RETENTION OF NITROGEN IN SMALL STREAMS ARTIFICIALLY POLLUTED WITH NITRATE

A simple method was developed to test hypotheses on nitrogen retention in first-order streams in an agricultural region near Oslo, SE Norway. A gravity-operated system added a nitrate solution to the streams continuously at a constant rate. Water samples were collected at fixed intervals downstream to follow the rate of decline in streamwater nitrate. Repeated sampling allowed calculation of regression lines from experiments with different levels of additions of nitrate. The experiments showed that removal of nitrate generally increased with higher initial nitrate concentration, regardless of temperature (range 8-16-degrees-C). Higher nitrate removal rates were found in a stream polluted by easily degradable organic matter than in a similar stream fed by groundwater. Experiments conducted in indoor channels lined with a layer of stream sediment gave reproducible, exponential rates of nitrate decrease in the recirculated water. The results are discussed in the framework of first-order streams as protective ecotones between agricultural areas and higher-order parts of the watersheds.