The chemical composition of phosphorus in a diversity of animal manures using solution 31P NMR and P K-edge XANES spectroscopy

Phosphorus (P) is an essential nutrient for crops. Mineral fertilisers have typically been applied to ensure an adequate supply of P to crops, but the cost of mineral fertilisers is increasing due to finite rock phosphate reserves. Manures are a renewable source of P that could partially offset the use of mineral fertilisers, but their agronomic value and behaviour in soil remain less clear due to limited information on their chemical composition. The aim of this study was to determine the chemical composition of P in a diversity of manures (chicken, pig, dairy, beef, goat, horse, sheep, and alpaca) using solution 31 P nuclear magnetic resonance (NMR) and synchrotron-based P K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Total P concentrations varied considerably among manures (4.4–38.8 g P kg −1 ). Inorganic P (orthophosphate) was predominant across all manures (64–97 % of total P), whereas organic P constituted a minor fraction (3–36 %), largely as phosphomonoesters. Drying increased the proportion of inorganic P while reducing organic P, with a shift towards less soluble forms. Composted and stockpiled manures also showed lower P solubility. Synchrotron-based XANES spectroscopy revealed that calcium-bound forms of P, mainly amorphous calcium phosphate and dicalcium phosphate, were predominant in most manures, whereas struvite was also important in dairy manures. These findings provide new insights on the chemical P composition of manures, which will help to understand their fate in soil, develop strategies to improve crop P uptake from manures and offset the use of mineral fertilisers.