Phosphate stable oxygen isotope variability within a temperate agricultural soil

In this study, we conduct a spatial analysis of soil total phosphorus (TP), acid extractable phosphate (PO4) and the stable oxygen (O) isotope ratio within the PO4 molecule (δ18OPO4) from an intensively managed agricultural grassland site. Total P in the soil was found to range from 736 to 1952 mg P kg− 1, of which between 12 and 48% was extractable using a 1 M HCl (HClPO4) solution with the two variables exhibiting a strong positive correlation. The δ18OPO4 of the extracted PO4 ranged from 17.0 to 21.6‰ with a mean of 18.8‰ (± 0.8). While the spatial variability of Total P has been researched at various scales, this is the first study to assess the variability of soil δ18OPO4 at a field-scale resolution. We investigate whether or not δ18OPO4 variability has any significant relationship with: (i) itself with respect to spatial autocorrelation effects; and (ii) HClPO4, elevation and slope - both globally and locally. Results indicate that δ18OPO4 was not spatially autocorrelated; and that δ18OPO4 was only weakly related to HClPO4, elevation and slope, when considering the study field as a whole. Interestingly, the latter relationships appear to vary in strength locally. In particular, the δ18OPO4 to HClPO4 relationship may depend on the underlying soil class and/or on different field managements that had operated across an historical north-south field division of the study field, a division that had been removed four years prior to this study.