Retranslocation, Plant, and Soil Recovery of Nitrogen-15 Applied to Bareroot Black Walnut Seedlings

Although retranslocation or nitrogen (N) derived from plants (NDFP) may account for more than 50% of the annual N demand in new growth of conifer seedlings, the proportional contribution of NDFP vs. current uptake or N derived from fertilizer (NDFF) in new growth of hardwood seedlings is relatively unknown. The current uptake was labeled with ammonium sulfate [(15NH4)2SO4] at the rate of 1.56gNplant-1 and reared for 90 days in sand culture under greenhouse conditions, and NDFP vs. NDFF was quantified in new growth of half-sib bareroot black walnut (Juglans nigra L.) seedlings. Nitrogen derived from plants accounted for 68 to 83% of the total N demand in new shoot growth of black walnut seedlings vs. NDFF (17 to 32%). Recovered applied fertilizer was 43% in soil and 9% in plants. The greater proportion of NDFP in new growth demonstrates the importance of retranslocation in meeting early N demand of transplanted black walnut seedlings.