Phosphorus in fresh and dry dung of grazing dairy cattle, deer, and sheep: sequential fraction and phosphorus-31 nuclear magnetic resonance analyses.

Knowledge of phosphorus (P) fractions in dung of animals (dairy cattle, deer, sheep) grazing pasture is important for soil fertility and the potential for P transport in runoff and subsequent surface water quality deterioration. We used sequential fractionation and 31P nuclear magnetic resonance (NMR) spectroscopy to determine P forms in fresh and air-dried (to simulate field conditions during grazing) dung. Sheep dung was richest in P (8 g kg(-1)), and cattle dung poorest (5.5 g kg(-1)). Data for sequential fractionation indicated that most P was extractable by water (15-36%) and bicarbonate (36-45%) in fresh dung, and shifted toward recalcitrant, HCl (12-28%), and residual P forms (15-31%) with drying. Organic P concentration in dung was poor (maximum of 15% of total P), probably due to the poor concentration of phytate in pasture. The 31P NMR spectra of NaOH-EDTA extracts supported this by detecting a low concentration of monoesters (9-19% of total P in extracts), of which phytate is a major component. The 31P NMR data also showed that changes in organic P concentration with drying could be due to the degradation of diesters. Data indicate the decreasing bioavailability of dairy cattle, deer, and sheep dung with drying and the need to consider this effect with respect to P returns for soil fertility and the potential for runoff.