Water treatment residuals (WTRs) and biosolids are byproducts from municipal water treatment processes. Both byproducts have been studied separately for land application benefits. There are possible environmental benefits of WTRs and biosolids co-application but these studies are limited. Our objectives were to determine relative long-term (13–15 yr) effects of a single and short-term (2–4 yr) effects of repeated WTR-biosolids co-applications on soil chemistry, microbiology, and plant community structure in a Colorado semiarid grassland. Only relative changes associated between co-applications were studied, as we assumed WTR application would only occur if used as a management practice. Three WTR rates (5, 10, and 21 Mg ha–1) were surface co-applied (no incorporation) with a single biosolids rate (10 Mg ha–1) once in 1991 (long-term plots) and again in 2002 (short-term plots). Soil 0- to 8-, 8- to 15-, and 15- to 30-cm depth pH, electrical conductivity (EC), NO3–N, NH4–N, total C, and total N were not affected by WTR application in 2004, 2005, or 2006. Ammonium-bicarbonate diethylenetriaminepentaacetic acid (AB-DTPA)- extractable soil Al was unaffected by WTR application, but extractable P and Mo decreased with increasing WTR rate because of WTR adsorption. Plant tissue P and Mo content decreased with specific plant species and years due to adsorption to WTR; no deficiency symptoms were observed. Plant community composition and cover were largely unaffected by WTR application. Soil microbial community structure was unaffected by WTR co-application rate (total ester-linked fatty acid methyl ester [EL-FAME] concentrations ranged from 33.4 to 54.8 nmol g–1 soil), although time since biosolids-WTR application affected a subset of microbial community fatty acids including markers for Gram-positive and Gram-negative bacteria. Overall, WTR-biosolids co-applications did not adversely affect semiarid grassland ecosystem dynamics.