Phosphorus availability in chicken manure is lower with increased stockpiling period, despite a larger orthophosphate content

Background and aims: The relative proportions of phosphorus (P) forms present in manure will determine the overall availability of manure P to plants; however, the link between the forms of P in manures and manure P availability is unclear. This study compares the bioavailability and P speciation of three manures of different stockpiling duration: less than 1 month, 6 months and 12 months; manures were collected concurrently from a single poultry farm. Methods: Bioavailability to wheat in a glasshouse trial was measured using an isotopic dilution method with manure added at an application rate equivalent to 20 kg P ha-1. Phosphorus speciation was measured by 31P nuclear magnetic resonance (NMR) spectroscopic analysis of NaOH-EDTA extracts of the manures. Results: The addition of all manures significantly increased shoot biomass and P concentration, with the fresh manure having the greatest effect. Addition of the fresh manure resulted in the largest labile P pool, highest manure P uptake and manure P recovery, while the manure stockpiled for 12 months resulted in the lowest manure P uptake and manure P recovery. NMR analysis indicated that there was more monoester organic P, especially phytate, in manure stockpiled for shorter periods, while the proportion of manure P that was orthophosphate increased with stockpiling time. Conclusions: Together, these results imply that although the proportion of total P in the manures detected as orthophosphate was higher with longer stockpiling, only a fraction of this orthophosphate was plant-available. This suggests the availability of P from orthophosphate in manures decreases with longer stockpiling time in much the same way that P from orthophosphate in mineral fertilizer becomes less available in soil over time. © 2013 Springer Science+Business Media Dordrecht.C. A. E. Peirce, R. J. Smernik, T. M. McBeat