Nitrogen Accumulation by Conifer Seedlings and Competitor Species From ¹⁵ Nitrogen‐labeled Controlled‐Release Fertilizer

Abstract

A major impediment to the establishment of outplanted conifer seedlings is competition for available soil N by early successional species. The objective of this field study was to determine the fate of controlled‐release fertilizer (CRF) N in soils with outplanted white spruce ( Picea glauca [Moench] Voss.) and jack pine ( Pinus banksiana Lamb.) seedlings, and the effect of weed control or vegetation management (VM) on fertilizer N accumulation. Nitrogen‐15 labeled CRF was placed next to the seedling root plug during planting at four boreal mixed wood sites. After one growing season in the control plots, fertilizer N recovery as a percentage of 15 N added was 4% in seedlings, 3% in competing vegetation, <1% leached, and 85% residual CRF. After two growing seasons, fertilizer N recovery was 15% in seedlings, 20% in competing vegetation, <1% leached, and 58% residual CRF. Overall, VM increased seedling fertilizer N uptake by almost 300% compared with conifer seedlings in control plots. In VM plots, fertilizer bags contained more N than in control plots after two growing seasons. In both treatments, >50% of the fertilizer N remained in the fertilizer bag, presumably remaining available in subsequent seasons. Calamagrostis ( Calamagrostis canadensis ) was the primary competitor for fertilizer N in both growing seasons, with minor competition from fireweed ( Epilobium angustifolium L.), and aspen ( Populus tremuloides Michx.). The use of a point source CRF delivery method resulted in high fertilizer use efficiency (FUE), and minimized losses to competing vegetation and leaching.