Freshwater Umbrella Presentations

Nitrate pathways in upland ecosystems: isotopic tracing from deposition to surface waters

Chris J. Curtis, Chris Evans, Tim H.E. Heaton, James Shilland, Gavin L. Simpson and Simon Turner


Studies of upland waters around the world have shown that even the most remote locations are being impacted by the deposition of anthropogenic sulphur (S) and nitrogen (N) compounds. The most obvious impact of S and N deposition is surface water acidification (Curtis et al., 2005a), but the deposition of anthropogenic N compounds has more insidious effects, because elevated nitrate (NO3-) leaching also affects the nutrient balances of aquatic ecosystems (Maberly et al., 2002; Sickman et al., 2003). Furthermore, a major problem in linking N deposition to effects has been the often subtle and long-term ecological response to nitrogen inputs.

N limitation is common in upland terrestrial ecosystems, and deposition onto plants and soils tends to be very strongly retained in the short term of months to years (Curtis et al., 2005b). Release of this retained N occurs through mineralization and nitrification processes which in turn may be mediated by carbon:nitrogen ratios in soil organic matter (e.g. Curtis et al., 2004) or hydrological processes linked to certain soil type and slope parameters in NO3- leaching zones (Evans et al., 2004). Hence the controls on NO3- leaching are complex but improved understanding is crucial to be able to model the fate of upland aquatic ecosystems.

Here we present the results of two parallel isotopic tracer studies at four intensively studied moorland catchments in the UK uplands (Lochnagar, Scotland; Scoat Tarn, English Lake District; River Etherow, Southern Pennines; Afon Gwy, mid-Wales):

  1. the dual isotope method (e.g. Campbell et al., 2002) using δ15N and δ18O to apportion leached NO3- to atmospheric and microbial sources and determine the relative importance of hydrological and microbial controls on NO3- leaching at the four sites; and
  2. the use of 15N labeled NO3- additions in combination with a conservative Br- (Bromide) tracer to determine leaching flowpaths in a plot based study at the Afon Gwy.

Results to date from the dual isotope study suggest a dominant microbial source of leached NO3- which contradicts earlier work suggesting that direct leaching of atmospheric NO3- could account for the NO3- observed in surface waters within these catchments. Here we explore the reasons for these conflicting results, including seasonality, hydrological flowpaths and problems of linking plot-based studies to catchment scale modelling.


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  • Campbell D.H., Kendall C., Chang C.C.Y., Silva S.R. & Tonnessen K.A. (2002) Pathways for nitrate release from an alpine watershed: Determination using δ15N and δ18O. Water Resources Research 38: 10.1029/2001WR000294 [ article ]
  • Curtis C.J., Emmett B.A., Grant H., Kernan M., Reynolds B. & Shilland E. (2005b) Nitrogen saturation in UK moorlands: the critical role of bryophytes and lichens in determining retention of atmospheric N deposition. Journal of Applied Ecology 42, 507–517 [ article ]
  • Curtis C.J., Evans C., Helliwell R.C. & Monteith D. (2005a) Nitrate leaching as a confounding factor in chemical recovery from acidification in UK upland waters. Environmental Pollution 137, 73–82 [ article ]
  • Curtis C.J., Emmett B., Reynolds B. & Shilland J. (2004) Nitrate Leaching from Moorland Soils: Can Soil C:N Ratios Indicate N Saturation?. Water, Air & Soil Pollution; Focus 4, 359–369 [ article ]
  • Evans, C.D., Reynolds, B., Curtis, C.J., Crook, H.D., Norris, D. & Brittain, S.A. (2004) A Conceptual Model of Spatially Heterogeneous Nitrogen Leaching from a Welsh Moorland Catchment. Water, Air & Soil Pollution; Focus 4, 97–105 [ article ]
  • Maberly S.C., King L., Dent M.M., Jones R.I. and Gibson C.E. (2002) Nutrient limitation of phytoplankton and periphyton growth in upland lakes. Freshwater Biology 47, 2136-2152. [ article ]
  • Sickman J.O., Melack J.M. and Clow D.W. (2003) Evidence for nutrient enrichment of high-elevation lakes in the Sierra Nevada, California. Limnology and Oceanography 48, 1885-1892. [ article ]