Cj Curtis

Nitrogen as a threat to European water quality

Grizzetti, B., Bouraoui, F., Billen, G., van Grinsven, H., Cardoso, A. C., Thieu, V., Garnier, J., Curtis, C., Howarth, R. W. and Johnes, P. (2011) Nitrogen as a threat to European water quality. In: Sutton, M. A., Howard, C. M., Erisman, J. W., Billen, G., Bleeker, A., Grennfelt, P., van Grinsven, H. and Grizzetti, B. (eds.) European Nitrogen Assessment. Cambridge University Press, UK, pp. 379-404. ISBN 9781107006126 Available at http://centaur.reading.ac.uk/20869/

Evidence that soil carbon pool determines susceptibility of semi-natural ecosystems to elevated nitrogen leaching

Deposition of reactive nitrogen (N) compounds has the potential to cause severe damage to sensitive soils and waters, but the process of ‘nitrogen saturation’ is difficult to demonstrate or predict. This study compares outputs from a simple carbon–nitrogen model with observations of (1) regional- and catchment-scale relationships between surface water nitrate and dissolved organic carbon (DOC), as an indicator of catchment carbon (C) pool; (2) inter-regional variations in soil C/N ratios; and (3) plot scale soil and leachate response to long-term N additions, for a range of UK moorlands. Results suggest that the simple model applied can effectively reproduce observed patterns, and that organic soil C stores provide a critical control on catchment susceptibility to enhanced N leaching, leading to high spatial variability in the extent and severity of current damage within regions of relatively uniform deposition. Results also support the hypothesis that the N richness of organic soils, expressed as C/N ratio, provides an effective indicator of soil susceptibility to enhanced N leaching. The extent to which current C/N is influenced by N deposition, as opposed to factors such as climate and vegetation type, cannot be unequivocally determined on the basis of spatial data. However, N addition experiments at moorland sites have shown a reduction in organic soil C/N. A full understanding of the mechanisms of N-enrichment of soils and waters is essential to the assessment of current sensitivity to, and prediction of future damage from, globally increasing reactive nitrogen deposition.

An Empirical Approach for Assessing the Relationship Between Nitrogen Deposition and Nitrate Leaching from Upland Catchments in the United Kingdom Using Runoff Chemistry

Samples were collected from 13 upland sites (main inflow and loch outflow) in the UK along an N deposition gradient of 12-50 kg ha-1 yr-1 to determine the relationship between N deposition and NO3- concentrations in surface waters. There was no direct correlation between NO3- leaching and soluble inorganic N deposition at these sites, but a significant relationship with NO3- was found using a deposition function incorporating dissolved organic carbon (DOC) flux from each catchment. A similar but less significant relationship was found between NO3- concentration and DOC:DON ratio in runoff water. Catchments showed evidence of N saturation, i.e., when mean NO3- concentration exceeded 5 µeq L-1, when the mean DOC:DON ratio fell below an approximate value of 25. Five other large loch sites (LLS) with multiple subcatchments were used to test these relationships and for four of these mostly heathland sites the predicted NO3- concentrations closely matched measured values. At the fifth site, where most subcatchments were forested, the deposition function and DOC:DON ratios gave conflicting predictions and both methods generally underestimated measured NO3- concentrations. If the capacity of these catchments to retain deposited N is determined by C supply then many upland catchments in the UK may experience increasing NO3- concentrations in runoff in the future at current or increased levels of N deposition.

Nitrogen flows from European regional watersheds to coastal marine waters

Approaches A comprehensive evaluation of net anthropogenic inputs of reactive nitrogen (NANI) through atmospheric deposition, crop N fi xation, • fertiliser use and import of food and feed has been carried out for all European watersheds. A database on N, P and Si fl uxes delivered at the basin outlets has been assembled. A number of modelling approaches based on either statistical regression analysis or mechanistic description of the processes involved • in nitrogen transfer and transformations have been developed for relating N inputs to watersheds to outputs into coastal marine ecosystems.

The impact of nitrogen deposition on upland surface waters in Great Britain: A regional assessment of nitrate leaching

A national dataset of water chemistry collected for critical loads mapping is used to make a regional assessment of surface water nitrate concentrations in Great Britain. The primary data are dominated by high concentrations in lowland regions Where N inputs are dominated by non-atmospheric sources. Land cover data are used to screen out sites with potential catchment sources of N, allowing the evaluation of nitrate leaching due to atmospheric deposition alone. In the screened dataset several upland regions show elevated nitrate concentrations, notably Wales, the Pennines, Cumbria, Galloway and the Cairngorms, and there is a clear relationship between surface water nitrate and total N deposition.

Reconstruction of Long-Term Changes in Lake water Chemistry, Zooplankton and Benthos of a Small, Acidified High-Mountain Lake: Magic Modelling and Palaeolimnogical Analysis

Starolesnianske pleso is a small and shallow acid lake in the High Tatra Mountains, situated at 2000 m above sea level, on granitic bedrock, with sparse and thin soil cover. When detailed measurements began in the 1980s Starolesnianske pleso had pH below 5 and only one species of cladoceran zooplankton, the ubiquitous Chydorus sphaericusPalaeolimnological investigations show changes in cladoceran zooplankton and chironomid zoobenthic assemblages since about 1920 and a major change in 1970–1980. The dynamic acidification model MAGIC was used to reconstruct changes in water chemistry over the past 150 years. The results from MAGIC agree well with the sediment record; pH levels gradually decreased from 6.5 in the mid-1800s to about 5.6 in 1920 (first response in biota) to below 5.0 by 1970, and concentrations of inorganic aluminium rose dramatically beginning about 1960. In the 1990s the lake water chemistry showed clear signs of reversal in acidification brought about by a major decline in S deposition.

The Prediction of Nitrate Leaching with the First-Order Acidity Balance (FAB) Model for Upland Catchment in Great Britain

The relative contribution of N deposition to the acidification of freshwaters in Great Britain has increased over the last few years as S deposition has fallen in line with reduced emissions. In certain high deposition areas of Great Britain, NO 3 − -based acidity can equal or exceed the contribution of SO 4 2− -based acidity in some upland waters. Here we apply the first-order acidity balance model (FAB) to predict the maximum N leaching from 13 study catchments at future steady state. Using mean water chemistry and catchment soils data, along with long-term default values for N sink processes, we predict NO3-N leaching at much higher rates than currently are being measured in surface waters, with a mean increase of 10.5 kg ha−1 yr−1. As a result, mean acid neutralizing capacity would decline to less than 0 meq L−1 at 4 sites. While there are uncertainties associated with model parameterization relating to the short-term storage of N within catchment soils and vegetation, model outputs do indicate much greater leaching of N at some time in the future as steady-state is achieved.

Evidence for the pollution of Loch Ness from the analysis of its recent sediments

Abstract For sites, such as Loch Ness, Scotland, which are often assumed to be pristine, it is difficult to assess from standard water-quality monitoring techniques whether long-term change due to pollution is taking place. In this study, the historical record held in the sediments was used to identify whether the Loch suffers from acidification or cutrophication. A sediment core was taken in 170 m of water and analyzed using palaeolimnological techniques. Evidence from spheroidal carbonaceous particles and artificial radionuclides indicates that the Loch has been contaminated by atmospherically deposited pollutants. Although diatom analysis shows that acidification has not occurred, there is evidence for a slight eutrophication during the past 20 years. A flood event which occurred in 1868 is recorded in the sediments. This did not have an impact on diatom communities within the lake. This paper illustrates the usefulness of palaeolimnological methods in providing an early warning of environmental change in lakes.

Rapid immobilisation and leaching of wet-deposited nitrate in upland organic soils

Nitrate (NO3-) is often observed in surface waters draining terrestrial ecosystems that remain strongly nitrogen (N) limited. It has been suggested that this occurs due to hydrological bypassing of soil or vegetation N retention, particularly during high flows. To test this hypothesis, artificial rain events were applied to 12 replicate soil blocks on a Welsh podzolic acid grassland hillslope, labelled with 15N-enriched NO3- and a conservative bromide (Br-) tracer. On average, 31% of tracer-labelled water was recovered within 4 h, mostly as mineral horizon lateral flow, indicating rapid vertical water transfer through the organic horizon via preferential flowpaths. However, on average only 6% of 15N-labelled NO3- was recovered. Around 80% of added NO3- was thus rapidly immobilised, probably by microbial communities present on the surfaces of preferential flowpaths. Transitory exceedance of microbial N-uptake capacity during periods of high water and N flux may therefore provide a mechanism for NO3- leaching.

Critical loads of acidity for Irish lakes

Abstract. Critical loads of acidity have been estimated for Irish lakes (277) using survey data from the Republic of Ireland and Northern Ireland, and compatible inputs and methods across both regions. This is the first trans-region application of the Steady-State Water Chemistry model for the island of Ireland. Results indicate that a small percentage of lakes (10.8%, 30 lakes) have low critical loads of acidity (<50.0 mmolc m–2 yr–1). Based on bulk sulphate deposition, exceedance of critical load was observed in 6.9% of the sampled lakes (19 lakes). Inferences relating to the whole lake population cannot be made, as statistical lake surveys were not carried out. However, as the surveys were weighted towards acid-sensitive lakes, the current study suggests that Irish lakes are not under a significant threat from anthropogenic acidification. The acidifying impact of nitrogen was incorporated into the exceedance calcula tion by superimposing the present leaching of nitrogen onto sulphate deposition. This increased exceedance by 40% (i.e., from 11 to 19 lakes). Despite the potential uncertainties associated with using default values and relationships in the Steady-State Water Chemistry model, uncertainty analysis indicated that the critical loads were relatively stable. The critical acid neutralising capacity limit (specified water quality criterion) had the greatest effect on the critical load. In the current study, a catchment dependent limit was employed (ranged: 0–50 μmolc L–1; mean: 35 μmolc L–1) rather than a single fixed value.