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 P and P is limited to special research sites and not allowed in natural environments. Moreover, only one stable isotope of P, namely P, exists. One alternative tool to measure P acquisition in the field could be the use of O-labelled P-phosphate ( P O ). Phosphate (P ) uptake rates calculated from the O enrichment of dried root material after application of P O via nutrient solution was always lower compared to P incorporation, did not show increasing rates of P uptake at P deficiency under controlled conditions, and did not reveal seasonal fluctuations in the field. Consequently, a clear correlation between P-based and O-based P uptake by roots could not be established. Comparison of P uptake rates achieved from P-P and O-P application led to the conclusion of high P metabolism in roots after P uptake. The replacement of O by O from water in O-P during root influx, but most probably after P uptake into roots, due to metabolic activities, indicates high and fast turnover of P . Hence, the use of O-P as an alternative tool to estimate P acquisition of trees in the field must consider the increase of O abundance in root water that was disregarded in dried root material.