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 (δ18OPO4) may help elucidate some phosphorus sources and cycling. Accurately determined and isotopically distinct source values are essential for this process. The δ18OPO4 of animal wastes have, up to now, received little attention. METHODS: Phosphate (PO4) was extracted from cattle faeces using anion resins and the contribution of microbial PO4 was assessed. The δ18OPO4 of extracted PO4 was measured by precipitating silver phosphate and subsequent analysis on a thermal conversion elemental analyser at 1400°C, with the resultant carbon monoxide mixed with a helium carrier gas passed through a GC column into a mass spectrometer. Faecal water oxygen isotope ratios (δ18OH2O) 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 extracted PO4 was not derived directly from the gut fauna lysed during the extraction of PO4 from the faeces. Assuming faecal δ18OH2O values represented cattle body water the predicted pyrophosphatase equilibrium δ18OPO4 (Eδ18OPO4) values ranged between +17.9 and +19.9‰, while using groundwater δ18OH2O gave a range of +13.1 to +14.0‰. Faecal δ18OPO4 values ranged between +13.0 and +15.4‰. CONCLUSIONS: Fresh faecal δ18OPO4 values were equivalent to those reported elsewhere for agricultural animal slurry. However, they were different to the Eδ18OPO4 value calculated from the faecal δ18OH2O value. Our results indicate that slurry PO4 is, in the main, derived from animal faeces although an explanation for the observed value range could not be determined.