The ability of some plants to grow and develop in the soils with limited access to oxygen results not so much from the biochemical differences in their root metabolism, but from the access of oxygen from the aboveground plant parts via internal transport. By the same route, harmful gases (e.g., carbon dioxide and methane) get into the plant from the soil. Their toxic properties are neutralised in the aboveground plant parts or these gases are released into the atmosphere. The studies were carried out on a peat-muck soil (Eutric Histosol – pH in KCl 7.2, C org. 326 g kg–1). The study plot was divided into three sections (A,B,C), in which three irrigation treatments were applied. The aim of the present study was to determine the contribution of a mixture of grasses (with Alopecurus pratensis, Phalaris arundinacea, Festuca pra- tensis as dominant species) to methane transportation via internal routes from the soil irrigated with municipal waste water after the second stage of purification (mechanical and biological treatment) during two flooding cycles. In spring, an increase in the methane emission from the mixture of grasses irrigated with a single and a double municipal waste water dose, up to the range 42.84–57.12 mg m–2h–1 was observed after 3 days from the onset of the stress conditions. In summer, an increased level of methane emission up to 47.84 and 57.12 mg m–2h–1 was observed 24 h after irrigation in the plots irrigated with a double and single dose of municipal waste water, respectively. Redox potential was a signal of the above changes as at a depth of 70 cm, it decreased to –150 mV.