Bioaccessibility of phosphorus in the subsoil (SubsoilP)

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Plants may take up between 10 and 80 % of their P demand from subsoils, but the forms and accessibility of subsoil P as affected by crop sequence management are currently hardly understood. This project aims at (i) quantifying the P stocks and binding forms in different subsoil compartments of the central field trial (CeFiT) with varying biopore systems, (ii) elucidating the contribution of earthworm burrows to P uptake from subsoil horizons, and (iii) testing the feasibility of the δ18O signature of soil and plant phosphates as a novel tool for tracing back their origin and residence time. Samples are taken from rhizosphere, bulk soil, earthworm burrow linings and former root channels both from the central field trial (CeFiT) and central microcosm experiment (CeMiX). The P forms and concentrations are characterized by sequential P extraction procedures, high-resolution element mapping, and 31P-NMR spectroscopy. Isotope uptake and exchange studies using 33P and 18O labelled phosphate are used to evaluate P uptake rates from biopores and phosphate residence time in soil.