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 {NaHCO(3)} 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, {H(2)O-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 {(H(2)O-,} {NaHCO(3)-,} 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.