The dynamics of nitrogen forms in soil irrigated with municipal waste water was studied in order to determine its effect on soil redox potential. The soil (Eutric Histosol) was planted with a mixture of grasses, and irrigated 10 times per year with waste water purified by a two-step (mechanical and biological) treatment with a dose optimal for the plants, or doubled. Simultaneously, control soil (not flooded) with plant vegetation was studied. It was found that redox potential was influenced by nitrate (V) nitrogen in plots irrigated with municipal waste water at the optimal irrigation dosage for the plants and with double the optimal dose. Irrigation with waste water resulted in a decrease of Eh value in the whole soil profile, causing a reduction of redox potential value below the level of +200 mV, especially in lower horizons, corresponding to dissimilative reduction of nitrates (V) to the forms of N2O and N2.