Freshwater Umbrella Presentations

What are the most important sinks for deposited nitrogen over one year in UK moorlands?

Freshwater Umbrella posterAbstract

Only a small proportion of anthropogenic nitrogen (N) compounds deposited onto upland catchments in the UK is currently lost as a surface water leaching flux. The construction of N budgets to determine the fate of the large proportion of deposited N that is not leached has been the primary focus of several research programmes across Europe and North America, especially with regard to modelling the future contribution of N deposition to acidification and eutrophication in terrestrial (mostly forest) and aquatic ecosystems.

Here, we describe the key sinks and fluxes of nitrogen at four acid moorland catchments across a gradient of total inorganic N deposition (TIN Depn.) in the UK uplands. Bulk deposition chemistry, surface water chemistry and flow were measured at least 2-weekly in each of the four catchments and used to estimate annual mean input and output fluxes for inorganic N.

13 study areas were defined across the four catchments, based on major representative soil and vegetation types in each catchment. In each study area, 3 replicated plots were used for in situ field measurements of soil moisture and soilwater chemistry (2-weekly) and trace gas fluxes (monthly — on duplicated plots ± additions of NH4NO3). A 15N labelled NH4NO3 tracer was added 2-weekly. After one full year of field measurements, plots were destructively sampled for measurement of 15N abundance in soils and vegetation, C:N ratio of soils and potential mineralization, nitrification and denitrification fluxes from incubated soils.

The 15N tracer experiment shows that 65–99% of TIN inputs are retained by vegetation uptake or immobilisation in soils over one year of additions. Denitrification is a very minor sink for TIN even where high mineralization and nitrification potentials, soilwater nitrate and surface water leaching fluxes are found. High transient denitrification fluxes under optimal conditions may rapidly become carbon limited. The C:N ratio of the surface organic layer is inversely related to nitrate leaching except at the River Etherow.

The relative importance of biological and hydrological controls on nitrate leaching is likely to vary between sites and is the subject of ongoing research in UK moorlands. The balance of these processes is important for dynamic models of future N saturation and nitrate leaching.

Authors:

Chris Curtis, Bridget Emmett, Brian Reynolds and James Shilland

Publication Date:

June 2005

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