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 (δ¹⁸O_PO4 value) may help elucidate some phosphorus sources and cycling. Accurately determined and isotopically distinct source values are essential for this process. The δ¹⁸O_PO4 values 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 δ¹⁸O_PO4 value of the 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 being mixed with a helium carrier gas passed through a gas chromatography (GC) column into a mass spectrometer. Faecal water oxygen isotope ratios (δ¹⁸O_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 PO₄ was not derived directly from the gut fauna lysed during the extraction of PO₄ from the faeces. Assuming that the faecal δ¹⁸O_H2O values represented cattle body water, the predicted pyrophosphatase equilibrium δ¹⁸O_PO4 (Eδ¹⁸O_PO4) values ranged between +17.9 and +19.9‰, while using groundwater δ¹⁸O_H2O values gave a range of +13.1 to +14.0‰. The faecal δ¹⁸O_PO4 values ranged between +13.2 and +15.3‰.

Conclusions

The fresh faecal δ¹⁸O_PO4 values were equivalent to those reported elsewhere for agricultural animal slurry. However, they were different from the Eδ¹⁸O_PO4 value calculated from the faecal δ¹⁸O_H2O 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.