In this work, the effects of various dissolved organic matter (DOM) sources (piggery effluent [PigE], dairy effluent [DE], sewage effluent [SE], and stormwater [SW]) on the priming effect (PE) of soil C as affected by solid organic amendments (biochar [BC], biosolids [BS], compost, and poultry manure [PM]) and microbial activity were quantified using landfill, arable, and metal-contaminated field and spiked soils. The BC-amended soil caused significantly lower PEs than BS-, compost-, or PM-amended field soils due to its low DOM. A strong positive correlation was observed between the dissolved organic C content and glucose-induced PE of soil C. However, a negative correlation between the PE and dissolved N in different sources of DOM suggested that the PE may also be influenced by the quality of added C sources in the soils. The DE-treated soil with the highest dissolved N resulted in significantly lower PE than PigE-, SE-, and SW-treated soils. Compared with the uncontaminated soils, microbial activity as CO2 evolution and PE decreased markedly in the metal-contaminated soils, which may be attributed to the heavy metal toxicity. However, the distinct increase in microbial activity in the wastewater-treated contaminated soils suggests the capacity of wastewater to reduce metal toxicity in soils. The findings of this study suggest that although wastewater DOM may reduce the toxic effect to microorganisms, it can have an important effect on the source of CO2 by stimulating the decomposition of native soil organic matter.