Plant growth and nutrient uptake response to increased fertilization can be conceptually described by cur vi linear relationships depicting phases of nutrient deficiency, sufficiency, luxury consumption, and toxicity to rationalize fertilizer prescriptions and improve nutrient diagnosis. We validated this model to determine optimum nitrogen storage of young black spruce (Picea mariana (Mill.) BSP). Container seedlings were supplied with a mixed nitrogen–phosphorus–potassium (N–P–K) fertilizer at rates ranging from 0 to 80 mg N/seedling and reared in a greenhouse for one growing season. Plant growth and nutritional parameters of the plants exhibited classic responses of N deficiency, luxury consumption, and toxicity that were corroborated by vector diagnosis and appeared consistent with the conceptual model. Seedling biomass production was maximized at sufficiency (30 mg N/seedling), whereas N content of tissues peaked at the optimum loading rate (64 mg N/seedling). Toxicity occurred at the 80 mg N/seedling dose rate that increased plant N concentration (5%) but reduced growth (17%) and N content (14%) relative to the optimum level. Plant N content was raised 150% by optimum loading, exemplifying the effectiveness of this practice for building internal N reserves prior to planting. The newly validated model will help refine fertilizer recommendations and nutrient diagnosis for other species or cultural systems.

Nutrient loading stimulates N retranslocation, an important mechanism of N reuse in plants to support new growth. We quantified N retranslocation in young black spruce [Picea mariana (Mill.) BSP] using tracer and nontracer techniques to examine enhanced field performance after nutrient loading. Nursery reared seedlings were transplanted to sand‐filled pots fertilized with 15NH4 15NO3 at rates equivalent to 0 and 200 kg N ha−1 simulating poor and rich soils. After one growing season (120 d), biomass increased (118%) on the poor soil without N gain demonstrating the significance of internal N reserves for retranslocation to new growth. Nutrient loading improved retranslocation (218%) and new biomass (156%) after planting confirming the advantage of higher preplant N reserves (175%) for later nutrient demand. Enhanced N availability in the rich soil accelerated growth (236%), N uptake (258%), and retranslocation (23%) in seedlings. Retranslocation increased with time reflecting higher N demand as seedlings become larger and suggest the process is driven by sink strength. Nontracer estimates of N retranslocation in seedlings fell short of isotopic determinations because of inability to discriminate between soil and plant derived N in tree components. Although fertilization promoted N uptake (125–258%), 15N recovery in plants averaged 12 to 19% indicating low fertilizer efficiency in young trees. Total reliance of unfertilized plants on internal N reserves for growth on the poor soil affirms the importance of retranslocation to meet plant N demands, and also exemplifies initial short‐term independence on soil N for newly planted seedlings that can be prolonged by nutrient loading.

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-We quantified and characterized fertility targets for nursery culture of container northern red oak (Quercus rubra L.) seedlings. Plants were supplied with a 15N-5P 2 O 5 -15K 2 O fertilizer at eight rates ranging from 0-150 mg N plant -1 and reared for 18 wk in a greenhouse. Plant growth and nutritional response to increased fertilization followed a curvilinear pattern depicting phases that ranged from deficiency to toxicity. Seedling dry mass production was maximized at sufficiency (25 mg N plant -1 season -1 ) while optimum N and P uptake occurred at 100 mg N plant -1 season -1 . The 150 mg N plant -1 seasonal dose rate induced N and P toxicity, but resulted in antagonistic K interaction. Nutrient loading raised plant N and P contents by 27 and 55%. This new approach demonstrates promise to help refine fertility targets for nursery production of Q. rubra planting stock and may have application to other hardwood species or cultural systems.antagonistic interaction / exponential fertilization / growth / luxury uptake / northern red oak / vector diagnosis Résumé -Caractérisation des objectifs de fertilité et des interactions multiéléments chez des semis de Quercus rubra cultivés en pépinière. Des objectifs de fertilité ont été quantifiés et caractérisés pour des semis de chêne rouge d'Amérique (Quercus rubra L.) cultivés en pépinière. Les semis ont été alimentés avec un engrais 15N-5P2O5-15K2O selon huit taux de 0-150 mg N plant -1 et ont poussés pendant 18 semaines dans une serre. La croissance des semis, leur réponse nutritionnelle à un accroissement de la fertilisation a suivi un modèle curvilinéaire décrivant des phases rangées depuis la carence jusqu'à la toxicité. La production en matière sèche des semis a été maximale à la dose suffisante correspondant à 25 mg N plant -1 saison -1 , tandis que l'optimum de consommation s'est situé à 100 mg N plant -1 saison -1 . La dose saisonnière de 150 mg N plant -1 a induit une toxicité N et P, mais il en est résulté une interaction antagoniste avec K. Le prélèvement de nutriments par les plants a augmenté le contenu en N et P de 27 % et 55 %. Cette nouvelle approche démontre la possibilité d'espérer perfectionner les objectifs de fertilité pour une production en pépinière de plants de Quercus rubra et peuvent avoir une application pour d'autres espèces feuillues et d'autres systèmes culturaux. interaction antagoniste / fertilisation exponentielle / croissance / consommation de luxe / chêne rouge / vecteur diagnose

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