Nitrogen Retranslocation Response of Young Picea mariana to Nitrogen‐15 Supply

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 15 NH 4 15 NO 3 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%), 15 N 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.