Tradeoffs of nitrogen investment between leaf resorption and photosynthesis across soil fertility in Quercus mongolica seedlings during the hardening period

The most important process before leaf senescence is nutrient resorption, which reduces nutrient loss and maximizes plant fitness during the subsequent growth period. However, plants must retain certain levels of nitrogen (N) in their leaves to maintain carbon assimilation during hardening. The objective of this study was to investigate the tradeoffs in N investment between leaf N resorption and N for photosynthesis in seedlings with increased soil fertility during the hardening period. A field experiment was conducted to determine if and how soil fertility treatments (17, 34, or 68 mg N seedling⁻¹) affected N resorption and allocation to the photosynthetic apparatus in Quercus mongolica leaves during the hardening period. Seedlings were sampled at T₁ (after terminal bud formation), T₂ (between terminal bud formation and end of the growing period), and T₃ (at the end of the growing period). Results showed that photosynthetic N content continued to rise in T₂, while N resorption started from non-photosynthetic N. Leaf N allocation to the photosynthetic apparatus increased as soil fertility increased, delaying N resorption. Additionally, soil fertility significantly affected N partitioning among different photosynthetic components, maintaining or increasing photosynthetic traits during senescence. This study demonstrates a tradeoff in N investment between resorption and photosynthesis to maintain photosynthetic assimilation capacity during the hardening period, and that soil fertility impacts this balance. Q. mongolica leaves primarily resorbed N from the non-photosynthetic apparatus and invested it in the photosynthetic apparatus, whereas different photosynthetic N component allocations effectively improved this pattern.