Nitrogen Fertilization: Field Performance of an Amino-Acid-Based Fertilizer in Sessile Oak Reforestation

Early seedling survival is a key determinant of reforestation success under increasingly variable climatic conditions. Fertilizers used to mitigate nutrient limitations are believed to mitigate early establishment stress, but their effectiveness under heterogeneous field conditions remains uncertain. This study specifically tests whether an amino-acid-based nitrogen fertilizer can provide a more efficient and ecologically sustainable Nitrogen source compared with conventional mineral formulations. Using a dataset of 6238 seedlings from seven operational Austrian reforestation sites, we quantify amendment performance and examine interactions with relief, soil depth, water availability, and management practices. We apply CatBoost to identify influential predictors of mortality and summarize results across repeated evaluations. Further, for the reported settings, we can reliably predict tree seedling mortality for three out of four seedlings, with an average model accuracy of 76.4% and an AUC of 0.82 across sites. The arginine-based fertilizer increased survival probabilities by up to 15% on moist, deep soils but showed no consistent benefit under shallow or drought-prone conditions. The results highlight the potential of amino-acid-based N supply as a more ecologically aligned alternative and support operational decisions on when and where fertilizers may improve oak establishment under changing climatic conditions.