Relationship between groundwater levels and oxygen availability in fen peat soils

•Ground water levels cannot be directly translated into oxygen availability.•The hypoxic zone is of different extent above groundwater levels (GWL).•Full oxygen saturation never occurred in 20 cm soil depth, even at GWL of −80 cm.•The offset between groundwater level and oxygen availability is site specific.•Higher groundwater levels are needed for anoxic conditions on extensified sites. Groundwater levels (GWL) are a major controlling factor for aeration and organic matter turnover in wetland soils but little is known about this relationship under field conditions. This study tested how the O2 availability in fen peat soils is related to groundwater levels. The study encompassed five sites over a wide range of land use intensity. Ground water levels and soil oxygen saturation in 5 cm and 20 cm depth were measured biweekly in three replicates per site over periods of 2–3 years. The O2 levels were not linearly proportional to the GWL, but changed sharply from anoxic to nearly atmospheric levels depending on the positions of water table. Binary logistic regression analyses (LRA) were calculated for the individual sites in order to predict the threshold GWL for defined probabilities of hypoxic or oxic conditions in 5 cm depth. The GWLs for 95% probability of oxic conditions were markedly lower for the managed grasslands (−116 cm and −89 cm to surface level, respectively) than for the unmanaged pasture and the sedge fen (−60 cm and −38 cm). Hypoxic conditions required GWLs close to the surface (7 cm and −2 cm for the pasture and the restored site, respectively) while in 5 cm soil depth managed grasslands remained hypoxic even at GWLs of −8 cm and −28 cm. In 20 cm soil depth, full oxygen saturation never occurred even at GWL as low as 80 cm. Threshold GWL required for 95% probability of oxic conditions was higher with increasing porosity and rooting density. The offset between GWL and oxic conditions can be used for hydrological wetland management, especially for restoration efforts.