Air pollution

Nitrogen in catchments and surface waters

Lochnagar, Grampian Mtns. Nitrate makes up around one third of acid anions in the lake water; Surface water nitrogen (N) pollution in lowland agricultural and urban areas is generally associated with nitrate (NO3-) and ammonium (NH3) in runoff from point sources, e.g. fertilizer applications and sewage works discharges. The resulting nutrient enrichment of rivers and lakes is known as eutrophication. Upland waters in sparsely populated areas are generally remote from these point sources of N pollution, while the low-intensity grazing practiced across much of the UK uplands results in little leaching of N into surface waters. Instead, the major source of N in upland waters may arise from the deposition of anthropogenic forms of both reduced N, ammonia (NH4+) and NH3, usually derived from local agricultural sources, and oxidized N (NOx) produced by fossil fuel combustion in motor vehicles, domestic heating and industrial processes. The ubiquitous nature of the N problem has been reflected in recent years by major publications on the global impacts of N (e.g. the Nitrogen Cascade of Galloway et al., 2003; Something in the air, Hooper, 2006) and thematic research initiatives such as the NERC GANE programme.

The ECRC and ENSIS have for over 15 years managed a programme of research into the effects of the deposition of sulphur (S) and N compounds on surface waters, funded by the Air Quality division of DEFRA (previously the DoE and DETR). While this work initially focused on the acidification of surface waters caused by S deposition and the modeling of critical loads for acid deposition, the widespread occurrence of elevated nitrate concentrations in surface waters remote from point source pollution indicated that N deposition was also impacting their chemistry (Allott et al., 1995).

Enhanced nitrate leaching may adversely affect upland waters in two ways, through the processes of acidification and nutrient enrichment.

Nitrate leaching and acidification

When the NO3- anion is leached from soils it is accompanied by a hydrogen ion or proton (H+) which contributes to the acidity of the surface waters receiving the runoff from the catchment. There may be resultant decrease in acid neutralizing capacity (ANC) and pH.

Nitrate leaching and lake nutrient cycles

Recent work under the NERC GANE thematic programme found that contrary to the long-held assumption that upland lakes tend to be phosphorus (P) limited, the phytoplankton and epilithon of many sites show a growth response to N additions as well as P additions in bioassays (Maberly et al., 2002). Of the 30 lakes studies, around one third were N limited and one third we co-limited by N and P. Work under the Freshwater Umbrella programme has found further evidence of N limitation or co-limitation in upland sites in the UK, and current efforts are aimed at expanding the N limitation dataset to improve models for the prediction of N limitation from catchment characteristics and N deposition data.

Palaeolimnology has provided a further strand of evidence for the impacts of N deposition on lake nutrient cycles. Following the approach developed by Wolfe et al. (2001) in alpine lakes of the Rocky Mountains, N isotope measurements in UK lake sediments have shown a widespread depletion in δ15N towards the surface, i.e. in recent decades. This pattern may reflect both an increase in deposition inputs of isotopically depleted N from anthropogenic sources and nutrient cycling changes in the utilisation of deposited N. Ongoing research efforts aim to understand the processes responsible for these common patterns, in particular the potential biological impacts of increased N availability beyond the phytoplankton growth responses observed in the bioassay study.

Modelling nitrate leaching

Potential N leaching_pathways Given the potential for NO3- leaching to cause acidification or prevent its recovery and to change surface water nutrient balances, there is a clear need to be able to model NO3- leaching into the future as N deposition levels change. However, there are several pathways and mechanisms for NO3- leaching with biological retention mechanisms in catchment soils and plants such as mosses being particularly important (Curtis et al., 2005). Several approaches are being developed concurrently under the Freshwater Umbrella programme in collaboration with the National Isotope Geochemistry Laboratory at BGS Keyworth and CEH Bangor, including:

  • The use of the dual isotope approach (δ15N and δ18O) to identify sources of leached NO3- (Chang et al., 1999);
  • The application of δ15N labeled tracers to determine the rate and degree of leaching;
  • Empirical catchment models linking NO3- leaching to catchment soils, vegetation type, carbon (C) and N pools and C:N ratio (e.g. Curtis et al., 1998; Evans et al., 2006; Rowe et al., in press).

It is hoped that with improved understanding of the biogeochemistry of N saturation and leaching processes, better NO3- leaching models will result, allowing for the prediction of acidification and nutrient enrichment impacts of anthropogenic N deposition. The global nature of the N pollution problem suggests that such models could have very widespread uses.

References

  • Allot T.E.H et al. (1995) TBC. TBC.
  • Chang, C.C.Y., Langston, J., Riggs, M., Campbell, D.H., Silva, S.R., and Kendall, C. (1999) A method for nitrate collection for δ15N and δ18O analysis from waters with low nitrate concentrations. Canadian Journal of Fisheries and Aquatic Sciences 56(10): 1856–1864
  • Curtis, C.J., Allott, T.E.H., Reynolds, B., and Harriman, R. (1998) The prediction of nitrate leaching with the first-order acidity balance (FAB) model for upland catchment in Great Britain. Water, Air and Soil Pollution 105(1-2): 205–215
  • Curtis, C.J., Emmett, B.A., Grant, H., Kernan, M., Reynolds, B., and Shilland, E. (2005) Nitrogen saturation in UK moorlands: the critical role of bryophytes and lichens in determining retention of atmospheric N deposition. Journal of Applied Ecology 42(3): 507–517
  • Evans, C.D., Reynolds, B., Jenkins, A., Helliwell, R.C., Curtis, C.J., Goodale, C.L., Ferrier, R.C., Emmett, B.A., Pilkington, M.G., Caporn, S.J.M., Carroll, J.A., Norris, D., Davies ,J., and Coull, M.C. (2006) Evidence that soil carbon pool determines susceptibility of semi-natural ecosystems to elevated nitrogen leaching. Ecosystems 9(3): 453–462
  • Galloway, J.N., Aber, J.D., Erisman, J.W., Seitzinger, S.P., Howarth, R.W., Cowling, E.B. and Cosby, B.J. (2003) The nitrogen cascade. Bioscience 53: 341–356.
  • Hooper, R. (2006) Something in the air. NewScientist 21stJanuary, Issue 2535: 40–43.
  • 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.
  • Wolfe, A.P., Baron, J.S., and Cornett, R.J. (2001) Anthropogenic nitrogen deposition induces rapid ecological changes in alpine lakes of the Colorado Front Range (USA). Journal of Paleolimnology 25: 1–7.