A Comparison of Ecosystem Recovery of Applied Nitrogen from Urea and Enhanced Efficiency Fertilizers in Mid-Rotation Loblolly Pine Plantations (Pinus taeda L) of the Southern United States.

The ecosystem partitioning and recovery of applied nitrogen (N) was compared for four N containing fertilizer (urea, three enhanced efficiency fertilizers (EEFs)) treatments enriched with ¹⁵N  in two separate studies in mid-rotation loblolly pine stands (Pinus taeda L.) across the southern United States. All fertilizer treatments were labeled with ¹⁵N (~370 permil, 0.5 AP). In the first study, fertilizer N recovery for the four fertilizer treatmets was compared after two different application seasons (spring vs. summer) to test seasonal effects on ecosystem partitioning of fertilizer N for each treatment. In the second study, a larger geographic area was used to test whether differences existed in the recovery of fertilizer N based on regional differences. In the first study, the total ecosystem fertilizer N recovery was greater for all EEFs (78% to 84%) compared to urea (52%) with no differences among individual EEFs or between seasons. Fertilizer N recovery in the soil (forest floor + 0-30cm) was greater for EEFs (36% to 43%) compared to urea (21%), with no treatment differences in fertilizer N recovery in the canopy (14% to 22%), stem (6% to 8%), or soils (10% to 13%). In the second study, total fertilizer N recovery ranged from 77% to 80% for EEFs compared to 56% for urea. Crop tree recovery for urea was 34% compared to EEFs which ranged from 33% to 42%. In the soil, fertilizer N recovery in study 2 for EEFs ranged from 36% to 43% compared to urea which was 21%. In both studies, numerous treatment differences occurred in individual ecosystem components for ¹⁵N values and N content in the same season, with minor differences between seasons. This research highlights increased fertilizer N recovery and ecosystem partitioning of fertilizer N using EEFs compared to urea in southern loblolly pine plantations, potentially increasing the fertilizer N use efficiency of these pine plantations.