An exponential fertilization dose–response model to promote restoration of the Mediterranean oak Quercus ilex

Abstract

Nursery nitrogen (N) fertilization influences seedling N reserves, morphology, photosynthesis rate and stress tolerance and frequently enhances outplanting performance. Although mineral nutrition is a critical aspect of seedling quality, fertility targets of Mediterranean sclerophylous species have not been thoroughly quantified. We sought to define those fertility targets for seedlings of Quercus ilex, a key species in Mediterranean areas. Nine fertility treatments, ranging from 0 to 200 mg N seedling−1 applied under an exponential regime were tested in a greenhouse dose response trial in which phosphorus (P) and potassium (K) were increased in the same proportion as N (15N:5P:15K). Height and diameter growth were measured weekly, and biomass and nutritional status were analyzed at the end of culture (24 week). Plant growth and nutritional response to increased fertilization followed a curvilinear pattern depicting phases that ranged from deficiency to luxury consumption. Seedling dry mass production was maximized at 125 mg N seedling−1 (sufficiency level). N content and concentration increased with fertilization, reaching a maximum at 200 mg N seedling−1 (luxury consumption). P and K concentrations peaked at 75 and 25 mg N, respectively, suggesting a dilution effect of these nutrients. Root volume increased linearly up to 100 mg N and declined thereafter. The sufficiency level for Q. ilex (125 mg of applied N seedling−1) is notably higher than for other Quercus species from other biomes but intermediate to other Mediterranean Quercus species. No toxicity was observed at the highest treatment rate (200 mg N) suggesting that increased exponential N rates along with greater P and K proportions than those used in our experiment may further maximize nutrient storage.