The stable oxygen isotope ratio of resin extractable phosphate derived from fresh cattle faeces

RATIONALE: Phosphorus losses from agriculture pose an environmental threat to watercourses. A new approach using the stable oxygen isotope ratio of oxygen in phosphate (δ(18)O(PO4) value) may help elucidate some phosphorus sources and cycling. Accurately determined and isotopically distinct source values are essential for this process. The δ(18)O(PO4) values of animal wastes have, up to now, received little attention. METHODS: Phosphate (PO(4)) was extracted from cattle faeces using anion resins and the contribution of microbial PO(4) was assessed. The δ(18)O(PO4) value of the extracted PO(4) was measured by precipitating silver phosphate and subsequent analysis on a thermal conversion elemental analyser at 1400°C, with the resultant carbon monoxide being mixed with a helium carrier gas passed through a gas chromatography (GC) column into a mass spectrometer. Faecal water oxygen isotope ratios (δ(18)O(H2O) values) were determined on a dual‐inlet mass spectrometer through a process of headspace carbon dioxide equilibration with water samples. RESULTS: Microbiological results indicated that much of the extracted PO(4) was not derived directly from the gut fauna lysed during the extraction of PO(4) from the faeces. Assuming that the faecal δ(18)O(H2O) values represented cattle body water, the predicted pyrophosphatase equilibrium δ(18)O(PO4) (Eδ(18)O(PO4)) values ranged between +17.9 and +19.9‰, while using groundwater δ(18)O(H2O) values gave a range of +13.1 to +14.0‰. The faecal δ(18)O(PO4) values ranged between +13.2 and +15.3‰. CONCLUSIONS: The fresh faecal δ(18)O(PO4) values were equivalent to those reported elsewhere for agricultural animal slurry. However, they were different from the Eδ(18)O(PO4) value calculated from the faecal δ(18)O(H2O) value. Our results indicate that slurry PO(4) is, in the main, derived from animal faeces although an explanation for the observed value range could not be determined.