Water treatment residuals (WTR) and biosolids are by-products from municipal treatment processes. Both products have been studied separately for their benefits on land application as an alternative method of beneficial reuse. WTR land application concern is the amorphous metal oxides adsorbing tremendous P amounts, thus significantly reducing plant-available P via surface adsorption. A supplemental P source, such as biosolids, should negate the adsorptive effects. Co-application effects in terms of P fractionation have not been studied.


The objectives of the project were to understand both the long-term effects of a single co-application of biosolids and water treatment residuals (WTR) and the short-term impacts of a repeated co-application on soil inorganic and organic P dynamics. Test plots were 7.5 x 15 m with treatments consisting of three different WTR rates co-applied with a single biosolids rate (0, 10, and 21 Mg WTR ha-1 coapplied with 10 Mg biosolids ha-1) in either 1991 or 2002. Soil samples were collected in 2003 and 2004 from the 0-5-cm depth within each plot. Soils were analyzed for pH, EC, total C and N, inorganic C, soluble/loosely bound P, Al-P, Fe-P, occluded-P, Ca-P, labile organic P, microbial biomass P, humic and fulvic acid P, resistant organic P, oxalate extractable P, Al, Fe, and phosphorus saturation index, Mehlich-III, Bray-I, and Olsen's extractable P. Comparisons were made between all inorganic and organic P fractions to identify P cycling in the system over time.