Bioaccessibility and turnover of phosphorous in forest subsoils

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This project has been designed to test the hypothesis that subsoils increasingly contribute to the P acquisition of trees with increasing P deficiency in the surface soil. Tracing the oxygen isotope composition of HCl extractable soil phosphates showed that their delta18O values were in equilibrium with biological cycled P in topsoils, but not in deeper subsoils. In particular, our data pointed to an increased biological transformation of bedrock P in very deep subsoils (several m) of P limited sites, which was not the case in soils where P was not limited. Intriguingly, our data also suggested that the utilization of subsoil P might be largely restricted by the supply of additional C and N sources rather than by specific P bondings in stable pools. In the second phase of this priority program, we thus aim at substantiating these findings by i) relating changes in phosphate oxygen isotope signatures down to 29 m soil depth to changes in P bonding forms, ii) testing the hypothesis that microbial turnover of (deep) subsoil P is enhanced by additional supply with C and N, iii) assess the role of subsoil minerals on microbial and abiotic phosphate 18O exchange rates, and build on our expertise from the first phase to use iv) the oxygen isotope composition of phosphate in xylem sap as an indicator for P uptake from the subsoil in P limited sites. To achieve these aims, we perform onsite soil and xylem sap monitoring as well as ex-situ incubation experiments from different profile depths with 18O-labelled water, and we will continue to monitor oxygen isotope ratios in soil phosphates by Isotope Ratio Mass Spectrometry, Secondary ion mass spectrometry at nanoscale, and Raman Spectroscopy using near infrared and deep ultraviolet laser excitation.