Fractionation of phosphorus in a mollisol amended with biosolids

Information about soil P fractions is useful to predict the bioavailability of P in soil as well as to predict the likelihood of its transport, In this study, we used a sequential fractionation procedure to investigate the forms of P in a Mollisol amended at the soil surface with biosolids (i.e,, anaerobically digested sewage sludge). Soil samples from three depths (0-5, 5-20, and 20-35 cm) were collected from a Cumulic Vertic Endoaquoll in a field experiment with three biosolid application rates, two vegetation treatments [hybrid poplarcottonwood trees (Populus x euramericnna - clone NC-5326) and switchgrass (Panicum virgatum L.)], and four replications per treatment. The Hedley fractionation scheme (dividing soil P into six empirical fractions [water-soluble, NaHCO3-soluble inorganic and organic P; NaOH-soluble inorganic and organic P; HCl-soluble P, and residual P)] was employed. After 6 yr of continuous application of biosolids to poplar plots, the absolute concentrations of all P fractions at the 0- to 5-cm depth increased significantly (P < 0.05), Some P fractions at the 5- to 20-cm depth increased significantly, whereas at the 20- to 35-cm depth, none of the fractions was affected by biosolids amendment. At the 0- to 5-cm depth of both poplar tree and switchgrass plots, the relative concentrations of some of the P fractions (e.g,, I-ICI-P, NaOH-OP, and residual P) decreased rather than increased. Because NaHCO3-IP and H2O-P increased in the biosolids-amended soil at rates disproportionate to their concentrations in the biosolids, we conclude that HCl-P applied with biosolids was transformed to more labile forms.