Measurements of ¹⁸ O-P_i uptake indicate fast metabolism of phosphate in tree roots

Phosphorus (P) nutrition of beech ecosystems depends on soil processes, plant internal P cycling and P acquisition. P uptake of trees in the field is currently not validated due to the lack of an experimental approach applicable in natural forests. Application of radiolabelled tracers such as ³³ P and ³² P is limited to special research sites and not allowed in natural environments. Moreover, only one stable isotope of P, namely ³¹ P, exists. One alternative tool to measure P acquisition in the field could be the use of ¹⁸ O-labelled ³¹ P-phosphate (³¹ P¹⁸ O₄ ^3- ). Phosphate (P_i ) uptake rates calculated from the ¹⁸ O enrichment of dried root material after application of ³¹ P_i ¹⁸ O₄ ^3- via nutrient solution was always lower compared to ³³ P incorporation, did not show increasing rates of P_i uptake at P deficiency under controlled conditions, and did not reveal seasonal fluctuations in the field. Consequently, a clear correlation between ³³ P-based and ¹⁸ O-based P_i uptake by roots could not be established. Comparison of P_i  uptake rates achieved from ³³ P-P_i and ¹⁸ O-P_i application led to the conclusion of high P_i metabolism in roots after P_i uptake. The replacement of ¹⁸ O by ¹⁶ O from water in ¹⁸ O-P_i during root influx, but most probably after P_i uptake into roots, due to metabolic activities, indicates high and fast turnover of P_i . Hence, the use of ¹⁸ O-P_i as an alternative tool to estimate P_i acquisition of trees in the field must consider the increase of ¹⁸ O abundance in root water that was disregarded in dried root material.