Improved photosynthesis and growth of poplar during nitrogen fertilization is accompanied by phosphorus depletion that indicates phosphorus remobilization from older stem tissues

Abstract Natural ecosystems are exposed to increasing atmospheric nitrogen (N) deposition since many decades but the input of other nutrients including phosphorus (P) is negligible. Consequences could be nutritional imbalances that may impair growth of the individual tree. High N input into forests, however, has been observed to stimulate tree growth, but was also accompanied by increasing foliar N/P ratios that imply the beginning of P deficiency. At the long-term perspective, N induced P deficiency will affect the P nutrition of the whole tree, especially P reserves of the bark and wood with the consequence of reduced tree growth. To test this assumption, young poplar plants, which were established in pots with ˜1 L sand at low N and medium P supply, were subjected to high N load via fertilization. Improved N supply resulted in depleted P pools of older stem sections where mature and senescent leaves were present. Simultaneously, mature leaves showed enhanced rate of CO 2 fixation that enables enhanced growth. Obviously, poplars grown at low N were N but not P limited despite N:P ratios indicating P deficiency for mature leaves. To show limitation of P remobilization, poplars which were established at high N and medium P supply were subsequently withhold from continuous phosphate (P i ) fertilization. These poplars showed decreased growth increment and diminished photosynthesis of mature leaves. At the whole plant level, this was accompanied by restricted P remobilization from older stem sections. Nevertheless, P contents of fine roots remained mostly unaffected. In conclusion, if P reserves are present, P compounds can be mobilized from stem bark and wood and can compensate for an enhanced P demand at high N load. However, this compensation is limited upon prolonged excess N load and might lead to P limitation that restricts growth in the long run.