Seasonal Related Internal P and N Nutrient (Re)Cycling Strategies of European Beech Saplings are Element Specific

Excessive N deposition to natural ecosystems such as temperate beech forests leads to the conversion of previously N limited forest ecosystems to N saturated systems. The input of N can result in decreased foliar P contents and hence in N induced P deficiency. Furthermore, upon N deposition growth increased and enhanced the demand of P. This might be realized by an impact on internal P (re)cycling.Hence, the present study tested whether N addition induces phosphorus deficiency in beech saplings that resists by enhanced internal P (re)cycling as well as the hypothesis that addition of P, mitigates the N induced P deficiency. Furthermore, the hypothesis that higher light intensity reduces P contents in tissues/organs of highly shade tolerant beech saplings because of enhanced growth realized by enhanced internal P (re)cycling was tested.Therefore, a mesocosm experiment with beech saplings and soil (O and A horizon) originating from a P-poor forest was conducted. Mesocosms were cultivated for two growing seasons at free-air conditions. N, P, or N plus P was added directly after mesocosm establishment. Another group of saplings was exposed to higher light intensity to enhance CO 2 assimilation and growth. Soil and beech saplings were harvested during the second growing year in spring, summer, autumn and winter.As the main result internal nutrient (re)cycling of N and P was found to be 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 perennial tissues 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