FACTORS INFLUENCING DISSOLVED NITROGEN CONCENTRATIONS AND LOADINGS IN UPLAND STREAMS OF THE UK

The major factors influencing dissolved nitrogen concentrations in upland streams are reviewed. Nitrate is the main form of inorganic nitrogen encountered in upland streams although dissolved organic nitrogen may constitute an important component of the dissolved nitrogen load in some catchments. In general nitrate concentrations in upland streams are low, typically less than 0.5 mg N l(-1). However, landuse change such as agricultural improvement of acid grassland, afforestation with exotic conifers and forest harvesting can increase nitrate concentrations substantially. Some of these increases (e.g. those due to forest harvesting) may be relatively transient (c. 5 years) whilst the effects of agricultural intensification are of longer duration. Atmospheric inputs are a major source of nitrogen to upland catchments. For semi-natural acid moorland catchments, nitrogen inputs in bulk precipitation generally exceed outputs so that the catchments act as net sinks for nitrogen. There is evidence that nitrate leaching from forest catchments is positively related to forest age, with older forests (40-50 years) acting as net sources of nitrate. Atmospheric deposition appears to determine nitrate leaching losses from forests more than 30 years old. Patterns of nitrate leaching are particularly affected by extremes of climate, such as drought which can have a long lasting, major effect on stream water nitrate concentrations. There are relatively few published long-term data sets to indicate trends in dissolved nitrogen concentrations for upland catchments. Any such data must be interpreted with care, due to the large influence of climatic variability on stream water nitrate concentrations.