Differences in the recovery of four different nitrogen containing fertilizers after two application seasons in pine plantations across the southeastern United States

• Ecosystem fertilizer N recovery greatest for enhanced efficiency fertilizers (EEFs).
• Spring and summer ecosystem fertilizer N recovery similar for all EEFs.
• High fertilizer N ecosystem loss for urea compared to all EEFs.
• Higher fertilizer N ecosystem loss for urea in summer than spring.
• Use of EEFs in intensively managed forests could increase nitrogen use efficiency.

The ecosystem recovery of four nitrogen (N) containing fertilizers (urea and three enhanced efficiency fertilizers [EEFs]) enriched with 15N were compared at five different sites with single treatment replication per site for both a spring and summer application in mid-rotation loblolly pine (Pinus taeda L.) stands across the southeastern United States. Total ecosystem fertilizer N recovery was greater for all EEFs (78–84%) compared to urea (52%) with no differences among individual EEFs or between seasons. Fertilizer N recovery in the soil (forest floor + 0–30 cm) was greater for EEFs (36–43%) compared to urea (21%), with no significant treatment differences in fertilizer N recovery in the canopy (14–22%), stem (5–8%), or roots (11–14%). When individual ecosystem components were analyzed, the greatest difference in fertilizer N recovery occurred in the 0–15 cm mineral soil for EEFs (27–31%) compared to urea (13%). Differences for the same fertilizer treatment between the season of application were generally minor. Despite these minor differences between season of application, there was a general trend for higher fertilizer N recovery in the crop trees for all treatments for the spring fertilization compared to a higher fertilizer N recovery in the soil after the summer fertilization. Numerous treatment differences also occurred in individual ecosystem components for δ15N values (‰) and N concentrations within the same season, but only occasional differences between seasons. This research highlights an increased fertilizer N recovery and ecosystem partitioning of fertilizer N using EEFs compared to urea in southeastern pine plantations, regardless of the application season, and potentially translate to an increase in the fertilizer N use efficiency of these pine plantations.