Nitrogen competition using 15 N between early successional plants and planted white spruce seedlings

The ability of noncrop plants to compete with white spruce (Picea glauca (Moench) Voss) seedlings for applied fertilizer N is not well understood. Therefore, the objective of this study was to examine the efficacy of fertilizer N uptake using 15 N by planted white spruce seedlings and understory vegetation. Double 15 N-labeled NH 4 NO 3 was broadcast in 1-m 2 plots for control, disc-trenched, and disc-trenched + manual brushing treatments. The fate of applied 15 N in white spruce and noncrop plants was determined at the end of the first two growing seasons. The major competitors for fertilizer 15 N were Populus tremuloides Michx. and grasses during the first growing season, and Populus tremuloides, Epilobium angustifolium L., and Achillea millefolium L. during the second growing season. Disc-trenching plus manual brushing significantly increased the fertilizer use efficiency of white spruce seedlings by limiting competition; however,