Uptake Efficiencies of 15n Enriched Enhanced Efficiency Fertilizers in a Pinus Taeda L. Stand.

Nitrogen (N) fertilization is a commonly used treatment to increase the productivity of southeastern loblolly pine plantations.  Though many loblolly pine plantations show increased growth after N fertilization, only about 10 to 25% of applied N as urea is taken up by crop trees.  Enhanced efficiency fertilizers are designed to increase N uptake efficiency by delaying the release of and/or stabilizing the applied N.  Stable isotope techniques allow researchers to determine the fate and quantify the amount of N moving through an ecosystem. In this study ¹⁵N enriched fertilizers were used to determine the uptake of applied N in an 8 year old loblolly pine stand.  Four 0.5 atom % enriched fertilizer treatments were applied to single tree plots in the spring of 2011 at a rate of 150 kg N ha^-1: 1) urea; 2) urea + NBPT; 3) Map coated urea; 4) polymer coated urea.  Uptake efficiencies of the applied N were determined using mass-balance techniques.  Foliar, branch, stem, fine root, forest floor, and mineral soil samples were collected pre-treatment to determine the ¹⁵N natural abundance. At the end of the 2011 growing season the study trees was destructively harvested and each component of the trees were weighed and subsampled.  Samples were analyzed for δ¹⁵N using an IsoPrime100 IRMS.  The percent uptake of applied ¹⁵N within each site was calculated using the following equation: %N df_source1= ((δ¹⁵N_x – δd¹⁵N_y)/(δ¹⁵N_x – δ¹⁵N_c)) x 100.  It is expected that the uptakes efficiencies of the fertilizer treatments will be as follows: urea < urea + NBPT ≤ MAP coated urea < polymer coated urea.  Foliage is expected to be the largest pool of applied N followed by soil and forest floor.