Nitrogen retranslocation of young Picea mariana to varied nitrogen supply and plant nutrient reserves

Newly planted seedlings depend on retranslocation of internal nitrogen (N) reserves for new growth because initial slow root development limits uptake from the soil. Nursery nutrient loading stimulates retranslocation, seedling growth and eventually plantation development. A dose response model for nutrient loading was validated for optimizing plant N reserves in black spruce [ Picea mariana (Mill.) BSP] crops for nursery culture. Subsequently, loaded and conventionally fertilized seedlings were planted on simulated soil fertility gradients under field and greenhouse conditions to examine impacts of nutrient loading and site fertility on nutrient remobilization and growth processes. Nutrient loading induced luxury uptake that increased plant N content (150--200%), demonstrating the effectiveness of this practice to build up plant N reserves before outplanting. On field sites, accelerated net N retranslocation (569%) in loaded trees was associated with higher plant N reserves. In the greenhouse, direct N retranslocation was quantified using labelled isotopes and compared with net estimates to confirm improved retranslocation by loaded trees. Nutrient loading increased tree growth (156%) and N retranslocation (218%), corroborating higher retranslocation under field conditions. In contrast to diminished net N retranslocation with soil fertility noted on field sites, soil N supply accelerated retranslocation (23%), and seedling growth (236%). Apparently, net N retranslocation estimates fell short of isotopic determinations because of the inability to discriminate between plant and soil derived N in tree tissues. Retranslocation also increased with seedling development reflecting higher N demand as plants become larger, suggesting the process is driven by sink strength. Results were integrated into conceptual models that quantified N uptake, distribution and retranslocation dynamics in tree components under different management scenarios, such as nutrient loading or site fertilization. Such knowledge can be used to promote growth of newly planted seedlings and trees in plantations.