Nitrogen in rainfall, cloud water, throughfall, stemflow, stream water and groundwater for the Plynlimon catchments of mid-Wales

doi:10.1016/S0048-9697(03)00100-1

Science of The Total Environment

Volumes 314–316, 1 October 2003, Pages 121-151

https://doi.org/10.1016/S0048-9697(03)00100-1 Get rights and content

Abstract

An extensive study of acidic and acid sensitive moorland and forested catchments in mid-Wales is used to show the water quality functioning with respect to nitrate and ammonium. For this, long-term records of rainfall, cloud water, throughfall, stemflow and stream water (up to 18 years of weekly data) are combined with shorter duration information on stream water associated with small tributary sources and drainage ditches, ground water from a network of exploratory boreholes and paired control and felled catchments. The ratio of nitrate to ammonium is about one in rainfall, cloud water, throughfall and stemflow but the concentrations are much lower in rainfall (∼25 μM l⁻¹) than in cloud water (∼300 μM l⁻¹) while throughfall and stemflow are intermediate (∼80 μM l⁻¹). Within the streams draining moorland and forested areas, nitrate concentrations are close to the mean value in rainfall while ammonium concentrations are often over an order of magnitude lower in the stream than in rainfall and are typically only about a fifth that of nitrate. With felling, stream water nitrate concentrations increase for podzolic soils but show a variable response for gley soils. For the streams draining forested podzols, the concentrations of nitrate can be up to an order of magnitude higher for the first few years after felling compared to than pre-fell values but in later years, concentrations decline to pre-fell and even lower levels. Felling for the podzolic soils barely leads to any changes in ammonium concentration. For the gley soils, felling results in an order of magnitude increase in nitrate and ammonium for a small drainage ditch, but the pulse barely reaches the main stream channel. Rather, within-catchment and within-stream processes not only take up the nitrate and ammonium fluxes generated, but in the case of nitrate, concentrations with- and post-felling are lower than pre-felling concentrations. Groundwater concentrations of nitrate for the moorland and forested catchments are slightly lower than for the streams while for ammonium the reverse is the case: ammonium concentrations in groundwater are about a tenth those of nitrate. With felling, groundwater nitrate concentrations show sporadic increases. For two boreholes, these increases occur during wet periods when groundwater levels are at their shallowest; for one other borehole, there is a gradual and sustained increase over several years. The results are explained in relation to the dominant hydrogeochemical processes operative.

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 SO_x 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 SO_x concentrations decline. Indeed, the proportion is increasing as NO_x 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

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Nitrogen in rainfall, cloud water, throughfall, stemflow, stream water and groundwater for the Plynlimon catchments of mid-Wales

Abstract

Introduction

Section snippets

Study area

General characteristics of the area

Monitoring points and sampling programme

Chemical analysis

Results

Flow, temporal and felling related changes in concentration

Nitrate and ammonium in streams for felling areas

The effects of felling on nitrate and ammonium concentrations in groundwater

Nitrate and ammonium fluxes

Discussion

Nitrate

Ammonium

Dissolved organic nitrogen

Conclusion

Future research needs

Sci Total Environ

J Hydrol

J Hydrol

Constrained multivariate trend analysis applied to water quality variables

Environmetrics

A comparison of river load estimation techniques: an application to dissolved organic carbon

Environmetrics

Atmospheric solute processes

Land use and water quality issues in the uplands with reference to the Plynlimon study

Hydrol Earth Syst Sci

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