Carbon dynamics and potential nutrient limitations in an oak forest under experimental canopy nitrogen deposition

Increasing global air pollution necessitates better understanding of Nitrogen (N) deposition impacts on forest ecosystems. Shifts in N availability may influence forest health and productivity while reshaping the rest of its elemental profile. Unlike conventional ground fertilization studies, the current research presents a canopy N application approach, using a conservative N rate for a more realistic simulation of atmospheric deposition scenarios. The experiment, conducted since 2015 in a pure mature Sessile oak (Quercus petraea L.) forest, compares both above and below canopy N application methods against unfertilized control plots. After eight years of continuous N inputs with minimal evidence of growth effects, we explored possible productivity limitations in the forest stand by assessing nutrient dynamics through annual analyses of green and abscised foliage, and soil. Indications of inherent nutrient imbalances were detected as suboptimal leaf N, Phosphorus (P), and Potassium concentrations, with a significantly elevated N:P ratio by one of the N treatments. Soil analysis revealed decreased C:N and inorganic P concentrations under both N treatments compared to the control, with enzymatic stoichiometry suggesting overall P limitation. Additionally, Aluminum levels in the leaves and Ca:Al ratios in the soil indicated potential stress. Our results suggest unfavorable conditions, which may prevent a prosperous environment for the monitored oaks. With these findings we emphasize the importance of comprehensive nutrient status assessments in forest research and management, particularly when evaluating potential future impacts of atmospheric pollutants. Furthermore, we highlight the relevance of distinguishing between the different N application methods for accurate ecological interpretations.