Denitrification in sediment of lowland streams: Regional and seasonal variation in Gelbæk and Rabis Bæk, Denmark

Seasonal variations of denitrification activity and the relative importance of controlling factors like NO₃⁻ concentration and abundance of organic matter were determined in three different sediments from Danish lowland streams. In Gelbæk, which is a stream fed by NO₃⁻-rich drainage water, highest denitrification activity (about 9 mmol N m⁻² d⁻¹) was observed in spring (May), together with high productivity and accumulation of benthic micro-algae on the sediment surface. During the summer, the diffusion flux of NO₃⁻ from the overlying water was a limiting factor for the decreasing denitrification activity. Both a non-vegetated, sandy sediment and a Batrachium peltatum-vegetated, organic-rich sediment were investigated in Rabis Bæk, which is a stream fed primarily by NO₃⁻-rich groundwater. In the sandy sediment, a denitrification maximum (about 4 mmol N m⁻² d⁻¹) was recorded in late spring (June), in combination with the bloom of benthic micro-algae. The availability of organic matter seemed to be the important limiting factor throughout the year. Denitrification also increased during the spring in the vegetated sediment, apparently in response to the growth of the plants; a maximum (about 11 mmol N m⁻² d⁻¹) was found in summer (July), immediately before the plants were cut to enhance water flow in the stream. Nitrification activity in the root zone of the plants was here the major NO₃⁻ source for denitrification, and the availability of NO₃⁻ limited the denitrification in this sediment.