Spatial and Seasonal Variations in Nitrogen Leaching and Acidity across Four Acid-impacted Regions of the UK

Helliwell, R. C.; Davies, J. J. L.; Evans, C. D.; Jenkins, A.; Coull, M. C.; Reynolds, B.; Norris, D.; Ferrier, R. C.

Spatial and Seasonal Variations in Nitrogen Leaching and Acidity across Four Acid-impacted Regions of the UK

2007

This study examines the seasonal variability in levels of nitrogen (N) leaching and acidity in four acid impacted upland regions of the UK: the South Pennines, Snowdonia, Galloway and the Mourne Mountains. All regions are acidified, with median pH values of <5.5. The South Pennines exhibit excess sulphate concentrations an order of magnitude higher than those for other regions and have the lowest pH values of all the regions. Nitrate concentrations are highest in the South Pennines (seasonal medians 20-26 μeq l-¹) and the Mourne Mountains (seasonal medians 9-26 μeq l-¹). Surface water in the Mourne Mountains is the most highly N impacted in terms of the proportional contribution of N to acidity. All N species exhibit seasonality, with greater retention by catchment soils and vegetation in August. This seasonality is most marked in Snowdonia and least marked in the South Pennines. This implies that the South Pennines have reached an advanced stage of N saturation. Despite the reductions in S deposition, xSO₄ is the dominant anion for all sites in the South Pennine and Snowdonia regions. A strong negative relationship between the contribution of NO₃ to total acidity and DON was observed for all regions except the South Pennines. It is hypothesized that catchments dominated by organic rich soils produced more DON and have an increased capacity to immobilise NO₃. Seasonal nitrogen budgets show that over 60% of the N inputs are retained within catchments, and no site retains all incoming N throughout the year.

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