Nitrous oxide production in the river Swale–Ouse, North–East England

Nitrous oxide production was measured monthly using intact sediment cores at five sites along the Swale–Ouse river system, North–East England, together with one site on a highly eutrophic tributary (Wiske) from August 1995 to December 1996. Both spatial and seasonal trends were evident. There was a marked change in rate on passing down the river, with a maximum value at the most upstream site of 14 μmol N2O-N m−2 h−1 (i.e. 14 μmol N as N2O), 96 μmol N2O-N m−2 h−1 at a mid-river site and 100 μmol N2O-N m−2 h−1 at the most downstream site (145.0 km down the main river and at the freshwater tidal limit). The highest rate on most sampling dates was for the Wiske (maximum of 497 μmol N2O-N m−2 h−1). (All values refer to unit area of sediment and not the whole river bed, since extensive parts of the bed consist of rocky substrata at the upstream sites.) The downstream increase is probably explained largely by an increase in ambient nitrate concentration. Higher rates were evident in spring and summer at most downstream sites, including the Wiske; such seasonal changes are probably largely due to changes in temperature.