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 poplar–cottonwood trees ( Populus × euramericana — 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, NaHCO 3 ‐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., HCl–P, NaOH–OP, and residual P) decreased rather than increased. Because NaHCO 3 –IP and H 2 O–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.