Interaction of nitrogen deposition and land use on soil and water quality in Scotland: issues of spatial variability and scale

Over large areas of the Scottish uplands anthropogenic sulfur (S) deposition is declining in response to stringent national and European controls on S emissions. At the same time, however, the relative contribution of nitrogenous (N) compounds to the total anthropogenic deposition loading has increased. To investigate the significance of N deposition on the potential acidification of surface waters, national, regional, and catchment databases were developed to assess the relationships between N deposition, soil C/N ratios, land use and surface water NO3 concentrations. National classification schemes for land use and soils were used as only limited empirical data are available at such large spatial scales. Data were screened to eliminate areas where N inputs are dominated by non-atmospheric sources. From these screened datasets, it was apparent that areas with the highest risk of N leaching were situated predominantly in the upland areas of south-west and west Scotland (areas with low soil C/N ratios). At the regional scale, surface-water NO3 concentration in afforested catchments was negatively correlated with soil C/N ratios below 20. This relationship was not evident in moorland catchments, where NO3 leaching was strongly related to N deposition and the loch/catchment ratio, rather than the soil C/N ratio. Temporal trends of regional water quality highlighted as increasing loch NO3 concentrations between 1988 and 1996-1997, presumably reflecting an increase in N deposition, enhanced leaching losses from the terrestrial component of the catchment, or altered in-lake processes. The hydrochemical records for two catchments in NE Scotland (Lochnagar and Allt a Mharcaidh) highlight the importance of within catchment process in controlling the nitrogen response observed in surface waters. The potential mechanisms through which vegetation and soils may modify incoming deposition are discussed.