Bioaccessibility and residence time of phosphorous in forest subsoils

Plants may take up between 10 and 80 % of their P demand from subsoils, but the P forms, their accessibility and particularly the residence time of oxygen in phosphates of forest subsoils are currently hardly understood. This project aims at finally testing the hypothesis that at increased P deficiency in the surface soil there is an increased utilization of subsoil P. As a basis for testing this hypothesis, we aim at (i) quantifying the P stocks, binding forms, and delta¹⁸O signature of phosphates in different subsoil compartments of the central field sites, (ii) tracking the effect of plant roots on P properties in the subsoil, (iii) elucidating the accessibility of subsoil P for delta¹⁸O exchange reactions; (iv) contributing to the understanding of the residence form of subsoil phosphates on the basis of position-specific exchange kinetics of stable oxygen isotopes in labelled inorganic P residues, and (v) relating this information to the P uptake by subsoil root systems. The P forms and concentrations are characterized by sequential P extraction procedures, XANES, high-resolution NanoSims element mapping, and liquid-state ³¹P-NMR spectroscopy. Isotope analyses and exchange studies are performed using a combination of Raman and isotope-ratio mass spectrometry. Quantum chemical calculations will help to interpret the spectroscopic measurements.