Understanding Ecosystem Fate of Applied Nitrogen in Loblolly Pine Plantations through the Use of Stable Isotopes.

Research focused on understanding the fate of applied nitrogen (N) in loblolly pine plantations in the southern United States indicates only 10 to 25% of the N is utilized by the crop trees.  Large N losses, primarily through volatilization, may occur immediately following application, reducing the overall efficiency and effectiveness of fertilization.  A large degree of uncertainty exists regarding the partitioning of the remaining applied N in these systems.  An improved fundamental understanding of applied N cycling in these systems is critical to enhance productivity and reduce potentially adverse environmental impacts.  This research is using fertilizers enriched with the stable isotope ¹⁵N (0.5 atom %;~370 ‰) to determine differences in applied N cycling in loblolly pine stands between conventional (urea) and three enhanced efficiency fertilizers [EEFs: polymer coated urea (PCU), urea+ N-(n-butyl) thiophosphoric triamide (NBPT), monoammonium phosphate coated urea (CUF)] with a spring and summer application.  Eighteen sites were established across the entire range of loblolly pine plantations in the southern United States in 2011 and 2012.  Five 100 m² circular plots were installed at each site with five treatments (no fertilizer, urea, CUF, NBPT, PCU) randomly assigned to each plot.  All primary ecosystem components were sampled at the end of the first growing season after application.  Results indicate significant differences in N losses (gaseous, vertical soil movement) using a microcosm experiment between EEFs (5 to 25%) compared to urea (25 to 45%).  Total ¹⁵N ecosystem recovery rates were highly variable ranging from 45 to 100%, dependent on site, treatment and season.  Foliage generally had the greatest percentage of ¹⁵N for aboveground biomass, whereas the mineral soil had the largest percentage of ¹⁵N for the entire ecosystem.