 Uptake of fertilizer nitrogen will be determined based upon the enrichment (or depletion) of ¹⁵N following fertilization relative to pre-fertilization ¹⁵N values. Total nitrogen and δ¹⁵N were analyzed with an elemental analyzer interfaced to an isotope ratio mass spectrometer. Foliage and soil samples (including forest floor) were collected prior to fertilization and analyzed to determine background levels of δ¹⁵N.  Following fertilization, foliage was collected at regular intervals and a final sampling at one year sample included soil, forest floor, foliage, bark, bole, and understory vegetation.  By calculating the mass of ¹⁵N in each of these components in addition to their mass (computed using allometric equations) we can determine the amount of fertilizer nitrogen distributed to each component.  Any remaining nitrogen not accounted for will be attributed to loss from the system via volatilization or other gaseous loss using mass balance. Results are in progress and unavailable as of yet.