The objectives of this research are to determine the nitrogen uptake efficiency and loss of four urea fertilizers in Pacific Northwest Douglas-fir plantations. In addition, we hope to expand our understanding of nitrogen cycling in Pacific Northwest Douglas-fir ecosystems by determining the fate of added nitrogen after one year.

The proposed methods for this study are to apply four treatments of ¹⁵N enriched urea fertilizers to 100m² plots at 10 sites in western Oregon and Washington over two years (five sites each year). Above and bellow ground ecosystem component will be sampled to determine d¹⁵N concentrations after one year, and compared to background d¹⁵N derived from a control plot to quantify the amount of fertilizer nitrogen in each component one year after application. The four treatments will be:

1.)Untreated urea pellets

2.)Urea+NBPT: Urea pellets coated with N(n-butyl) thiophosphoric triamide, commercially available as Agrotain. The NBPT binds to the active site of the enzyme urease, inhibiting the enzyme’s capacity to catalyze the hydrolysis of urea.

3.)Urea CUF: Urea pellets coated in a boron solution that inhibits urease and acts as a binding agent for secondary nutrients like Phosphorous in the commercial variety Arborite.

4.)ESN urea: Polymer coated pellets of urea commercially available under the brand Agrium designed to delay urea release to the soil by creating a permeable physical barrier that breaks down in time.

The results of this study are expected to assists foresters in selecting the best urea formulation for Douglas-fir fertilization in western Oregon and Washington.