Kinetic Parameters of Denitrification in a River Continuum

Kinetic parameters for nitrate reduction in intact sediment cores were investigated by using the acetylene blockage method at five sites along the Swale-Ouse river system in northeastern England, including a highly polluted tributary, R. Wiske. The denitrification rate in sediment containing added nitrate exhibited a Michaelis-Menten-type curve. The concentration of nitrate for half-maximal activity (Kmap) by denitrifying bacteria increased on passing downstream from 13.1 to 90.4 µM in the main river, but it was highest (640 µM) in the Wiske. The apparent maximal rate (Vmaxap) ranged between 35.8 and 324 µmol of N m-2 h-1 in the Swale-Ouse (increasing upstream to downstream), but it was highest in the Wiske (1,194 µmol N m-2 h-1). A study of nitrous oxide (N2O) production at the same time showed that rates ranged from below the detection limit (0.05 µmol of N2O-N m-2 h-1) at the headwater site to 27 µmol of N2O-N m-2 h-1 at the downstream site. In the Wiske the rate was up to 570 µmol of N2O-N m-2 h-1, accounting for up to 80% of total N gas production.