Dehydrogenase and catalase activities were measured in a Eutric Histosol irrigated with municipal wastewaters (Lublin, Poland) purified by a two-step (mechanical and biological) treatment. Soil enzyme activity was used to test the biochemical status of the soil-plant system, the last step of wastewater purification. Three experimental fields, each of I ha area, were planted with Populus nigra, Salix americana, and grasses (with Alopecurus pratensis, Phalaris arundinacea, Festuca pratensis as dominating species). The fields were divided into three parts: not flooded control (A), flood-irrigated 10 times per year with 60-75 mm (B), and flood-irrigated 10 times per year with 120-150 mm (C) of wastewater per irrigation. The enzyme activity was measured several times during the first 2 years of wastewater application in soil sampled from control and flooded plots (0-10, 10-30, 30-50, 50-70 cm depth). Simultaneously, redox potential at the same depths was measured with permanently installed Pt electrodes. Irrigation with municipal wastewater elevated soil dehydrogenase activity on average by 44% and 27% for the low and high irrigation doses, respectively, but reduced catalase activity at the high irrigation dose on average by 12.4% (significant at P < 0.001). Plant cover significantly influenced soil dehydrogenase and catalase activities.