Conifer Seedling Growth Response to Soil Type and Selected Nitrogen Availability Indices

Although N is typically a growth‐limiting factor in northeastern forest soils, few studies have demonstrated the best analytical approach for determining available N in this heavily forested region. To address this information need, seedlings of balsam fir [ Abies balsamea (L.) Miller], red spruce ( Picea rubens Sarg.), and white pine ( Pinus strobus L.) were grown for 26 wk in the greenhouse using soil materials representative of commercial forest sites in Maine. Soils included two O horizons and two B horizons from Spodosols derived in till, and an Ap and B horizon from an Inceptisol derived in marine sediments. Six different indices of available N were evaluated for each horizon, and results were compared with seedling‐growth parameters. Three available‐N indices were chemical and three were biological. Seedlings grew markedly better in O horizons than in mineral soils. Of the species examined, white pine had the greatest dry‐weight yields, and was the only species to exhibit visible evidence of root mycorrhizal infection. Although N‐availability indices were correlated with seedling‐growth parameters, the 1‐ and 2‐wk anaerobic incubations generally exhibited the highest correlation coefficients. Of the chemical indices, the simple KCl extraction appeared best correlated with seedling growth. Of the seedling characteristics examined, foliar N concentration was not highly correlated with soil N availability due to dilution effects. In contrast, foliar N uptake was highly correlated with N availability.