15N Stable Isotope Research

The FPC has been utilizing stable isotopes of 15N to improve our understanding of nutrient dynamics in managed forest ecosystems across the southeastern United States for close to a decade. By using 15N enriched fertilizers, we have improved our quantification of the proportion of fertilizer nitrogen (N) used by a crop tree over a single, and multiple, growing seasons. Understanding how much, and where, fertilizer N is incorporated in a managed forest is important to continue refining fertilizer N recommendations on an individual site basis. Use of 15N labeled fertilizers has been combined with enhanced efficiency fertilizer products (EEFs) to continue to improve the sustainability of fertilization in managed forests. The FPC has been a research leader in combining the use of 15N with enhanced efficiency products to improve fertilizer stewardship and understanding nutrient dynamics in relation to forest productivity. Over the last decade, research highlights from intensively managed forests across the southeastern United States include:

  • EEFs reduce fertilizer N losses from managed forests 

  • EEFs increase fertilizer N remaining in the forest system

  • EEFs increase available N to crop trees

  • EEFs increase productivity in managed forests

  • Initiated a study investigating the competitiveness of understory vegetation for mid-rotation fertilizer N using 15N in Appomattox, VA.

The proportion of fertilizer nitrogen lost, retained, and partitioned for mid-rotation, thinned loblolly pine forests across the southeastern United States after a spring and summer fertilization with minimal understory competition.

Minimizing N Losses After Winter Fertilization 

In a recent FPC study, N losses were compared between urea and urea treated with the urease inhibitor N-(n-Butyl) thiophosphoric triamide (urea + NBPT) after a surface application in winter at three thinned mid-rotation (age 15-20 years) loblolly pine plantations in Virginia (VA), South Carolina (SC), and Florida (FL). Losses following fertilization with urea (24% to 50%) were greater at all sites compared to urea + NBPT (12% to 22%). Fertilizer N losses were greater in FL than in SC and VA, although N loss following urea fertilization was still 25% in VA. The loss of fertilizer N was consistently lower on beds compared to interbeds for both urea (bed = 25%, interbed = 40%) and urea + NBPT (bed = 12%, interbed = 23%). This research highlights the value of using NBPT to reduce fertilizer N losses after a winter application and the greater potential loss in the interbed on wetter sites. 

Destination of Fertilizer N Among Competing Vegetation

In 2020, we established a study in Appomattox, Virginia which will measure fertilizer N sequestered by competing vegetation in a mid-rotation loblolly pine stand. The study site is located on a former chemical site preparation study which was established by Dwight Lauer and Harry Quicke in 2005. Jerre Creighton, of the Virginia Department of Forestry has been stewarding the project, enabling an investigation into the long-term effects of various chemical site preparation treatments. The site prep experiment resulted in experimentally varied amounts (i.e. basal area, stem count, and species) of competing vegetation. We delineated three levels of competing vegetation (high, medium and low) and we installed three treatments: vegetation control (VC) before fertilizer application, VC 1-year after fertilizer application, and no VC. Treatments are equally distributed among the levels of competing vegetation. In the spring of 2022, we collected samples of soil, forest floor, foliage, competing vegetation biomass, and loblolly pine biomass. We expect competing  species to behave similarly and acquire more fertilizer N as their size increases. We expect greater quantities of competing vegetation to be associated with less fertilizer N in loblolly pine crop trees. Conversely, we expect greater initial quantities of competing vegetation to be associated with a larger, post-VC “pulse” of fertilizer N in loblolly pine crop trees.