To understand the initial changes in the microbial activities of wetland soil after construction, dehydrogenase activity (DHA) and denitrification potential (DNP) of soil from 1 natural wetland and 2 newly constructed wetlands were monitored. Soil samples were collected from the Daepyung marsh as a natural wetland, a treatment wetland in the West Nakdong River, and an experimental wetland in the Pukyong National University, Busan. The results showed that the DHA of the natural wetland soil was 6.1 times higher than that of the experimental wetland and similar to that of the treatment wetland 6 months after wetland construction (fall). Few differences were observed in the DNP between the soil samples from the natural wetland and 2 constructed wetlands four months after wetland construction (summer). However, 6 months after the construction (fall), the DNP of the soil samples from the natural wetland was 12.9 times and 1.8 times higher than that of the experimental wetland and the treatment wetland, respectively. These results suggested that the presence of organic matter as a carbon source in the wetland soil affects the DHA of wetland soil. Seasonal variation of wetland environment, acclimation time under anaerobic or anoxic wetland conditions, and the presence of carbon source also affect the DNP of the wetland soil. The results imply that the newly constructed wetland requires some period of time for having the better contaminant removal performance through biogeochemical processes. Therefore, those microbial activities and related indicators could be considered for wetland management such as operation and performance monitoring of wetlands.