Excessive N deposition leads to the conversion of previously N limited to N saturated forest ecosystems. The input of N can result in severe N/P imbalance and N-induced P deficiency. The present study investigates whether N addition induces P deficiency in beech saplings or if this can be counteracted by enhanced internal P (re)cycling. Furthermore, it was tested whether addition of P can mitigate N induced P deficiency. In addition, higher need of P due to enhanced growth may be realized either by enhanced P uptake or by improved internal P (re)cycling. This assumption was tested with beech saplings growing at higher light intensity.

Therefore, a mesocosm approach with beech saplings and soil (O and A horizon) originating from a P-poor forest was conducted. Mesocosms were cultivated for two growing seasons in a garden. Addition of N, P, or combined addition of N and P took place during the first year and another group of saplings was exposed to higher light intensity to enhance CO₂ assimilation and growth. Soil horizons and beech saplings were harvested during the second year of growth in spring, summer, autumn, and winter.

The results show that internal (re)cycling of N and P is different and mostly determined by the season. The seasonal (re)cycling of P was driven by the metabolic P demand of tissues/organs characterized by shifting P between the perennial tissues bark and wood of branches and coarse roots and deciduous leaves, fine roots and long-distance transport paths without using major storage resources. In contrast, seasonal (re)cycling of N is characterized by N storage in perennial tissues during dormancy and by N mobilization from the entire trunk, i.e. branch, stem and coarse roots, in spring. Furthermore, these seasonal dynamics were found to be independent of the treatment.