Wetland soils are known for the largest carbon (C) stock in terrestrial ecosystems, conversely, potential C source through greenhouse gases emission; therefore, understanding C stock and processes of wetland soils is very important. High spatial variability of soil properties in a wetland along hydrological gradient may affect wetland soil C stock and characteristics. We investigated soil C stocks with vertical profiles and examined soil C processes in a temperate forested wetland dominated by Alnus japonica, in central Korea. One upland site (US) and three wetland sites, named drained site (DS), poorly drained site (PDS) and surface saturated site (SSS) were selected based on the hydrological gradient. In total, 20 soil cores with 1-m depth were taken and sampled at 10-cm intervals. Then, C concentration and stock, d¹³C, proportion of density fraction, extracellular enzymes activity, bulk density, and soil water content of each sample were determined. Soil C stock (Mg C ha^-1) was highest in DS (222.6) followed by PDS (200.1), SSS (194.3), and US (137.3). The largest soil C stock was observed at medium soil water content (about 1.5 g/g) due to the combined effects of C concentration increase and bulk density decrease with soil water content. Moreover, d¹³C enrichment with increasing soil depth was lower in DS (1.98 ‰) than in US (4.64 ‰), PDS (4.76 ‰), and SSS (4.47 ‰), and lower C turnover rate of DS which was expected by d¹³C enrichment may result in the largest C stock of DS. In addition, b-1,4-glucosidase and b-1,4-xylosidase activities (0–40 cm) were suppressed by 15–50% in wetland sites compared to US whereas cellobiohydrolase activity did not change. These results corresponded to the other soil C processes studies such as soil respiration and litter decomposition, and would be valuable for estimating C stocks and processes in wetland soils.