Phosphorus distribution in dairy manures

The chemical composition of manure P is a key factor determining its potential bioavailability and susceptibility to runoff. The distribution of P forms in 13 dairy manures was investigated with sequential fractionation coupled with orthophosphate-releasing enzymatic hydrolysis. Among the 13 dairy manures, manure total P varied between 4100 and 18 300 mg kg(-1) dry matter (DM). Water-extractable P was the largest fraction, with inorganic phosphorus (P-i) accounting for 12 to 44% of manure total P (1400-6800 mg kg(-1)) and organic phosphorus (P-o) for 2 to 23% (130-1660 mg kg(-1)), respectively. In the NaHCO3 fraction, P-i varied between 740 and 4200 mg P kg(-1) DM (4-44% of total manure P), and P-o varied between 340 and 1550 mg P kg(-1) DM (2-27% of total manure P). In the NaOH fraction, P-i fluctuated around 200 mg P kg(-1) DM, and P-o ranged from 130 to 630 mg P kg(-1) DM. Of the enzymatically hydrolyzable P-o in the three fractions, phytate-like P dominated, measuring 26 to 605 mg kg-1 DM, whereas monoester P and DNA-like P were relatively low and less variable. Although concentrations of various P forms varied considerably, significant correlations between manure total P and certain P forms were observed. For example, H2O-extracted P-i was correlated with total manure P (R-2 = 0.62), and so was NaOH-extracted P-o (R-2 = 0.81). Data also show that the amount of P released by a single extraction with sodium acetate (100 mM, pH 5.0) was equivalent to the sum of P in all three fractions (H2O-, NaHCO3-, and NaOH. extractable P). Thus, a single extraction by sodium acetate buffer could provide an efficient evaluation of plant-available P in animal manure, while the sequential fractionation approach provides more detailed characterization of manure P.