Biosolids and water treatment residuals (WTR) land co-application has not been extensively studied, but can be beneficial by sorbing excess biosolids-borne or soil P onto WTR, reducing the likelihood of off-site movement. We utilized 7.5 × 15 m test plots with treatments consisting of three different WTR rates with a single biosolids rate (5, 10, and 21 Mg WTR/ha; 10 Mg biosolids/ha) surface co-applied once in 1991 or re-coapplied in 2002. Control plots consisted of those which received no WTR-biosolids co-applications. Previous soil enzymatic research showed that soil phosphodiesterase and acid phosphatase activity decreased and increased in WTR-amended plots as compared to controls (no co-application), respectively. Reductions in phosphodiesterase activity suggested lower microbial activity and thus less P mineralization from biomass sources. Increased acid phosphatase activity indicated that WTR application possibly triggered a P deficiency response, causing plants to secrete this enzyme. However, the dominant plant species, western wheatgrass (Pascopyrum smithii), contained greater P concentrations in co-amended versus control plots. To further understand differences in plant P chemistry and enzymatic activity, we characterized the microbial community structure using ester-linked fatty acid methyl ester (EL-FAME) analysis. Results will be discussed.