Fertilization with nitrogen (N) is a treatment commonly used to increase the productivity of loblolly pine plantations in south.  Urea is the most widely used source of N in forestry.  However, only 10 to 25% of the N applied as urea is taken up by the crop trees.  Enhanced efficiency fertilizers, which are designed to delay the release of or stabilize the N, may increase N uptake efficiency in forests.  This study compared the N release rates of six fertilizers: 1) urea; 2) urea + NBPT (20%); 3) urea + NBPT (0.06%); 4) MAP coated urea; 5) MAP coated urea + NBPT; and 6) polymer coated urea.  Nitrogen availability in the soil in the various fertilizer treatments was measured using ion exchange membranes.  Cation and anion exchange membranes were inserted into the surface 10 cm of mineral soil and replaced every two weeks.  The urea, urea + NBPT (20%), urea + NBPT (0.06%), MAP coated urea, and MAP coated urea + NBPT all had similar patterns of NH₄⁺ availability in the mineral soil which peaked 26 days after fertilization and declined back to baseline levels 85 days after fertilization.  The pattern of NH₄⁺ availability following application of the polymer coated urea was delayed and extended over a longer period of time.  NO₃^- availability in the soil was greatest in the urea and the MAP coated urea + NBPT was greater than the other enhanced efficiency fertilizer treatments.