Availability and fractionation of phosphorus in sewage sludge-amended soils

A laboratory incubation study was conducted with the objective of determining the effects of time, temperature, and soil properties on availability and chemical fractions of phosphorus (P) in sewage sludge-amended soils. Anaerobically digested sewage sludge was mixed with three soils (Crofton silty clay loam, Moody silty clay loam, and Thurman sandy loam) at a rate equivalent to 80 Mg sludge ha^-1. The mixtures were incubated at 25 and 37°C for up to 120 days. Phosphorus in sewage sludge-treated soils was extracted with iron-oxide impregnated filter paper strips (strip-P). Phosphorus also was fractionated chemically into four components by sequential extraction. Phosphorus concentration in all fractions increased with sludge application. Strip-P concentration was higher at 25°C in all soils than at 37°C over the entire period of incubation. Soluble-P was greatest in sandy Thurman soil at both temperatures over all incubation times. A sharp increase in non-occluded P (NOC-P) concentration occurred in all soils with time at both temperatures. Occluded P (OC-P) in all soils decreased more rapidly with time at 37°C than at 25°C. Calcium-P (Ca-P) concentration was unaffected by time and was highest in calcareous Crofton soil. Soil texture and the presence of carbonates strongly influence the fate of P from applied sewage sludge. It was concluded, based on time trends that sludge as a P source on a P-limited soil should be applied well before the period of maximum plant demand. Elevated temperature (37°C) typical of mid-summer, promotes depletion of more available (strip-P and soluble P) fractions compared with lower temperature (25°C).