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Effect of nitrogen, phosphorus and potassium fertilization on the growth and development of pedunculate oak seedlings (Quercus robur L.)

Pedunculate oak (Quercus robur L.) is the most valuable commercial forest tree species in Croatia and the second best represented species in the timber fond of the Republic of Croatia (16%). Unfortunately, forest ecosystems of pedunculate oak are no longer as stable as they used to be and are currently growing in very complex stand and ecological conditions. According to the forest damage data for Croatia, pedunculate oak is the most severely damaged broadleaved species in the Republic of Croatia, with the percentage of severely damaged trees (crown defoliation > 25%) amounting to 30.6%. Seriously disturbed natural balance in oak stands, problems occurring during oak stand regeneration and frequent absence of regular fructification highlight the issue of nursery production. There is increasing demand for the production of good quality planting material, where the nutrition status is one of the main indicators of its quality. This approach to research will respond to the requirements of pedunculate oak seedlings for certain nutrients, at the same time satisfying the quality of planting material. The experiment was established according to the randomised block method with 32 treatments and 4 repetitions. Potassium (potassium-sulphate) and phosphorus fertilizers (triplex) were applied to the soil during basic fertilization, while nitrogen (Urea) was added as supplement nutrition. The first nitrogen factor had four different dosages, the second phosphorus factor two, and the third potassium factor also four dosages. The article presents nitrogen, phosphorus, potassium, calcium and magnesium concentrations in the leaves, the results of chemical soil analyses, the results of seedling measurements and concentrations of photosynthetic pigments. Fertilization with nitrogen, phosphorus and potassium did not affect their level in the soil. Applied fertilizers affected the chemical content of leaves, in the first place the increased concentrations of applied nutrients (N, P, K) in the leaves of pedunculate oak seedlings. Significantly higher concentrations of nitrogen and phosphorus were found in the leaves of pedunculate oak seedlings treated with 100 and 150 kg nitrogen/ha in relation to the control and treatments with 50 kg of nitrogen/ha. Statistically significant differences were found in the concentrations of potassium in the leaf of pedunculate oak seedlings between the control and fertilization with different potassium dosages. No differences were found between the control and phosphorus treatment in the concentration of phosphorus in the leaf. Fertilization with 100 and 150 kg potassium/ha resulted in a decreased concentration of calcium and magnesium in the leaf, but the decrease is still not critical. Significant differences in the heights, height increments, stem biomass and leaf biomass between nitrogen treatments point to strong correlation between nitrogen fertilization and the variables mentioned above. An increase in nitrogen dosage led to an increase in the height, height increment, stem biomass and leaf biomass. The applied nitrogen, phosphorus and potassium fertilization did not affect the root collar diameter, the increment of the root collar diameter, the mass of 100 leaves and the concentration of photosynthetic pigments. According to the conducted research, fertilization with 100 kg/ha of nitrogen and phosphorus and 150 kg/ha of potassium is recommended to produce pedunculate oak seedlings of satisfactory quality.

Pedunculate oak; fertilisation; nutriant status; growth

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