EVALUATION OF ORGANIC PHOSPHORUS IN ANIMAL MANURE BY ORTHO-PHOSPHATE RELEASING ENZYMATIC HYDROLYSIS

The chemical composition of phosphorus in manure significantly impacts its transport and potential bioavailability. As much as 50% of the phosphorus in animal manure is present in organic forms (Po). Therefore, characterization of Po and its bioavailability may contribute to effective manure P management. Phosphatases catalyze reactions that release orthophosphate (Pi) from various types of organic phosphorus compounds. The possibility of using those enzymes for classifying manure hydrolysable Po and for evaluating Po bioavailability was explored. Swine manure and cattle manure were first sequentially fractionated into water-soluble, NaHCO3-soluble, and NaOH-soluble P. Fractions were then incubated with phytase, acid phosphatase, alkaline phosphatase, nuclease P1, nucleotide pyrophosphatase, or their combinations. The released Pi was determined by a molybdate blue method. The difference in Pi determined after incubation in the presence and absence of an enzyme reflected the corresponding type and amount of Po in the sample. Based on this research, we selectively applied acid phosphatase from potato, acid phosphatases from potato and wheat germ, and both phosphatases plus nuclease P1 to identify and quantify simple monoester P, phytate (inositol hexaphosphate)-like P, and DNA-like P, respectively. With this approach, we examined the distribution of the three types of Po in 11 dairy manures. Phytate-like P was present as the major hydrolysable Po in all three fractions. Its concentration in the H2O fraction was positively related to total manure P. However, the concentration in the NaHCO3 and NaOH fractions was more correlated, respectively, to the NaHCO3 – or NaOH–extracted Po. We also investigated the change in manure P distribution after 1 yr storage at 4oC (swine manure) and 20oC (cattle manure) using enzymatic hydrolysis. Our data indicate that the amounts of bioavailable P (Pi and enzyme hydrolysable Po) in these manures remained relatively constant during storage. Soluble but enzymatically unhydrolysable Po, however, increased significantly. Our study demonstrates that enzymatic hydrolysis is an effective approach to identify and quantify hydrolysable Po in animal manure.