Nitrogen in rainfall, cloud water, throughfall, stemflow, stream water and groundwater for the Plynlimon catchments of mid-Wales
Introduction
Over the past 30 or more years, there has been a major drive to study nitrogen dynamics within soils and stream waters of the UK uplands in relation to concerns over land use change and atmospheric deposition of nitrogen oxides adversely impacting water resources and stream ecology (Hudson et al., 1997a).
The primary environmental concerns of land use change in relation to nitrogen are associated with fertilizer applications for grassland improvement and the introduction of conifer forests onto moorland areas. Under grassland improvement, the main concern is the rapidity of fertilizer leaching through the soils and into the stream. Forestry issues are more indirect, being associated with nutrient generation following land disturbance during the planting and harvesting phases of the plantation forestry cycle, due to disruption of the biogeochemical cycle over the past 50 years (Neal et al., 1998a, Neal et al., 1998b). Plantation forestry has been a major feature of upland agricultural development. The developments started between and post the two World wars in response to the strategic requirements of homegrown timber and many of the afforested areas are now reaching maturity.
The UK uplands are frequently acidic, acid sensitive and they have been directly impacted by SOx emissions and by forestry development (UKAWRG, 1988). Nitrogen oxide and ammonium add to the acid deposition loading. While nitrogen oxides and ammonium have not been the main components of acidification to date, it is of increasing importance in terms of acidification recovery associated with national and international legislation for acid emissions reduction from industry. Thus, the proportion of acid deposition from nitrogen sources increases as SOx concentrations decline. Indeed, the proportion is increasing as NOx emissions continue at current levels due to the opposing trends of decreasing industrial emissions and increasing vehicle emissions (RGAR, 1997). The situation for ammonium inputs, particularly from agricultural emissions, remains uncertain (RGAR, 1997). More recently, there has been the concern over the generation of nitrogen containing ‘greenhouse’ gases.
In this paper, the dissolved concentration and flux of inorganic forms of nitrogen (ammonium and nitrate) is examined within the context of a detailed study of the hydrogeochemistry of upland acidic catchments in mid-Wales; the Plynlimon study (Neal, 1997a, Neal, 1997b, Neal et al., 2001). The work integrates the findings for nitrate and ammonium within the context of the most detailed study of rainfall, cloud water, stream water and ground water undertaken in the UK that links closely with studies on nutrient dynamics and fluxes and soil solution chemistry (Reynolds et al., 1988, Reynolds et al., 1989, Reynolds et al., 1992, Reynolds et al., 1995, Stevens et al., 1990, Stevens et al., 1994, Stevens et al., 1997). New information is presented on how conifer harvesting affects nitrate and ammonium water quality, which is of strategic management concern in relation to forestry policy and practice, for a key type locality in the UK uplands.
Section snippets
Study area
This paper relates to wide ranging studies at the Centre for Ecology and Hydrology catchment research sites at Plynlimon in mid-Wales. Key features of the hydrology, biology and water quality of these sites, and the Plynlimon research have been described earlier (Neal, 1997a, Neal, 1997b).
General characteristics of the area
The study area is the headwater catchments of the River Severn, with its three main tributaries, the Afon Hafren, the Afon Hore and the Nant Tanllwyth. The streams drain a hill top plateau dominated by acid moorland in the upper portion of the catchment (Pumlumon Fawr) and the Hafren Forest. Catchment areas for these sites vary from approximately 50 to 300 ha. The moorland and forest catchments represent a mixture of upland acid soil types dominated by peaty podzols with subsidiary peaty gleys.
Monitoring points and sampling programme
Nitrate and ammonium data is available for three types of water quality monitoring site:
(1) Long-term monitoring sites for rainfall, cloud water and the main tributaries of the headwaters of the River Severn. Rainfall was collected weekly from open gauges at the top and bottom of the Hafren/Hore catchment. Cloud water was collected using a passive lidded ‘harp type’ system at one site near the top of the catchment on a weekly basis (Wilkinson et al., 1997). All three tributaries of the upper
Chemical analysis
For the chemical analysis, the samples were first filtered using 0.45 μM membranes. In the case of the stream waters, the samples were filtered in the field while all other samples were filtered soon after return to the laboratories. The samples were stored in chromic acid washed glass bottles at 4 °C in the dark prior to nitrate and ammonium analysis using standard colorimetric techniques. The nitrate analytical methodology involves the reduction of nitrate in alkaline solution by hydrazine in
Results
Within this paper, a large amount of data is presented. For example, long-term information is provided for rainfall, cloud water and five major streams, together with more specialized information for four other streams and nineteen groundwaters, all of which need tying in with issues such as heterogeneous behaviour, varying degrees of deforestation and climate variability. Thus, the presentation has had to rely on a concise analysis of the data. To do this, the information is summarized in the
Flow, temporal and felling related changes in concentration
The variations in nitrate and ammonium concentrations in the atmospheric inputs, the streams and the boreholes are related to hydrology, time and extent of felling activity and it is difficult to provide a simple scheme for describing the information. Here, the atmospheric inputs are first considered as they have unique characteristics, then the stream and ground waters are dealt with in terms of moorland/forested sites and felled sites.
Nitrate and ammonium in streams for felling areas
With felling, the hydrobiogeochemical cycling is disrupted. For the majority of the catchments studied, the primary change in nutrient chemistry with felling is associated with nitrate generation (e.g. Bormann and Likens, 1994). In the case of drainage from forested catchments with podzolic soil, felling results in relatively little change in ammonium concentrations in runoff. However, for forested catchments draining gley soils under some circumstances felling can lead to a marked increase in
The effects of felling on nitrate and ammonium concentrations in groundwater
With felling of both the podzolic and the gley areas studied, nitrate concentrations in groundwater changed as a function of time and groundwater level, but ammonium concentrations remained unaffected (Fig. 8). There were two types of nitrate response. Firstly, at one of the sites with podzolic soils (HA4 borehole), nitrate concentrations increased from approximately 20 to 180 μM l−1 with the felling. The increase was gradual and the concentrations seem to have flattened after approximately 2
Nitrate and ammonium fluxes
The study of nitrogen fluxes for such hydrologically and hydrobiochemically dynamic catchments is difficult. The difficulty comes about for four reasons. Firstly, the input of nitrogen fluxes comprises rainfall, cloud water, particle and gaseous deposition and while rainfall inputs may be reasonably well determined from the hydrometric network of gauges, the other components are much more difficult to gauge. Secondly, because of the dynamic nature of the hydrograph and nitrogen chemograph, a
Discussion
Dissolved inorganic nitrogen in the moorland and forested catchments at Plynlimon are determined by a complex set of hydrological and biological reactions. There are two main inorganic forms of nitrogen in the streams, nitrate and ammonium of which nitrate predominates. The hydrobiogeochemical behaviour differs for these two components. The salient points are discussed separately below followed by a comment on dissolved organic carbon.
Nitrate
There are strong seasonal variations in nitrogen concentrations within the streams across all the catchments. In terms of nitrate fluxes, there may be a net uptake—to, a net balance or a net efflux—from the catchment relative to the rainfall and cloud water input depending upon the particular catchment. The highest variations in nitrate concentrations and fluxes are probably associated with climate variability and felling activity. In the case of climate, it seems that certain perhaps drier
Ammonium
Ammonium inputs are largely retained within the Plynlimon catchments even with felling activity, but ammonium concentrations in stream runoff showed very erratic behaviour. Felling was followed by increased in ammonium concentrations in the stream and this was most prevalent for the gley sites. The clearest indication of the increase was for the small drainage areas in the gley where the felling response lasted for a relatively short period with a sharp decay during the year of felling.
Dissolved organic nitrogen
Although in this paper the emphasis is on inorganic forms of nitrogen, it is important to flag that nitrogen is also present in dissolved organic forms (DON) as well. Information on DON is fragmentary, but there have been some new regional studies in the UK uplands including at Plynlimon (Chapman et al., 1998, Chapman et al., 1999, Chapman et al., 2001). The results for Plynlimon (Chapman et al., 1999), based on monthly sampling of streams draining moorland and partly forested catchments from
Conclusion
This paper provides a major overview of the nitrate and ammonium dynamics within rainfall, cloud water streams and groundwaters for the Plynlimon catchments. The results show that there are significant inputs of both nitrate and ammonium in rainfall and cloud water and these concentrations show major changes from year to year. These inputs are largely ‘pollution’ related and concentrations show strong dilution with increasing volume of catch, a ‘washout’ effect. While there is no data for
Future research needs
The results presented in this paper provide important clues for the hydrobiogeochemical functioning of nitrogen within UK upland acidic and acid sensitive moorland and forested catchments. However, there is a clear need to investigate the details of the changes involved and undertake a more in-depth study of the processes and flux transfers involved through the hydrological cycle. These needs include:
(1) The acquirement of much longer data runs. This is required for two reasons. Firstly, there
References (32)
- et al.
The nitrogen composition of streams in upland Scotland: some regional and seasonal differences
Sci Total Environ
(2001) - et al.
Baseflow buffering of stream water acidity in five mid-Wales catchments
J Hydrol
(1986) - et al.
Variations in stream water nitrate concentrations and nitrogen budgets over 10 years in a headwater catchment in mid-Wales
J Hydrol
(1992) - et al.
- Chapman PJ, Edwards AC, Reynolds B, Cresser MS, Neal C. Hydrology, Water Resources and Ecology in Headwaters....
- Chapman PJ, Edwards AC, Reynolds B, Neal C. The nitrogen composition of streams draining grassland and forested...
- et al.
Constrained multivariate trend analysis applied to water quality variables
Environmetrics
(2002) - et al.
A comparison of river load estimation techniques: an application to dissolved organic carbon
Environmetrics
(2002) Atmospheric solute processes
- et al.
Land use and water quality issues in the uplands with reference to the Plynlimon study
Hydrol Earth Syst Sci
(1997)
The Plynlimon water balance 1969–1995: the impact of forest and moorland vegetation on evaporation and streamflow in upland catchments
Earth Syst Sci
Fractal stream chemistry and its implications for contaminatn transport in catchments
Nature
Catchment-scale advection and dispersion as a mechanism for fractal scaling in stream tracer concentrations
J Hydrol
Sodium and chloride levels in rainfall, mist, streamwater and groundwater at the Plynlimon catchments, mid-Wales: inferences on hydrological and chemical controls
Hydrol Earth Syst Sci
Cited by (36)
Sources of major elements and nutrients in the water cycle of an undisturbed river basin – Samothraki Island, Greece
2023, Science of the Total EnvironmentForms and subannual variability of nitrogen and phosphorus loading to global river networks over the 20th century
2018, Global and Planetary ChangeContribution of transboundary air pollution to ionic concentrations in fog in the Kinki Region of Japan
2009, Atmospheric EnvironmentCitation Excerpt :Fog water deposition through the interception of fog droplets by vegetation can be an important part of the hydrologic budget of forests (Vong et al., 1991; Dawson, 1998; Eugster et al., 2006). Ionic concentrations in fog water are much higher than those in rain water (Igawa et al., 1998; Neal et al., 2003; Aikawa et al., 2006). Consequently, fog can contribute significantly to the deposition of ionic compounds in mountainous forest areas (Kobayashi et al., 2001; Baumgardner et al., 2003).
Long-term nitrate increases in two oligotrophic lakes, due to the leaching of atmospherically-deposited N from moorland ranker soils
2008, Environmental PollutionCitation Excerpt :Time-series nitrate data for 2005–2006 are plotted in Fig. 2. A seasonal trend in [NO3] is apparent, that has been observed for streams draining similar catchments in the Lake District (Tipping et al., 2000; Cresser et al., 2004), and at other UK locations (e.g. Neal et al., 2003). The low concentrations in summer are attributed to greater use of N in plant growth.
Groundwater nitrogen composition and transformation within a moorland catchment, mid-Wales
2008, Science of the Total Environment