SOLUBLE PHOSPHORUS IN A FOREST SOIL AP HORIZON AMENDED WITH MUNICIPAL WASTE-WATER SLUDGE OR COMPOST

Soluble P in forest soils amended with municipal wastewater sludges may exist as orthophosphate or organic P, and microbial or chemical reactions controlling the distribution between the two forms will affect its availability to plants and concentration in soil solution at the C-rich, forest floor-mineral horizon boundary. A laboratory leaching study identified time-dependent changes in soluble orthophosphate and organic P (operationally-defined as labile and nonlabile soluble P; LSP and NLSP, respectively) in samples of Chester soil A horizon (fine-loamy, mixed, mesic, Typic Hapludult) amended with municipal wastewater sludge or sludge compost. The soil was sampled from a hardwood forest typical of the Piedmont physiographic region of Maryland. The sludge materials were added to the soil with or without leaf litter that was also sampled from the site. Concentrations of total soluble P (TSP) in leachates decreased over 51 d of the incubation, and the percentage of TSP as LSP increased to nearly 100%, while NLSP was preferentially adsorbed by the soil or immobilized by microbial uptake. The only increases measured during the incubation were in LSP in the compost treatments, and LSP and NLSP decreased with time in all other treatments. Leaf litter additions decreased quantities of P released or enhanced P removal in all treatments over time. The results suggested that compost may be a better source of plant-available P than municipal wastewater sludge, even though compost principally contains insoluble, organic P and sludge contains higher levels of soluble orthophosphate.