Combined effects of pre-hardening and fall fertilization on nitrogen translocation and storage in Quercus variabilis seedlings

Abstract

Maintaining proper seedling nitrogen status is important for outplanting success. Fall fertilization of evergreen conifer seedlings is a well-known technique for averting nitrogen (N) dilution caused by continued seedling growth during hardening. For deciduous seedlings, this technique is much less understood, and regardless of foliage type, the interaction of N status prior to fall fertilization and the rate of fall fertilization have yet to be fully explored. Therefore, we fertilized Quercus variabilis container seedlings with either 25, 100, or 150 mg total N seedling−1, applied exponentially, during a 23-week pre-hardening regime, followed by either 0, 12, or 24 mg total N seedling−1 applied during hardening (i.e., fall fertilization) in equal aliquots for 4 weeks. For seedlings without supplemental N during hardening, N concentration in stems and roots increased significantly despite substantial growth. The absence of N dilution was attributed to N translocation from foliage to these tissues, which was independent of pre-hardening N status. Overall, 32 % of foliar N was translocated and accounted for 75 % of the total N increase in stems and roots. Final stem N status was a function of pre-hardening fertilization, whereas root N concentration was affected by the interaction of pre-hardening and fall fertilization. Roots appear to be the main site of N storage, and root N content was significantly affected by pre-hardening and fall fertilization, but not their interaction. A combination of pre-hardening and fall fertilizer at a rate of 100 and 24 mg total N seedling−1, respectively, yielded seedlings with the largest root systems.