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) depositio n is declining in response to stringent national and European controls on S emissions. At the same time, however, the relative contribution of nitroge nous (N) compounds to the total anthropogenic deposition loading has increa sed. To investigate the significance of N deposition on the potential acidi fication of surface waters, national. regional, and catchment databases wer e developed to assess the relationships between N deposition, soil C/N rati os, 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 ar eas where N inputs are dominated by non-atmospheric sources. From these scr eened datasets, it was apparent that areas with the highest risk of N leach ing 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-w ater NO3 concentration in afforested catchments was negatively correlated w ith soil C/N ratios below 20. This relationship was not evident in moorland catchments, where NO3 leaching was strongly related to N deposition and th e loch/catchment ratio, rather than the soil C/N ratio. Temporal trends of regional water quality highlighted as increasing loch NO3 concentrations be tween 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 o f within catchment process in controlling the nitrogen response observed in surface waters. The potential mechanisms through which vegetation and soil s may modify incoming deposition are discussed. (C) 2001 Elsevier Science B .V. All rights reserved.