Nitrate behaviour in streamflow from a grassland catchment in Devon, U.K.

Abstract

NO 3N levels and loadings in the River Dart, Devon, U.K. have been intensively monitored during the study period February 1975–January 1983, and NO 3N behaviour in streamflow is compared with that of Mg which has predominantly contrasting origins and controls in runoff. A strongly developed annual regime characterises daily mean NO 3N concentrations which peak at 4.5 mg l −1 in December and reach a minimum of 1.1 mg l −1 in early autumn, and this regime is reflected in a positive relationship between NO 3N concentration and discharge. Storm-period NO 3N behaviour also exhibits a marked seasonal variation, and “dilution” responses are typical of the winter period whereas a “concentration” effect is more characteristic of summer months and is attributed to the influence of soil throughflow. Spring and autumn are associated with a transition in storm-period behaviour, and the reaction of NO 3N concentrations during individual events or over a sequence of storms at any time of the year may also vary in response to a complex interplay of factors. NO 3N transport is more strongly biased towards storm events than is Mg, and more than 50% of the total NO 3N load was removed in only 10% of the study period. The mean annual NO 3N load for the study basin (24 kg ha −1) is higher than that recorded in many grassland catchments of the U.K. but does not reflect an unusually high rate of fertiliser application. Unlike several other arable and grassland drainage basins studied in Britain, there is no evidence in the River Dart of a significant upward trend in NO 3N concentrations and loads since 1975.