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Hydrology, Water Resources and Ecology in Headwaters (Proceedings of the HeadWater'98 Conference
held at Meran/Merano, Italy, April 1998). IAHS Publ. no. 248, 1998.
443
The nitrogen content of rivers in upland Britain:
the significance of organic nitrogen
P. J. CHAPMAN, A. C. EDWARDS
Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
B. REYNOLDS
Institute of Terrestrial Ecology, Deiniol Road, Bangor, Gwynedd LL57 2UP, UK
M. S. CRESSER
Department of Plant and Soil Science, University of Aberdeen, Aberdeen, UK
C. NEAL
Institute of Hydrology, Crowmarsh Gifford, Wallingford, Oxfordshire OX19 8BB, UK
Abstract The influence of catchment characteristics on the form and
concentration of nitrogen (N) in upland rivers is currently being assessed.
Since April 1997, river water from 61 catchments located throughout the
uplands of Britain have been regularly sampled and analysed for total
dissolved nitrogen (TDN), nitrate (N03-N), ammonium (NH4-N) and
dissolved organic nitrogen (DON). Concentrations of TDN were small,
generally less than 1.2 mg l'1, and varied significantly among upland
regions. The relative contribution of N03-N, NH4-N and DON to mean
TDN also varied among regions. The proportion of N03-N ranged from
25% to 84%, while the proportion of DON ranged from 14% to 69%.
Ammonium contributed less than 7% to TDN in each region. Results show
that DON can contribute significantly to TDN. Thus, assessment of
anthropogenic impacts on TDN losses from upland ecosystems need to
consider not only the dissolved inorganic species but also DON.
INTRODUCTION
The characteristics of upland areas make them sensitive to any kind of perturbation.
Two aspects currently of particular concern are; (a) the effect of increased N
deposition and (b) the effect of land-use changes, such as afforestation, on the
acidification and N status of upland soils and surface waters. In Britain, several
catchment and regionally specific studies have indicated that afforestation and/or
increased N deposition have lead to elevated N0 3 -N concentrations and loads in
upland streams and rivers (e.g. Reynolds et al., 1994). However, little consideration
has been given to organic forms of N and the contribution they may make to TDN
losses from upland ecosystems.
The lack of data for organic N in British rivers was recognized by the Impacts of
Nitrogen Deposition in Terrestrial Ecosystems report (INDITE, 1994), which stated
that: (a) the distribution of organic N in UK running waters is impossible to assess as
few data have been collected ... a more extensive database of organic N levels in
upland streams is needed, and (b) the impacts of increased organic N concentrations
on the functioning of aquatic ecosystems needs to be defined. The present study,
444
P. J. Chapman et al.
therefore, was undertaken to investigate: (a) the spatial distribution of N forms and
concentrations in upland rivers of Britain in relation to catchment attributes, and
(b) the relative contribution of DON to TDN concentrations. The preliminary results
of this study are presented in this paper.
THE UPLANDS OF BRITAIN
In Britain, upland and marginal upland landscapes as defined in Bunce & Howard
(1992) represent 37% of the total land area and are located predominantly in the
north and west. The climate is cool and wet, with annual rainfall ranging between
1000 and 3000 mm. The dominant soils include brown earths, gleys, podzols and
peats. These soils are generally acidic, imperfectly drained, organic rich and nutrient
poor, although they contain substantial amounts of organic carbon, nitrogen and
phosphorus in the upper horizons. Consequently streams draining upland areas are
usually acidic, organic-rich and contain small concentrations of nutrients (Reynolds
& Edwards, 1995).
Land use is dominated by heather moorland and semi-natural acid-grassland
which is largely sustained by low-intensity sheep grazing. Agricultural improvement
of upland areas has occurred for many years. The methods of improvement have
varied, but generally involved the addition of lime and fertilizers (Newbould, 1985).
However, current land-use policy does not favour the continued improvement of
upland areas. The major land-use change in the uplands of Britain has been the
conversion of semi-natural vegetation to plantation forest.
Table 1 Characteristics of catchments in different upland regions of the UK.
Region
1.
2.
3.
4.
5.
6.
7.
8.
Regional
rainfall*
Number of
catchments
Average catchment:
Rainfall"
(mm)
Runoff
(mm)
Area
(km2)
Altitude
N deposition' Dominant
1
1
range (m) (kg ha" year" ) geology
Southwest
England
South
Wales
North
Wales
Pennines
Southeast
Scotland
Southwest
Scotland
Northeast
Scotland
1173
6
1935
1392
30
244-498
10-20
Granite
1313
5
1652
1190
51
121-676
20-30
Sandstone
1313
10
1995
1614
185
104-727
25-30
Shales
834
969
10
6
1089
1627
806
1098
281
182
212-743
187-771
15-20
20-25
Limestone
Shales
1419
6
1862
1340
141
28-629
20-25
Shales
973
10
1161
749
216
223-1002 10-15
Scottish
Highlands
1761
8
1970
1470
170
23-843
"Average rainfall 1961-1990 (Institute of Hydrology, 1993).
"Average for 1986-1990 (Institute of Hydrology, 1993).
Total deposition of N, 1989-1992 (INDITE, 1994).
10-15
Metamorphic
and
granite
Metamorphic
The nitrogen content of rivers in upland Britain: the significance of organic nitrogen
445
METHODS
As the uplands of Britain are distributed from the southwest of England to northern
Scotland, the climatic, vegetational, physical and other characteristics of the uplands
regions vary markedly within the country. Catchments, therefore, were selected:
(a) to include a wide range of upland regions, and (b) to cover a wide range of total
atmospheric-N deposition. Selection of the catchments for the survey was constrained
by three factors: (a) land use had to be dominated by semi-natural vegetation, (b) the
river had to be flow gauged, and (c) the river had to be routinely monitored for water
quality by the Environment Agency and the Scottish Environment Protection
Agency. In total, 61 catchments, representing eight upland regions, were included in
the survey, the characteristics of which are presented in Table 1.
Since April 1997, monthly samples of river water have been collected in 1-litre
polyethylene bottles from all catchments except those in northeast England and
southeast Scotland, where samples were collected bimonthly. On return to the
laboratory, samples were filtered through pre-washed, 0.45 um membrane filters and
all analyses was carried out less than one week after sample collection. Nitrate-N and
NH4-N were determined colorimetrically using a Technicon TRAACS auto-analyser.
Total dissolved N (TDN) was determined as N03-N after oxidation with alkaline
potassium persulphate (Williams et al, 1995). Dissolved organic N (DON) was
calculated as the difference between TDN and N03-N plus NH4-N.
Statistical analysis
Analysis of variance (ANOVA) was performed using "Genstat 5" (1990). Unless
otherwise stated levels of statistical significance were for P < 0.05. Box and whisker
plots were used to show the range of N concentrations in river water from each
upland region. The middle horizontal line of the box represents the median value.
Fifty percent of the data points lie within the box. The ends of each box delineate the
upper and lower quartiles. The whiskers show the spread of data. Outliers are
represented by a closed circle.
RESULTS
Nitrogen concentrations
The mean, median, minimum and maximum concentration of N species in all river
samples collected between April and July 1997 are summarized in Table 2. Concentrations of TDN ranged from 0.05 to 3.32 mg l"1 with a median of 0.5 mg l"1, and
90% of the samples contained less than 1.2 mg l"1. The variability of N03-N in upland
river samples was similar to that of TDN; concentrations ranged from below detection
to 3.05 mg l"1 and 90% of the samples contained less than 1.0 mg l"1. Ammonium
concentrations were typically very small (<0.12 mg l"1) and were below the detection
limit in 35% of the samples. For river samples in which NH4-N was detectable, the
median concentration was 0.023 mg l"1 and 50% of the samples contained between
P. J. Chapman et al.
446
Table 2 The mean, standard error of the mean, median, minimum and maximum concentration
(mg l"1) of nitrogen forms in upland river samples.
TDN
0.635
0.034
0.497
0.05
3.32
201
NO r N
Mean
0.443
Standard error
0.032
Median
0.318
Minimum
< 0.005
Maximum
3.05
Number of samples'
198
"Number of samples in which N fraction was detectable
NH4-N
0.027
0.002
0.023
< 0.005
0.12
130
DON
0.175
0.008
0.157
0
0.75
193
0.01 and 0.035 mg l"1. Concentrations of DON ranged from below detection to
0.78 mg l"1, and 80% of samples contained between 0.05 and 0.325 mg l"1.
Regional variation
The range of TDN concentrations in rivers samples collected from different upland
regions is shown in Fig. 1. A significant difference in TDN concentrations was
observed between regions. In particular, concentrations were smallest (mean = 0.28
mg l"1) and the least variable in the Highlands of Scotland. Low ranges and mean
concentrations of TDN also were observed in northeast Scotland, southwest England
and the Pennines. In contrast, the data from the remaining regions showed
significantly higher ranges and mean concentrations of TDN. Rivers in south Wales
had the largest concentrations of TDN (mean = 1.23 mg l"1).
Figure 2 shows the variability in N03-N, NH4-N and DON concentrations among
upland regions of Britain. Nitrate-N concentrations varied significantly among
regions (Fig. 2(a)) and displayed a similar regional pattern to that observed for TDN.
The mean concentration of N03-N was more than 12 times greater in rivers draining
the uplands of south Wales (mean = 1.02 mg l"1) than the Scottish Highlands
(mean = 0.08 mg l'1). The concentrations of NH4-N and DON showed considerably
less variation among regions and more within a region as indicated by the box and
whisker plots in Fig. 2(b) and (c). In addition, the mean concentrations of NH4-N
o
o
3 -
2 -
0
E
Q
i
1 -
0 -
èT
1
2
i
k,
3
T Ç $
4
5
6
7
8
Region
Fig. 1 Box and Whisker plots summarizing concentrations of TDN in samples of
river water collected from different upland regions of Britain. Regions as defined in
Table 1.
The nitrogen content of rivers in upland Britain: the significance of organic nitrogen
447
and DON were only 2.5 times greater in southwest Scotland (NH4-N = 0.037,
DON = 0.30 mg l'1) than the Scottish Highlands (NH4-N = 0.015 mg l1) and
northeast Scotland (DON = 0.12 mg l"1), respectively.
The relative contributions of individual N fractions to mean TDN concentrations
varied significantly among regions (Fig. 3). The proportion of N03-N varied from
25% in the Scottish Highlands to 84% in south Wales. Despite the similarity in DON
concentrations among regions, the actual contribution of DON to TDN varied from
14% in south Wales to 69% in the Scottish Highlands. Ammonium-N contributed
less than 7% to TDN in each region. Based upon the composition of TDN, the
regions can be divided into three groups: (a) those where N03-N dominates (>65%),
(b) those with approximately equal amounts of N03-N and DON, and (c) those where'
the DON fraction dominates (>65%). Southwest England, south Wales and north
Wales fall into the first group, the Scottish Highlands belong to group three, while
the remaining regions are in group two.
3 -
(a)
E
0 -
Region
(b)
0.10
0.05
o.oo -
z
o
Q
0.0 -
Region
Fig. 2 Box and Whisker plots summarizing concentrations of (a) N03-N, (b) NH4-N
and (c) DON in samples of river water collected from different upland regions of
Britain. Regions as defined in Table 1.
448
P. J. Chapman et al.
DISCUSSION
Concentrations and forms of nitrogen in upland rivers
As the majority of river water sampling programmes in Britain only analyse for
NO3-N, and in some cases NH4-N, there is a paucity of information on TDN and
DON. Therefore, the initial results from this study provide the first information on
TDN and DON concentrations in upland rivers countrywide. Concentrations of TDN
were small, generally less than 1.2 mg l"1, and varied significantly between upland
regions. The variability in TDN was accounted for by N03-N concentrations, which
were generally less than 1.0 mg l"1 and displayed a similar regional pattern to TDN.
Ammonium-N was present in very small concentrations, as observed in other studies
of upland ecosystems (e.g. Reynolds et al., 1994). Detectable levels of DON
occurred in 96% of river samples and the median concentration was 0.157 mg l"1.
Compared to N03-N, concentrations of DON displayed more variation within a
region than between regions.
The NO3-N concentrations observed in the present study were comparable with
those reported by Betton et al. (1991), who analysed river N03-N concentrations for
1980-1986 at 743 sites in Britain. Betton et al. (1991) observed that mean N03-N
concentrations were less than 2.5 mg l'1 in upland areas and were less than 1 mg l"1 in
the highlands of Dartmoor, central and north Wales, the Pennines, the Lake District
and the Southern Uplands and Highlands of Scotland. The regional pattern observed
for N03-N in the present study was consistent with that observed across the UK Acid
Waters Monitoring Network sites (Patrick et al., 1995), where N03-N concentrations
at sites in northwest and central Scotland had small mean concentrations with small
standard deviations, while sites in Wales had larger means and larger standard
deviations.
Differences in N03-N concentrations in rivers draining upland catchments may
reflect atmospheric deposition patterns (Allott et al., 1995), but they also are likely
to reflect the variability in upland catchment characteristics, which include climate,
geology, soil type and land use.
The significance of organic forms of nitrogen
This preliminary analysis of the data show that DON can contribute significantly to
TDN concentrations in upland rivers, although the proportion of DON varied among
regions, from a minimum of 14% in south Wales to a maximum of 69% in the
Scottish Highlands. This observation has a number of important implications.
Nutrient cycling at the catchment scale is usually quantified using the massbalance approach, which generally is computed as the net difference between total
inputs and outputs. This approach has been used to assess the effects of acidic
deposition and changes in land use, such as afforestation on upland ecosystems
(Hornung et al., 1990). While substantial information exists on dissolved inorganic
N fluxes from upland catchments (e.g. Reynolds & Edwards, 1995), little
consideration has been given to quantifying the contribution organic N forms make to
total catchment losses. This information is needed to accurately assess losses of N in
The nitrogen content of rivers in upland Britain: the significance of organic nitrogen
449
Results are expressed
as a percentage of TDN
NO3-N
•
NH4-N
I
I %DON
0 " Circle area is proportional ^
to mean concentration
1
(mgNI- )
'^t?ë?
la§f;
Upland regions
©
©
©
©
©
®
©
South -west England
South Wales
North Wales
« b
rffe
Pennines
SE Scotland
SW Scotland
NE Scotland
Scottish Highlands
Upland and marginal upland landscape
as defined in Bunce & Howard (1992)
'Mi
'./^.'••".Sift-'*t
150krr
X : «
^
Fig. 3 Map showing the relative contribution of N fractions to the mean total
dissolved N concentration in rivers draining different upland regions of Britain.
drainage waters from upland catchments (INDITE, 1994). The need for these data
has been emphasized by recent studies where increased leaching of organic N was
observed after clear-felling (Stevens & Wannop, 1987) and from intact peat turfs
collected along an increasing atmospheric N deposition gradient (Yesmin et al.,
450
P. J. Chapman et al.
1995). Consequently assessment of environmental impacts on TDN losses from
upland ecosystems need to consider not only the dissolved inorganic species but also
the contribution from DON. The possible implications that increased leaching of
DON may have on water quality and, in particular, eutrophication have not been
assessed adequately (INDITE, 1994).
Acknowledgements This research is funded by the Natural Environment Research
Council (Grant GT5/96/2/FS) and the Scottish Office Agriculture, Environment and
Fisheries Department. Water samples are collected by the Institute of Hydrology,
Institute of Terrestrial Ecology, the Environment Agency in England in Wales and
the Scottish Environment Protection Agency; thanks to all the staff involved for their
help. Thanks also to Henry and Lucy Chapman for collecting water samples and
Yvonne Cook for help with the chemical analysis.
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