Using δ¹⁸O(PO4) for historical source apportionment of inorganic phosphates in the eutrophic lake Baldegg, Switzerland

The identification of phosphorus (P) sources is critical for implementing effective eutrophication mitigation strategies. Lake Baldegg (Switzerland) has a history of excessive phosphorus inputs leading to severe eutrophication. Here, we utilise the oxygen isotopic composition of inorganic phosphate (δ¹⁸O(PO₄)) to discriminate soil-bound phosphate sources (orchard, arable, grasslands and forest; effluents from the local wastewater treatment plant and manure).
Previously, source apportionment using δ¹⁸O(PO₄) has been limited by the number of sources exceeding the number of tracers. In attempt to resolve this issue, additional tracers (C, N and geochemical elements) have been incorporated into the mixing models. As these tracers may originate from different sources and/or undergo different biogeochemical cycling than phosphate, their use for phosphate apportionment can potentially lead to erroneous results.
To overcome this issue, we analysed the δ¹⁸O(PO₄) values in multiple inorganic phosphate pools: NaOH-extractable (Fe/Al-bound), HCl-extractable (Ca/Mg-bound) and HNO₃-extractable residual inorganic P (modified Hedley sequence). The pools were purified using a zirconium-loaded resin, precipitated as Ag₃PO₄ and analysed for δ¹⁸O(PO₄) via high-temperature pyrolysis based isotope ratio mass spectrometry (TC/EA-IRMS).
Preliminary results show that δ¹⁸O(PO₄) values discriminate in each pool between land-uses: forest (NaOH: +10.2‰; HCl: +10.6‰), orchards (NaOH: +15.6‰; HCl: +14.7‰), arable fields (NaOH: +16.0‰; HCl: +14.9‰) and grassland soils (NaOH: +17.0‰; HCl: +16.8‰). As such, multiple pools can be potentially used as tracers for phosphate apportionment and remove the need for additional non-phosphate-specific tracers. While this study demonstrates the discrimination between different sources, analysis of the lake sediments is currently ongoing. We aim to reconstruct 130 years of inorganic phosphate sources and identify key moments the catchment’s history.