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 P 18 O 4 3− ). Phosphate (P i ) uptake rates calculated from the 18 O enrichment of dried root material after application of 31 P i 18 O 4 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.