Studies on the effect of high atmospheric N deposition report inconsistent results on forest productivity and N cycling which might be related to P availability in soil and subsequently affect tree P nutrition. We wanted to test the effects of (i) site i.e., a P-poor versus a P-rich site and of (ii) fertilization (N, P, N+P) on inorganic P (Pi) and organic P (Po) concentrations as well as on biologically cycled phosphate (inferred from the O isotope signature after adding an 18 O-enriched label) in xylem sap. We measured Pi and Po concentrations and the O isotope signature in phosphate (δ 18 O Pi) in xylem sap of beech (Fagus sylvatica L.) trees two and 14 days after addition of 18 O-enriched water to the organic layer in a full factorial fertilization experiment (control, +N, +P, +NP) at two sites differing in P availability. Higher P concentrations in xylem sap at the P-rich than at the P-poor site originated from accelerated biological P cycling indicated by incorporation of 18 O from the isotope label into phosphate in xylem sap shortly after labeling. At this site, δ 18 O W values of xylem sap after label application remained close to background δ 18 O W values of soil solution. We speculate that in contrast to P uptake, trees took up water from deeper (non-18 O-labeled) soil layers. At the P-poor site, the 18 O label was recovered both in xylem sap water and phosphate in xylem sap, the latter only after 14 days. These results imply that trees relied on the organic layer for P acquisition and water uptake. However, biological processes associated with an incorporation of 18 O from the label were slower at the P-poor than at the P-rich site. P addition (P, NP) increased Pi concentrations in xylem sap at the P-rich site. Based on δ 18 O Pi values in xylem sap, the additional P originated both from the fertilizer and from accelerated biological P cycling in soil. We conclude that P-poor sites likely suffer more from climate change in case of an increased frequency of droughts because as opposed to P-rich sites both water and nutrient uptake will be affected.