Evaluation of Soil Phosphorus Transformations by Sequential Fractionation and Phosphatase Hydrolysis

Increased understanding is needed on the interconversion among P species when manure and fertilizer are added to soils. To assess changes in P species affected by manure and fertilizer addition, a sandy loam (no established soil series designation; coarseloamy, mixed, frigid, Typic Haplorthod) and a Lamoine silt loam (fine, illitic, nonacid, frigid Aeric Epiaquept) with no P added, with fertilizer P, and with dairy manures were incubated at 22 °C for up to 108 days. Sequential fractionation with H2O, 0.5 M NaHCO3, and 0.1 M NaOH and phosphatase hydrolysis were used to monitor changes in inorganic P and enzymatically hydrolyzable and nonhydrolyzable organic P during the incubation period. Similar patterns of P dynamics were observed in the two soils. Added inorganic P from either chemical fertilizer or animal manure amendments was found mainly in the NaHCO3 and NaOH fractions. Changes in H2O, NaHCO3, and NaOH-extractable P were similar for P fertilizer–amended soils and unamended soils, indicating that soil properties played a major role in controlling P dynamics. In the H2O fraction, inorganic P attained a low and stable level after an initial rapid decrease. Concentrations of P species in other fractions fluctuated during the incubation period. Furthermore, the fluctuations were observed in complementary patterns between inorganic P in the NaHCO3 and NaOH fractions as well as between labile P (inorganic and hydrolyzable organic) and nonhydrolyzable organic P in the NaOH fraction. These complementary fluctuations implied an interchange of P species during incubation. This interchange could be an important mechanism for maintaining balance between labile and immobile P in soils. Further detailed examination of the patterns of interchange among P fractions in soils may provide more accurate prediction of soil P bioavailability, thus improving soil P management.