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An assessment of the representation of moorland nitrogen sinks in static critical load models for freshwater acidity

Curtis, Christopher James; (2003) An assessment of the representation of moorland nitrogen sinks in static critical load models for freshwater acidity. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The context of the thesis is the requirement for critical load models for total acidity (Sulphur plus nitrogen) to freshwaters. It has long been recognised that nitrogen leaching into acid-sensitive upland waters is widespread, and the contribution of nitrate associated acidity to freshwater acidification and the prevention of recovery is increasing in relative importance as sulphur deposition and leaching decline in response to international emissions reductions, while total nitrogen deposition remains relatively constant. What is less certain is that nitrate leaching could increase in absolute terms even under constant deposition. The widely-used static critical loads model for freshwater acidity, the First-order Acidity Balance (FAB) model, employs a number of default values for key nitrogen processes which provide sinks for deposition inputs of nitrogen. The model is used to determine critical loads of sulphur and nitrogen for use in international integrated assessment modelling to aid policy decisions on emissions reductions for sulphur and nitrogen within Europe. Application of the FAB mass balance to catchments with known input-output budgets for nitrogen shows that there is a very large difference between current leaching of nitrate into surface waters, which is generally a small proportion of nitrogen inputs, and that predicted at steady-state by the FAB model. The aim of this thesis is to experimentally test the assumptions of the FAB model for four moorland catchments at which the major nitrogen fluxes have been measured, across a gradient of total nitrogen deposition in the acid-sensitive UK uplands. In particular, the degree to which the representation of nitrogen immobilisation and denitrification processes are appropriate for these ecosystems is assessed by a combination of field measurements and laboratory experiments. The relative importance of mineralisation and nitrification in catchment soils as controls on nitrogen retention and leaching is tested through laboratory incubations of soils, and the implications of their absence from the FAB model are discussed. Finally, two methods for the determination of nitrogen saturation status, using stable isotopes (15N) and soil carbon/nitrogen ratios, are applied, in an attempt to determine the effects of nitrogen accumulation on the terrestrial catchments and evaluate the assumptions of nitrogen saturation and enhanced leaching of nitrate at steady-state which are implicit in the FAB model.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: An assessment of the representation of moorland nitrogen sinks in static critical load models for freshwater acidity
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Social sciences; Freshwater acidity; Nitrogen sinks
URI: https://discovery.ucl.ac.uk/id/eprint/10100765
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