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 O value) may help elucidate some phosphorus sources and cycling. Accurately determined and isotopically distinct source values are essential for this process. The δ O values of animal wastes have, up to now, received little attention.

Methods: Phosphate (PO ) was extracted from cattle faeces using anion resins and the contribution of microbial PO was assessed. The δ O value of the extracted PO 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 O 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 was not derived directly from the gut fauna lysed during the extraction of PO from the faeces. Assuming that the faecal δ O values represented cattle body water, the predicted pyrophosphatase equilibrium δ O (Eδ O ) values ranged between +17.9 and +19.9‰, while using groundwater δ O values gave a range of +13.1 to +14.0‰. The faecal δ O values ranged between +13.2 and +15.3‰.

Conclusions: The fresh faecal δ O values were equivalent to those reported elsewhere for agricultural animal slurry. However, they were different from the Eδ O value calculated from the faecal δ O value. Our results indicate that slurry PO is, in the main, derived from animal faeces although an explanation for the observed value range could not be determined.