Fly ash and biosolid wastes can be mixed and applied to soil as a means of disposal. A significant decline in soil respiration following waste application indicates restricted activities of functional microbial populations. Weathering decreases salinity and neutralizes alkalinity in fly ash, but there is little information on the effects of unweathered fly ash and biosolid mixtures on soil carbon (C) mineralization. The objective of this study was to determine the effects of a weathered fly ash–limestone scrubber residue (LSR) mixed with an aerobically digested biosolid on soil respiration in a laboratory incubation study. Biosolids significantly increased carbon dioxide (CO2) production (p < 0.05), but up to 6.75% (w/w) fly ash did not. Mean total C mineralization was 770 mg CO2‐C kg−1 soil in the control and 3,810 mg CO2‐C kg−1 soil in the 6.75% (w/w) biosolid treatment. Fly ash with neutral pH and low salinity appears unlikely to affect soil and biosolid C mineralization.