Freshwater Umbrella Presentations

15N/14N + 18O/16O tracing of nitrate in UK upland waters

IsoEcol 5 posterAbstract

Although problems relating to nitrate in water are mainly associated with modern farming methods in more populated parts of the UK, nitrate also constitutes a potential problem in more remote, upland areas. Here it is nitrate in atmospheric deposition which has a particular impact due to a combination of factors:

  • these are high rainfall areas;
  • low concentrations of base cations in upland soils makes them particularly prone to acidification, and nitrate is soon likely to become the main acidifying agent in acid rain; and
  • upland ecosystems are adapted to low nutrient levels and are often N-limited.

Current trends suggest critical loads for acidity and N deposition will be exceeded in many upland areas by 2024, factors which threaten future compliance with the EU Water framework Directive. Predicting the response of upland areas to nitrate deposition, however, depends on knowledge of the extent to which their soil/plant ecosystems are already N‑saturated. In saturated systems, excess soil N is likely to undergo bacterial nitrification, and be released as nitrate into surface waters. To this end, being able distinguish between nitrate derived from atmospheric deposition, and nitrate formed by soil nitrification is particularly important.

In the first combined 15N/14N and 18O/16O study of nitrate in rainfall and surface waters in the UK, we analysed samples from four upland sites of the UK Acid Waters Monitoring Network: Afon Gwy (Wales), Lochnagar (Cairngorms), River Etherow (Peak District), and Scoat Tarn (Lake District). For the data available to date several very clear results emerge:

  • δ15N values for rainfall nitrate were typically in the range -2 to +3‰, and therefore not distinguishable from the values expected for nitrate in upland soils.
  • δ18O values for rainfall nitrate were typically in the range +60 to +80‰, and therefore very different, and distinguishable from the values of +2 to +4‰ which would be theoretically expected for bacterially-produced soil nitrate (based on one third of the oxygen being derived from atmospheric O2 = +23‰, and two thirds being derived from water = -8 to -6‰).
  • Nitrate in most of the streams had δ18O values corresponding very closely to the theoretical values for bacterial soil nitrate; implying that, at least during the sampled period, very little atmospheric nitrate passes directly through the soils into the streams.
  • Only the outflows of Lochnagar and Scoat Tarn showed significant atmospheric nitrate (c. 17%), and this may represent rainfall which has fallen directly onto the surface of these water bodies.
Authors:

Tim H.E. Heaton, Chris J. Curtis & Gavin L. Simpson

Publication Date:

August 2006

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