Seasonal Nitrogen Volatilization Differences Between Conventional and Enhanced Efficiency Fertilizers in Southern Loblolly Pine Plantations.

Nitrogen (N) loss from conventional N fertilizers (e.g. urea) in southern loblolly pine (Pinus taeda L.) plantations is currently an economic and environmental concern.  The primary N loss mechanism from these forested systems is generally considered to be volatilization, with loss estimates ranging from 20 to 50% of applied fertilizer.  Losses may be minimized by applying N fertilizer in late winter or early spring when environmental conditions are less conducive to volatilization mechanisms, although this timeframe may not coincide with seasonal N uptake of the crop trees.  Enhanced efficiency fertilizers (EEFs) have been shown to reduce N losses from agricultural systems, and more recently in forestry applications, by different physical and chemical mechanisms.  We examined whether differences in N losses exist across the range of loblolly pine plantations between conventional (urea) and EEFs (polymer coated urea, urea+NBPT, and monoammonium phosphate coated urea) by using a mass balance approach with ¹⁵N enriched EEFs (0.5 atom %;~370 ‰).  Each EEF treatment was applied twice (late winter-early spring and summer) to individual microcosm cores constructed of PVC.  Individual microcosms were removed from the field at 1, 15 and 30 days and divided into the forest floor, 0-10, 10-20, and 20+cm mineral soil for analysis of ¹⁵N.  The fertilizer ¹⁵N unaccounted for was considered removed from the system by volatilization or vertical soil movement.  The initial analysis of depth increments based on ¹⁵N (‰) enrichment values indicate N losses through volatilization are generally less for most EEFs when compared to urea at 1 and 15 days, with less of an observable difference on Day 30.  Furthermore, winter losses appear generally less compared to summer at most sites, although vertical movement of fertilizer ¹⁵N is greater in summer compared to winter.  This research indicates certain EEFs may reduce N volatilization losses in southern loblolly pine plantations, but attention is still required for optimizing timing and site application.