VARIATIONS IN STREAMWATER NITRATE CONCENTRATIONS AND NITROGEN BUDGETS OVER 10 YEARS IN A HEADWATER CATCHMENT IN MID-WALES

Between 1980 and 1989, samples of bulk precipitation, soil water and stream water have been collected regularly from a small catchment within the headwaters of the River Wye at Plynlimon. Subsequent analysis for nitrate-N and ammonium-N revealed that ammonium-N was present in bulk precipitation but concentrations were below detection limit ( < 0.1 mg l-1 N) in soil waters and streamwaters. Over the 10 year period, no consistent trends were observed in the concentration or wet deposition of inorganic nitrogen compounds. Between 1980 and 1983, soil water nitrate-N concentrations varied irregularly over time, in contrast to the regular pattern of summer minima and winter maxima observed in the stream. Following the dry summer of 1984, large increases in soil water nitrate-N concentrations were observed in the stream such that during 1985, winter maximum concentrations increased by a factor of two and summer minima by a factor of ten as compared with previous years. Winter maxima declined gradually over the following 3 years to values similar to those prior to 1984. The variations in streamwater nitrate concentration are discussed in relation to the processes controlling nitrate supply from the catchment and nitrogen depletion within the stream channel. The significance of inorganic nitrogen deposition at Plynlimon is discussed on the basis of streamwater nitrate-N data from an adjacent headwater catchment and a simple mass balance comparing wet deposition of inorganic nitrogen compounds with nitrate exports in the stream. Although nitrate leaching losses vary between 4 and 5 kg ha-1 year-1, there is a net retention of inorganic nitrogen within the catchment. The mass balance also indicates that nitrogen transformations within the catchment may generate an average of 0.34 kEq H+ ha-1 year-1.