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Jones et al 2004
598 Original Paper
Changes in Vegetation and Soil Characteristics in Coastal Sand
Dunes along a Gradient of Atmospheric Nitrogen Deposition
M. L. M. Jones1, H. L. Wallace2, D. Norris1, S. A. Brittain1, S. Haria1, R. E. Jones3, P. M. Rhind3,
B. R. Reynolds1, and B. A. Emmett1
1
Centre for Ecology and Hydrology Bangor, Orton Building, Deiniol Road, Bangor, Gwynedd LL57 2UP, UK
2
Ecological Surveys (Bangor), The school house, Canon Pyon, Herefordshire, HR4 8PF, UK
3
Countryside Council for Wales, Maes-y-Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW, UK
Received: February 13, 2004; Accepted: May 6, 2004
Abstract: A field survey was conducted to detect signals of at- approximately doubled in Europe over a similar timescale and
mospheric nitrogen (N) in 11 dune systems along a nitrogen is implicated in biological changes in other oligotrophic habi-
deposition gradient in the United Kingdom. In the mobile and tats such as Dutch heathlands (Heil and Diemont, 1983; van
semi-fixed dunes, above-ground biomass was positively related der Eerden et al., 1991) and the UK uplands (Haines-Young et
to N inputs. This increase was largely due to increased height al., 2000). While the changes in dune vegetation may also be
and cover of Ammophila arenaria. In the long term, this in- due to climatic influences, changes in management practices,
creased biomass may lead to increased organic matter accumu- and a general decline in natural and managed grazing (Dargie,
lation and consequently accelerated soil development. In the 1995; Radley, 1994; Ranwell, 1960), it was concern about the
fixed dunes, above ground biomass also showed a positive rela- effects of atmospheric nitrogen which initiated this study. Ear-
tionship with N inputs as did soil C : N ratio while soil available N ly nutrient addition experiments (Pemadasa and Lovell, 1974;
was negatively related to N inputs. Plant species richness was Willis, 1963; Willis, 1965) were of short duration, using large
negatively related to N inputs. In the dune slacks, while soil and dose, high concentration nutrient applications. While useful
bulk vegetation parameters showed no relationship with N in- in highlighting which species may respond to N enrichment,
puts, cover of Carex arenaria and Hypochaeris radicata increased. they are of limited use in understanding the long-term conse-
Site mean Ellenberg N numbers showed no relationship with N quences of continuous, low concentration atmospheric inputs,
deposition either within habitats or across the whole dataset. although work in the Netherlands has begun to address these
Neither abundance-weighting nor inclusion of the Siebel num- issues (ten Harkel et al., 1998; ten Harkel and van der Meulen,
bers for bryophytes improved the relationship. The survey re- 1996).
veals that the relationships of soil and vegetation with atmo-
spheric N deposition vary between sand dune habitats but, de- There is therefore a need to assess the broad impacts of N de-
spite this variability, clear correlations with N inputs exist. While position on dune habitats and this study utilised a field survey
this survey cannot establish causality, on the basis of the rela- approach to take samples from sites representing a gradient
tionships observed we suggest a critical load range of 10 ± of nitrogen deposition. The aims were to 1) detect signals of
20 kg N ha±1 yr±1 for coastal sand dunes in the UK. atmospheric N inputs in the soil and vegetation; 2) test the
suitability of Ellenberg N index values for assessing nitrogen
Key words: Critical loads, global change, Ellenberg indicator impacts in other dune systems in the UK and; 3) suggest a crit-
values, eutrophication, C : N ratios. ical load range for UK sand dunes.
Materials and Methods
Introduction Ten sites were initially chosen to encompass a gradient of N
deposition and a range of geographical locations around Eng-
Over the last 30 to 40 years, many UK sand dunes have land and Wales. Over the course of the field survey, one extra
changed from being actively mobile systems to landscapes site, Great Yarmouth, was included to increase representation
with fairly continuous vegetation cover and only minor areas of the lichen rich dune grassland communities in Norfolk. One
of blowouts or shifting dunes (Hope-Simpson, 1997; Rhind et year later, soil samples were obtained from three relatively low
al., 2001). This relatively rapid stabilisation, the loss of early N sites in the Outer Hebrides, Scotland for comparison. The
successional habitats, and the observed increase at many sites sites are characterised in Table 1.
of tall rank grassland and woody species is causing concern
among conservation agencies. Atmospheric N deposition has Calculating nitrogen deposition
The N deposition data for each site (Table 1) were calculated
Plant Biol. 6 (2004): 598 ± 605 using wet deposition data for the UK for 1997, extrapolated
Georg Thieme Verlag KG Stuttgart ´ New York from a network of monitoring sites (NEGTAP, 2001); combined
DOI 10.1055/s-2004-821004 ´ Published online July 21, 2004 with dry deposition data based on diffusion tube measure-
ISSN 1435-8603 ments of NO2 and NH3 concentrations at each dune site for
Nitrogen Deposition on Dunes Plant Biology 6 (2004) 599
Table 1 Characterisation of the eleven sand dune sites surveyed in England and Wales, and three sites from the Outer Hebrides, Scotland includ-
ed for comparison
Site name Number Approximate latitude pH range Range of available Calculated nitrogen
of sample and longitude soil phosphorus deposition
points (mg 100 g dry soil±1) (kg N ha±1 yr±1)
Wales
Newborough Warren, Ynys Mon 8 53808¢N, 4821¢W 4.5 ± 7.2 0.033 ± 0.234 12.35
Morfa Harlech, Meirionedd 8 52852¢N, 4808¢W 4.9 ± 7.1 0.062 ± 0.088 12.22
Pembrey Coast, Carmarthenshire 8 51840¢N, 4817¢W 5.7 ± 6.9 0.034 ± 0.065 10.31
Kenfig Burrows, Glamorgan 8 51831¢N, 3844¢W 5.7 ± 6.7 0.063 ± 0.088 11.02
Merthyr Mawr, Glamorgan 8 51829¢N, 3838¢W 5.8 ± 6.7 0.066 ± 0.092 15.38
England
Ainsdale, Lancashire 8 53835¢N, 3804¢W 4.5 ± 6.6 0.042 ± 0.422 12.86
Studland, Dorset 4 50840¢N, 1857¢W 3.1 ± 6.0 0.042 ± 0.359 15.63
Great Yarmouth, Norfolk 4 52838¢N, 1845¢E 3.8 ± 6.6 not measured 19.49 a
Winterton, Norfolk 7 52843¢N, 1842¢E 3.3 ± 6.5 0.068 ± 0.379 29.40
Seaton, County Durham 7 54839¢N, 1810¢W 5.7 ± 6.7 0.271 ± 0.345 25.22
Bamburgh, Northumberland 8 55836¢N, 1842¢W 5.5 ± 6.7 0.080 ± 0.117 19.33
Scotland (Hebrides)
Baleshare, North Uist 3 57831¢N, 7827¢W 6.3 ± 6.8 not measured 9.8 b
Borve, Benbecula 3 57826¢N, 7824¢W 6.4 ± 7.1 not measured 8.5 b
Tobha Mor, South Uist 3 57817¢N, 7822¢W 6.5 ± 7.0 not measured 6.9 b
a
Nitrogen deposition value for Great Yarmouth estimated using regression relationship of moss tissue N against calculated N inputs established from the other sites
b
Nitrogen deposition estimated from NEGTAP (2001)
May and July, 2001 which were taken to represent an annual tion clipped to ground level for above ground biomass and tis-
mean. This approach was validated by regression with tissue sue N determination. This was increased to 50 50 cm in the
N content of the moss Hypnum cupressiforme and gave a strong mobile dunes to ensure a representative sampling area. All soil
correlation (R2 = 36.6 %, p = 0.084). As NH3 and NO2 concentra- samples were stored in a 4 8C cold-room prior to laboratory
tions were not measured at Great Yarmouth, N deposition for analysis. At each site, a number of moss samples were taken
this site was calculated by regression using tissue N content for tissue N analysis. Hypnum cupressiforme was the only
of moss samples. Full details of estimating the N deposition widespread moss species and was present at all sites except
for this study can be found in Jones et al. (2002 a, b). Nitro- Seaton. Therefore, only this species was used for tissue N de-
gen deposition for the Hebridean sites was calculated from terminations.
NEGTAP (NEGTAP, 2001). These sites were not used in the anal-
ysis and are included for comparison only. Analysis of vegetation samples
Field survey methodology Biomass samples were oven dried at 65 8C to constant weight
before analysis. Sub-samples of total above-ground biomass
The field survey was conducted in May 2002, although some were ground to 0.5 mm and analysed for %N content on a Leco
additional soil samples collected from the Outer Hebrides in 2000 CHN analyser. Further sub-samples from the fixed dune
June 2003 are included in the results. Within each sand dune grasslands were analysed for nitrogen (%N) and phosphorus
system, sampling of soil and vegetation was conducted along (%P) content by Kjeldahl acid digest followed by dilution and
a transect inland from the coast, stratified to incorporate sam- analysis by ion chromatography, in order to determine N : P
ples, where present, from the following habitat types: mobile ratios. N pool data are presented as kg N ha±1. Samples of the
dunes; semi-fixed dunes; fixed dunes and established dune moss H. cupressiforme were prepared by cleaning and separat-
grassland; and dune slacks. Eight survey points were sampled ing the live portion of shoots (approximately 3 cm of shoot).
along each transect, although only four were conducted at The samples were then ground to a fine powder prior to analy-
Studland and Great Yarmouth, and seven conducted at Winter- sis for %N content on a Leco 2000 CHN analyser.
ton and Seaton due to the homogeneity of the vegetation com-
munities and the lack of dune slacks at these sites. At each sur- Analysis of soil samples
vey point an area of homogenous vegetation was identified
and two, 2 2 m, quadrats established within an area of ca. The two replicate cores from each vegetation quadrat were
20 m radius. Sward height at three random points was meas- bulked for chemical analysis. The soil horizons were not sep-
ured and percentage cover of all vascular plants, bryophytes, arated. Soil pH was measured in 0.01 M CaCl2 solution (1 : 2.5
and lichens within the quadrat was determined. On two cor- w.b.v.). Soil moisture and Loss on Ignition (LOI) were deter-
ners of each quadrat, a 5-cm diameter, 15-cm depth soil core mined by drying and re-weighing soil samples at oven temper-
was removed for soil analysis, and a 25 25 cm area of vegeta- atures of 105 8C and 375 8C for 12 hours, respectively (MAFF,
600 Plant Biology 6 (2004) M. L. M. Jones et al.
1986), with the oven temperature for LOI reduced to 375 8C to Table 2 Site mean values for soil and vegetation parameters meas-
minimise CO2 losses from CaCO3. Available NH4 and NO3 were ured during the survey, split by habitat type. Data in each cell show
determined by extraction with 1.0 M KCl. The filtrates were an- the mean across all sites (top line) and the variation between site
alysed using continuous segmented-flow colorimetry. Avail- means (minimum and maximum values ± bottom line). Parameters
showing a significant regression relationship with nitrogen inputs are
able PO4 was determined by the Olsen P method. The resulting
shown in bold. The sign (+/±) indicates the slope of the relationship.
filtrates were analysed on an auto analyser using the molyb- (+/±) p < 0.1; +/± p < 0.05; ++/±± p < 0.01; +++/±±± p < 0.001. Those
date blue method. The total N and total C contents of soil were with a hash (#) represent variables where the effect of soil pH was sig-
determined on air dried soil using a Leco 2000 CHN analyser. nificant and pH was included as the first term in the regression in order
However, total N contents in soils from the mobile and semi- to remove its effect from that of the nitrogen gradient
fixed dunes were below detectable limits for this machine
and N contents for these samples were determined by Kjeldahl Mobile and Fixed dunes Dune slacks
semi-fixed and dune
acid digest followed by dilution and analysis using ion chro-
dunes grasslands
matography. In all analyses, Quality Assurance was conducted
by inclusion of laboratory blanks, duplicates, and standard Vegetation
soils. N pool data are expressed as kg N ha±1 for the top 15 cm Biomass (g m±2) 1349 + # 1164 + # 1274
soil layer. All other data are expressed as mg 100 g dry soil±1. (571 ± 2463) (408 ± 1956) (664 ± 1848)
Sward height (cm) 57.5 (+) 22.0 27.5
Statistical analysis (27.5 ± 89.2) (2.0 ± 43.4) (16.2 ± 41.5)
The vegetation quadrat data were analysed by detrended cor- Tissue N content 0.84 1.21 1.19
respondence analysis (DCA) using CANOCO for Windows 4.5. (%) (0.62 ± 1.01) (1.03 ± 1.49) (0.94 ± 1.67)
The DCA revealed strong differences between the principal Vegetation N pool 111 ++ # 132 (+) 148
habitat types. Therefore for the majority of the analyses, the (kg N ha±1) (58 ± 200) (49 ± 202) (76 ± 223)
dataset of soil and vegetation parameters was subdivided into
three main categories on the basis of the groupings observed. Species richness 14.3 21.0 ± 18.8
These were 1) mobile and semi-fixed dunes, 2) fixed dunes (9.0 ± 20.5) (13.1 ± 28.8) (12.8 ± 28.5)
and established dune grasslands, and 3) dune slacks. As the Ellenberg N 3.9 3.7 3.8
broad aim of this study was to assess surveyed parameters for (3.1 ± 4.3) (2.5 ± 4.4) (3.6 ± 4.1)
effects of N deposition and as each site has only one deposition
value, measured parameters of soil and vegetation were aver- Soil
aged to give a mean for each habitat at each site. The relation- Available N (mg 0.24 0.65 ± 0.47
ship between site parameters and N deposition was examined 100 g dry soil±1) (0.12 ± 0.43) (0.21 ± 1.13) (0.16 ± 0.79)
using linear regression. Each parameter was also checked for
Available N 59.0 22.9 12.4
significant soil pH effects. Where significant relationships with (mg 100 g organic (33.2 ± 92.7) (11.2 ± 38.9) (2.9 ± 26.0)
pH occurred, pH was included as the first term of the regres- matter ±1)
sion to separate these effects from those of atmospheric N. El- Soil total N (%) 0.032 0.099 0.216
lenberg numbers for vascular plant species were taken from
(0.007 ± 0.070) (0.034 ± 0.158) (0.088 ± 0.385)
Hill s modified Ellenberg numbers for the UK (Hill et al., 1999)
and an equivalent index from Siebel (1993) was included for Soil N pool 674 1604 2578
bryophyte species. Mean and abundance-weighted values per (kg N ha±1) (146 ± 1251) (722 ± 2553) (1337 ± 4365)
quadrat were calculated for vascular plants on their own and Soil organic matter 0.64 3.34 5.13
vascular plants plus bryophytes. The relationships between El- (OM) (% loss on (0.22 ± 1.47) (1.20 ± 7.89) (2.74 ± 10.24)
lenberg N and a number of soil parameters were assessed us- ignition)
ing linear regression. Ellenberg N was strongly related to soil Soil C : N ratio not analysed 22.1 + 23.1
pH (R2 = 19.9 %, p < 0.001). Therefore, soil pH was included as (14.4 ± 37.7) (18.4 ± 28.4)
the first term of the regression in order to remove the effects
of pH from the analysis. The mean Ellenberg N indicator for
each site was also calculated and the relationship with N dep-
osition assessed using linear regression as above. Unless other- very low levels of soil %N and soil % C. Mean Ellenberg N values
wise stated, all statistical tests were carried out using Minitab in the mobile dunes were not related to N inputs. There was a
13.32. trend of increased percentage cover of Ammophila arenaria in
response to N inputs (Table 3) although this was not significant
at the 95 % level. None of the other species showed a significant
Results
response.
Mobile and semi-fixed dunes
Fixed dune grasslands
There was a significant positive relationship of N inputs with
above ground biomass (Table 2, Fig. 1 a), and consequently There was a significant positive relationship between N inputs
with the vegetation N pool. Although not significant at the and above ground biomass (Fig. 2 a) once the effects of pH had
95% level, sward height appeared to increase with N inputs to been removed, and a positive increase in the vegetation N pool,
a maximum of roughly 80 cm (Fig. 1 b). There were no signifi- although this was not significant at the 95% level (Table 2).
cant relationships between N inputs and any soil parameters. There was also a significant negative relationship between N
C : N ratios were not calculated for these habitats due to the inputs and species richness (Fig. 2 b). N inputs were negatively
Nitrogen Deposition on Dunes Plant Biology 6 (2004) 601
3000 120
Above ground biomass (g/m )
2
2500 100
Sward height (cm)
2000 80
1500 60
1000 40
500 20
0 0
0 10 20 30 40 0 10 20 30 40
a Calculated nitrogen deposition b Calculated nitrogen deposition
(kg N ha–1y–1) (kg N ha –1y–1)
Fig. 1 Data from the mobile and semi-fixed dunes in relation to N in- (cm). Filled diamonds represent calcareous sites, open squares repre-
puts showing (a) above ground biomass (gm±2) and (b) sward height sent acidic sites. Bars show 1 s.e.
Table 3 Site mean values of percentage cover of the four most com- Ellenberg N numbers and N : P ratios
mon species, split by habitat type. For description of layout, refer to
legend in Table 2 Using regression to allow for the effects of pH, and including
the full data set of all 156 quadrats and associated soil and veg-
Mobile and Fixed dunes Dune slacks etation samples, the mean Ellenberg N numbers were nega-
semi-fixed and dune
tively related to soil available N per gram organic matter in
dunes grasslands
the mobile dunes (R2 = 20.2 %, p = 0.039), and positively related
Ammophila arenaria 42.4 (+) 8.8 not analysed to soil %N (R2 = 36.2 %, p = 0.016) and to soil organic matter
(31.3 ± 55.5) (0 ± 30.1) (R2 = 36.3 %, p = 0.022) in the fixed dunes. Ellenberg N numbers
did not relate significantly to any of the soil and vegetation
Carex arenaria 2.7 # 4.8 + 2.3 +
parameters in the dune slacks. Despite the significant relation-
(0 ± 8.8) (0.3 ± 25.4) (0 ± 7.3)
ships of atmospheric N deposition with various soil and vege-
Festuca rubra 14.5 # 17.9 4.4 tation parameters in the mobile and fixed dunes, the mean
(5.3 ± 26.7) (0 ± 41.7) (0 ± 14.0) Ellenberg N number per site was not significantly related to
Hypochaeris radicata # 3.2 # 4.7 0.3 ++ atmospheric N deposition in any of the habitats, or across the
(0.3 ± 9.3) (0.3 ± 17.5) (0 ± 1.0) data set as a whole. Neither abundance-weighting nor inclu-
sion of the Siebel bryophyte numbers improved the relation-
ship of mean Ellenberg N with atmospheric N deposition. N : P
ratios of total above ground vegetation in the fixed dune grass-
lands ranged from 5.0 to 15.6 indicating borderline P limitation
related to soil available N (Fig. 2 c) and positively related to soil in some samples. However, ratios varied strongly both within
C : N ratio (Fig. 2 d). However, neither the total N in the soil nor sites and between sites. There was no significant relationship
the available N content of the organic matter showed any rela- between soil available P and N : P ratios in the vegetation, and
tionship to N inputs. Mean Ellenberg N values in the fixed there was no relationship between site mean N : P ratios and N
dunes were not related to N inputs. Carex arenaria showed a deposition.
significant positive response to N inputs and was the only spe-
cies in this habitat to do so (Table 3).
Discussion
Dune slacks Mobile and semi-fixed dunes
There were no significant relationships of N inputs with any of Results suggest that there is a relationship between N deposi-
the soil or vegetation parameters in the dune slacks (Table 2). tion and biomass in the mobile dunes, largely accounted for by
Mean Ellenberg N values in the dune slacks were not related to an increase in sward height and an increase in cover of the
N inputs. However, percentage cover of both C. arenaria and dominant species A. arenaria. Studies aimed at improving
Hypochaeris radicata showed a significant positive response dune stabilisation showed that the classic European dune-
to N inputs (Table 3). binding species (A. arenaria, Elytrigia juncea and Leymus are-
narius) all responded to fertiliser applications (Adriani and Ter-
windt, 1974; Greipsson and Davy, 1997), as did a study on Am-
602 Plant Biology 6 (2004) M. L. M. Jones et al.
3000 35
Above ground biomass (g/m )
2
30
2500
25
Species richness
2000
20
1500
15
1000 10
500 5
0 0
0 10 20 30 40 0 10 20 30 40
a Calculated nitrogen deposition b Calculated nitrogen deposition
(kg N ha –1y–1) (kg N ha–1y –1)
2 50
Available N (m g/100 g dry soil)
1.8 45
1.6 40
1.4 35
Soil C:N r atio
1.2 30
1 25
0.8 20
0.6 15
0.4 10
0.2 5
0 0
0 10 20 30 40 0 10 20 30 40
c Calculated nitrogen deposition d Calculated nitrogen deposition
(kg N ha –1 y –1 ) (kg N ha –1 y –1)
Fig. 2 Data from the fixed dunes and dune grasslands in relation to N diamonds represent calcareous sites, open squares represent acidic
inputs showing (a) above ground biomass (gm±2), (b) species richness, sites, triangles show soils from the Outer Hebrides for comparison
(c) soil available N (mg 100 g dry soil±1), and (d) soil C : N ratio. Filled (not included in the analysis). Bars show 1 s.e.
mophila breviligulata in Canada (Boudreau and Houle, 2001), Fixed dune grasslands
although a Canadian study on foredune vegetation showed no
increase in plant growth following N and P additions (Houle, The negative relationship of N inputs with available N is sur-
1997). Pot studies have also shown that A. arenaria increases prising. A plausible hypothesis is that atmospheric deposition
tillering and leaf length with increased nitrogen supply (Pavlik, is reducing the activity of nitrogen fixing organisms and there-
1983). The long-term implications of this increased biomass fore nitrate production. Inputs from N-fixation by cyanobacte-
may therefore be to accelerate rates of organic matter accumu- ria on damp sand in dune grassland have been estimated at
lation and consequently soil development. This will lead to a 25 kg N ha±1 yr±1 (Stewart, 1967), although extrapolation of
reduction in the area of the mobile components of these dune data from Johnson (Johnson, 1979) suggests that inputs from
systems, and more rapid stabilisation of an ecosystem whose legumes in dune grasslands may only be in the order of 5 kg N
long-term existence is dependent on the dynamics of wind- ha±1 yr±1. Nevertheless, these amounts are equivalent to inputs
blown sand. A further consequence of more rapid nitrogen ac- from atmospheric deposition and represent a significant part
cumulation in the soil may be to alter the course, or even the of the N budget. An alternative hypothesis may be that the
endpoint, of the usual dune succession. increased plant growth is leading to depletion of available N
in the soil. The soil C : N ratios show a similar range to those
reported by Kooijman and Besse (2002), although the positive
relationship with N inputs is again surprising. A similar in-
Nitrogen Deposition on Dunes Plant Biology 6 (2004) 603
crease in C : N ratios has been shown in some forest soils likely that P limitation has dramatically influenced the ob-
(Billett et al., 1990). While these relationships appear counter- served relationships with N deposition.
intuitive, they are reinforced by the soil data from relatively
pristine air-quality sites in the Outer Hebrides. Berendse The impacts of N deposition have been mentioned by several
(1998) suggests that during the course of succession, as more authors in relation to dunes in north west Europe (Dopheide
woody species appear, the litter contains more phenols and or- and Verstraten, 1995; Ketner-Oostra, 2001; Ovesen, 2001; van
ganic compounds and decomposition rates slow-down. How- der Laan, 1985) but these impacts have rarely been quantified,
ever, there was no obvious shift towards more woody species with the exception of Dopheide and Verstraten (1995). This is
at the high N sites. Increased N deposition can also result in therefore the first published systematic survey which identi-
slower long-term litter decomposition in N-saturated forest fies relationships between N inputs and the soil and vegetation
systems as the C-supply becomes limiting for microbial degra- of dune systems.
dation (Berg et al., 1998). There is clearly further research
needed to elucidate the mechanisms behind the observed rela- Critical Loads
tionship of N with C : N ratios.
This survey has shown relationships of a number of param-
Dune slacks eters with N inputs which include: increased biomass in the
mobile and fixed dunes; increased cover of certain species in
The lack of significant responses of biomass and soil param- the fixed dunes and dune slacks; reduced N availability and in-
eters in the dune slacks is surprising, since Willis (1963) creasing C : N ratio in the fixed dunes. While this survey only
showed that dune slack vegetation at Braunton Burrows re- shows an association and not causality, on the basis of these
sponded to N applications. However, the increases in cover of results it appears likely that the sites with higher N deposition
C. arenaria and H. radicata show that individual species may are exceeding the critical load for nutrient nitrogen for sand
be responding to increased N deposition, even if major vegeta- dunes. In many of the variables where significant relationships
tion changes are not evident across the N deposition gradient. with N inputs were observed, a threshold at around 15 kg N
This may, in part, be due to the absence of dune slacks at two of ha±1 yr±1 seemed to differentiate between the response of the
the high N sites. cluster of sites experiencing low N inputs and the responses
of those sites experiencing higher loads of N deposition. This
Ellenberg indicator values and N : P ratios reinforces findings by Dutch authors using a mesocosm ap-
proach (Tomassen et al., 1999). Therefore, acknowledging the
Ellenberg N values in this study did not reveal changes in the precautionary principle behind the critical load concept, and
vegetation in response to the N deposition gradient. As in other the need to account for different base status and degree of
studies (Hill and Carey, 1997; Melman et al., 1988; Pitcairn et management at sites (Achermann and Bobbink, 2003), a pre-
al., 2003), neither abundance-weighting nor inclusion of the liminary critical load range of 10 ± 20 kg N ha±1 yr±1 is proposed
Siebel indicators for bryophytes improved the relationship for mobile and fixed sand dune systems. There are not enough
with N deposition. Due to the variety of species and vegetation data to suggest a critical load for humid dune slacks. This sug-
communities encountered across the survey, it was hoped that gested critical load is an important first step towards providing
Ellenberg N values would be able to provide a unifying ap- protection of a valuable European habitat which, until now,
proach to analysing potential N responses. However, this ap- has not been included in the critical loads manual (Achermann
proach was not successful. As identified by Hill and Carey and Bobbink, 2003). However, further studies are necessary to
(1997) and Schaffers and Sykora (2000), Ellenberg N seems to validate the suggested critical loads for dry dune habitats and,
be a general indicator of soil fertility rather than specifically a in particular, to examine potential effects of N in dune slacks.
nitrogen index. Secondly, in habitats such as the mobile dunes,
the available species pool is restricted to those species capable In conclusion, the results of this field survey suggest that at-
of surviving the harsh physical conditions of water stress, sal- mospheric N deposition is having an effect on soil and vegeta-
inity, and disturbance, thus limiting potential changes to com- tion in some sand dune sites in England and Wales. However,
munity composition in response to N inputs. Despite their use- there does not appear to be one single indicator of excess N
fulness in detecting change over time in other dune systems deposition which is common to all sites, and different varia-
(van der Maarel et al., 1985) and eutrophication effects in for- bles respond in different dune habitats. Integrated indices
ests (Diekmann and DuprØ, 1997), they were of limited use in such as Ellenberg N values have not proved to be of use in this
this study, and a recent UK study along a local gradient of NH3 survey. There is considerable work required to improve our un-
deposition (Pitcairn et al., 2003) also showed no significant re- derstanding of the long term consequences of enhanced N
lationship with Ellenberg N values. deposition in the dry dunes and in dune slacks. There is also a
need to build on the results of this survey to develop reliable
Phosphorus availability may limit community responses to N indicators of excess N deposition in dune systems.
deposition and N : P ratios can be used to determine which
element is more limiting to plant growth, with ratios above Acknowledgements
16 indicating P limitation (Koerselman and Meuleman, 1996).
In this study the N : P ratios did not suggest greater P limitation This survey was co-funded by Countryside Council for Wales
with increasing N deposition, and observed vegetation re- (CCW) and English Nature (EN) and thanks are due to staff in
sponses were reasonably consistent across the N deposition these agencies for providing information and acting as central
gradient. The two sites with the highest N deposition had rela- contacts. Thanks are also due to wardens and staff at the study
tively high available P levels in the soil which may have con- sites who installed and collected the diffusion tubes. Thanks
tributed to these sites remaining N limited. Therefore it is un- go to colleagues at CEH Banchory for collecting soil sam-
604 Plant Biology 6 (2004) M. L. M. Jones et al.
ples from the Hebrides. Advice on interpretation of the NO2 Jones, M. L. M., Hayes, F., Brittain, S. A., Haria, S., Williams, P. D.,
and NH3 concentration data was helpfully provided by Mark Ashenden, T. W., Norris, D. A., and Reynolds, B. (2002 a) Changing
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Changes in Vegetation and Soil Characteristics in Coastal Sand
Dunes along a Gradient of Atmospheric Nitrogen Deposition
M. L. M. Jones1, H. L. Wallace2, D. Norris1, S. A. Brittain1, S. Haria1, R. E. Jones3, P. M. Rhind3,
B. R. Reynolds1, and B. A. Emmett1
1
Centre for Ecology and Hydrology Bangor, Orton Building, Deiniol Road, Bangor, Gwynedd LL57 2UP, UK
2
Ecological Surveys (Bangor), The school house, Canon Pyon, Herefordshire, HR4 8PF, UK
3
Countryside Council for Wales, Maes-y-Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW, UK
Received: February 13, 2004; Accepted: May 6, 2004
Abstract: A field survey was conducted to detect signals of at- approximately doubled in Europe over a similar timescale and
mospheric nitrogen (N) in 11 dune systems along a nitrogen is implicated in biological changes in other oligotrophic habi-
deposition gradient in the United Kingdom. In the mobile and tats such as Dutch heathlands (Heil and Diemont, 1983; van
semi-fixed dunes, above-ground biomass was positively related der Eerden et al., 1991) and the UK uplands (Haines-Young et
to N inputs. This increase was largely due to increased height al., 2000). While the changes in dune vegetation may also be
and cover of Ammophila arenaria. In the long term, this in- due to climatic influences, changes in management practices,
creased biomass may lead to increased organic matter accumu- and a general decline in natural and managed grazing (Dargie,
lation and consequently accelerated soil development. In the 1995; Radley, 1994; Ranwell, 1960), it was concern about the
fixed dunes, above ground biomass also showed a positive rela- effects of atmospheric nitrogen which initiated this study. Ear-
tionship with N inputs as did soil C : N ratio while soil available N ly nutrient addition experiments (Pemadasa and Lovell, 1974;
was negatively related to N inputs. Plant species richness was Willis, 1963; Willis, 1965) were of short duration, using large
negatively related to N inputs. In the dune slacks, while soil and dose, high concentration nutrient applications. While useful
bulk vegetation parameters showed no relationship with N in- in highlighting which species may respond to N enrichment,
puts, cover of Carex arenaria and Hypochaeris radicata increased. they are of limited use in understanding the long-term conse-
Site mean Ellenberg N numbers showed no relationship with N quences of continuous, low concentration atmospheric inputs,
deposition either within habitats or across the whole dataset. although work in the Netherlands has begun to address these
Neither abundance-weighting nor inclusion of the Siebel num- issues (ten Harkel et al., 1998; ten Harkel and van der Meulen,
bers for bryophytes improved the relationship. The survey re- 1996).
veals that the relationships of soil and vegetation with atmo-
spheric N deposition vary between sand dune habitats but, de- There is therefore a need to assess the broad impacts of N de-
spite this variability, clear correlations with N inputs exist. While position on dune habitats and this study utilised a field survey
this survey cannot establish causality, on the basis of the rela- approach to take samples from sites representing a gradient
tionships observed we suggest a critical load range of 10 ± of nitrogen deposition. The aims were to 1) detect signals of
20 kg N ha±1 yr±1 for coastal sand dunes in the UK. atmospheric N inputs in the soil and vegetation; 2) test the
suitability of Ellenberg N index values for assessing nitrogen
Key words: Critical loads, global change, Ellenberg indicator impacts in other dune systems in the UK and; 3) suggest a crit-
values, eutrophication, C : N ratios. ical load range for UK sand dunes.
Materials and Methods
Introduction Ten sites were initially chosen to encompass a gradient of N
deposition and a range of geographical locations around Eng-
Over the last 30 to 40 years, many UK sand dunes have land and Wales. Over the course of the field survey, one extra
changed from being actively mobile systems to landscapes site, Great Yarmouth, was included to increase representation
with fairly continuous vegetation cover and only minor areas of the lichen rich dune grassland communities in Norfolk. One
of blowouts or shifting dunes (Hope-Simpson, 1997; Rhind et year later, soil samples were obtained from three relatively low
al., 2001). This relatively rapid stabilisation, the loss of early N sites in the Outer Hebrides, Scotland for comparison. The
successional habitats, and the observed increase at many sites sites are characterised in Table 1.
of tall rank grassland and woody species is causing concern
among conservation agencies. Atmospheric N deposition has Calculating nitrogen deposition
The N deposition data for each site (Table 1) were calculated
Plant Biol. 6 (2004): 598 ± 605 using wet deposition data for the UK for 1997, extrapolated
Georg Thieme Verlag KG Stuttgart ´ New York from a network of monitoring sites (NEGTAP, 2001); combined
DOI 10.1055/s-2004-821004 ´ Published online July 21, 2004 with dry deposition data based on diffusion tube measure-
ISSN 1435-8603 ments of NO2 and NH3 concentrations at each dune site for
Nitrogen Deposition on Dunes Plant Biology 6 (2004) 599
Table 1 Characterisation of the eleven sand dune sites surveyed in England and Wales, and three sites from the Outer Hebrides, Scotland includ-
ed for comparison
Site name Number Approximate latitude pH range Range of available Calculated nitrogen
of sample and longitude soil phosphorus deposition
points (mg 100 g dry soil±1) (kg N ha±1 yr±1)
Wales
Newborough Warren, Ynys Mon 8 53808¢N, 4821¢W 4.5 ± 7.2 0.033 ± 0.234 12.35
Morfa Harlech, Meirionedd 8 52852¢N, 4808¢W 4.9 ± 7.1 0.062 ± 0.088 12.22
Pembrey Coast, Carmarthenshire 8 51840¢N, 4817¢W 5.7 ± 6.9 0.034 ± 0.065 10.31
Kenfig Burrows, Glamorgan 8 51831¢N, 3844¢W 5.7 ± 6.7 0.063 ± 0.088 11.02
Merthyr Mawr, Glamorgan 8 51829¢N, 3838¢W 5.8 ± 6.7 0.066 ± 0.092 15.38
England
Ainsdale, Lancashire 8 53835¢N, 3804¢W 4.5 ± 6.6 0.042 ± 0.422 12.86
Studland, Dorset 4 50840¢N, 1857¢W 3.1 ± 6.0 0.042 ± 0.359 15.63
Great Yarmouth, Norfolk 4 52838¢N, 1845¢E 3.8 ± 6.6 not measured 19.49 a
Winterton, Norfolk 7 52843¢N, 1842¢E 3.3 ± 6.5 0.068 ± 0.379 29.40
Seaton, County Durham 7 54839¢N, 1810¢W 5.7 ± 6.7 0.271 ± 0.345 25.22
Bamburgh, Northumberland 8 55836¢N, 1842¢W 5.5 ± 6.7 0.080 ± 0.117 19.33
Scotland (Hebrides)
Baleshare, North Uist 3 57831¢N, 7827¢W 6.3 ± 6.8 not measured 9.8 b
Borve, Benbecula 3 57826¢N, 7824¢W 6.4 ± 7.1 not measured 8.5 b
Tobha Mor, South Uist 3 57817¢N, 7822¢W 6.5 ± 7.0 not measured 6.9 b
a
Nitrogen deposition value for Great Yarmouth estimated using regression relationship of moss tissue N against calculated N inputs established from the other sites
b
Nitrogen deposition estimated from NEGTAP (2001)
May and July, 2001 which were taken to represent an annual tion clipped to ground level for above ground biomass and tis-
mean. This approach was validated by regression with tissue sue N determination. This was increased to 50 50 cm in the
N content of the moss Hypnum cupressiforme and gave a strong mobile dunes to ensure a representative sampling area. All soil
correlation (R2 = 36.6 %, p = 0.084). As NH3 and NO2 concentra- samples were stored in a 4 8C cold-room prior to laboratory
tions were not measured at Great Yarmouth, N deposition for analysis. At each site, a number of moss samples were taken
this site was calculated by regression using tissue N content for tissue N analysis. Hypnum cupressiforme was the only
of moss samples. Full details of estimating the N deposition widespread moss species and was present at all sites except
for this study can be found in Jones et al. (2002 a, b). Nitro- Seaton. Therefore, only this species was used for tissue N de-
gen deposition for the Hebridean sites was calculated from terminations.
NEGTAP (NEGTAP, 2001). These sites were not used in the anal-
ysis and are included for comparison only. Analysis of vegetation samples
Field survey methodology Biomass samples were oven dried at 65 8C to constant weight
before analysis. Sub-samples of total above-ground biomass
The field survey was conducted in May 2002, although some were ground to 0.5 mm and analysed for %N content on a Leco
additional soil samples collected from the Outer Hebrides in 2000 CHN analyser. Further sub-samples from the fixed dune
June 2003 are included in the results. Within each sand dune grasslands were analysed for nitrogen (%N) and phosphorus
system, sampling of soil and vegetation was conducted along (%P) content by Kjeldahl acid digest followed by dilution and
a transect inland from the coast, stratified to incorporate sam- analysis by ion chromatography, in order to determine N : P
ples, where present, from the following habitat types: mobile ratios. N pool data are presented as kg N ha±1. Samples of the
dunes; semi-fixed dunes; fixed dunes and established dune moss H. cupressiforme were prepared by cleaning and separat-
grassland; and dune slacks. Eight survey points were sampled ing the live portion of shoots (approximately 3 cm of shoot).
along each transect, although only four were conducted at The samples were then ground to a fine powder prior to analy-
Studland and Great Yarmouth, and seven conducted at Winter- sis for %N content on a Leco 2000 CHN analyser.
ton and Seaton due to the homogeneity of the vegetation com-
munities and the lack of dune slacks at these sites. At each sur- Analysis of soil samples
vey point an area of homogenous vegetation was identified
and two, 2 2 m, quadrats established within an area of ca. The two replicate cores from each vegetation quadrat were
20 m radius. Sward height at three random points was meas- bulked for chemical analysis. The soil horizons were not sep-
ured and percentage cover of all vascular plants, bryophytes, arated. Soil pH was measured in 0.01 M CaCl2 solution (1 : 2.5
and lichens within the quadrat was determined. On two cor- w.b.v.). Soil moisture and Loss on Ignition (LOI) were deter-
ners of each quadrat, a 5-cm diameter, 15-cm depth soil core mined by drying and re-weighing soil samples at oven temper-
was removed for soil analysis, and a 25 25 cm area of vegeta- atures of 105 8C and 375 8C for 12 hours, respectively (MAFF,
600 Plant Biology 6 (2004) M. L. M. Jones et al.
1986), with the oven temperature for LOI reduced to 375 8C to Table 2 Site mean values for soil and vegetation parameters meas-
minimise CO2 losses from CaCO3. Available NH4 and NO3 were ured during the survey, split by habitat type. Data in each cell show
determined by extraction with 1.0 M KCl. The filtrates were an- the mean across all sites (top line) and the variation between site
alysed using continuous segmented-flow colorimetry. Avail- means (minimum and maximum values ± bottom line). Parameters
showing a significant regression relationship with nitrogen inputs are
able PO4 was determined by the Olsen P method. The resulting
shown in bold. The sign (+/±) indicates the slope of the relationship.
filtrates were analysed on an auto analyser using the molyb- (+/±) p < 0.1; +/± p < 0.05; ++/±± p < 0.01; +++/±±± p < 0.001. Those
date blue method. The total N and total C contents of soil were with a hash (#) represent variables where the effect of soil pH was sig-
determined on air dried soil using a Leco 2000 CHN analyser. nificant and pH was included as the first term in the regression in order
However, total N contents in soils from the mobile and semi- to remove its effect from that of the nitrogen gradient
fixed dunes were below detectable limits for this machine
and N contents for these samples were determined by Kjeldahl Mobile and Fixed dunes Dune slacks
semi-fixed and dune
acid digest followed by dilution and analysis using ion chro-
dunes grasslands
matography. In all analyses, Quality Assurance was conducted
by inclusion of laboratory blanks, duplicates, and standard Vegetation
soils. N pool data are expressed as kg N ha±1 for the top 15 cm Biomass (g m±2) 1349 + # 1164 + # 1274
soil layer. All other data are expressed as mg 100 g dry soil±1. (571 ± 2463) (408 ± 1956) (664 ± 1848)
Sward height (cm) 57.5 (+) 22.0 27.5
Statistical analysis (27.5 ± 89.2) (2.0 ± 43.4) (16.2 ± 41.5)
The vegetation quadrat data were analysed by detrended cor- Tissue N content 0.84 1.21 1.19
respondence analysis (DCA) using CANOCO for Windows 4.5. (%) (0.62 ± 1.01) (1.03 ± 1.49) (0.94 ± 1.67)
The DCA revealed strong differences between the principal Vegetation N pool 111 ++ # 132 (+) 148
habitat types. Therefore for the majority of the analyses, the (kg N ha±1) (58 ± 200) (49 ± 202) (76 ± 223)
dataset of soil and vegetation parameters was subdivided into
three main categories on the basis of the groupings observed. Species richness 14.3 21.0 ± 18.8
These were 1) mobile and semi-fixed dunes, 2) fixed dunes (9.0 ± 20.5) (13.1 ± 28.8) (12.8 ± 28.5)
and established dune grasslands, and 3) dune slacks. As the Ellenberg N 3.9 3.7 3.8
broad aim of this study was to assess surveyed parameters for (3.1 ± 4.3) (2.5 ± 4.4) (3.6 ± 4.1)
effects of N deposition and as each site has only one deposition
value, measured parameters of soil and vegetation were aver- Soil
aged to give a mean for each habitat at each site. The relation- Available N (mg 0.24 0.65 ± 0.47
ship between site parameters and N deposition was examined 100 g dry soil±1) (0.12 ± 0.43) (0.21 ± 1.13) (0.16 ± 0.79)
using linear regression. Each parameter was also checked for
Available N 59.0 22.9 12.4
significant soil pH effects. Where significant relationships with (mg 100 g organic (33.2 ± 92.7) (11.2 ± 38.9) (2.9 ± 26.0)
pH occurred, pH was included as the first term of the regres- matter ±1)
sion to separate these effects from those of atmospheric N. El- Soil total N (%) 0.032 0.099 0.216
lenberg numbers for vascular plant species were taken from
(0.007 ± 0.070) (0.034 ± 0.158) (0.088 ± 0.385)
Hill s modified Ellenberg numbers for the UK (Hill et al., 1999)
and an equivalent index from Siebel (1993) was included for Soil N pool 674 1604 2578
bryophyte species. Mean and abundance-weighted values per (kg N ha±1) (146 ± 1251) (722 ± 2553) (1337 ± 4365)
quadrat were calculated for vascular plants on their own and Soil organic matter 0.64 3.34 5.13
vascular plants plus bryophytes. The relationships between El- (OM) (% loss on (0.22 ± 1.47) (1.20 ± 7.89) (2.74 ± 10.24)
lenberg N and a number of soil parameters were assessed us- ignition)
ing linear regression. Ellenberg N was strongly related to soil Soil C : N ratio not analysed 22.1 + 23.1
pH (R2 = 19.9 %, p < 0.001). Therefore, soil pH was included as (14.4 ± 37.7) (18.4 ± 28.4)
the first term of the regression in order to remove the effects
of pH from the analysis. The mean Ellenberg N indicator for
each site was also calculated and the relationship with N dep-
osition assessed using linear regression as above. Unless other- very low levels of soil %N and soil % C. Mean Ellenberg N values
wise stated, all statistical tests were carried out using Minitab in the mobile dunes were not related to N inputs. There was a
13.32. trend of increased percentage cover of Ammophila arenaria in
response to N inputs (Table 3) although this was not significant
at the 95 % level. None of the other species showed a significant
Results
response.
Mobile and semi-fixed dunes
Fixed dune grasslands
There was a significant positive relationship of N inputs with
above ground biomass (Table 2, Fig. 1 a), and consequently There was a significant positive relationship between N inputs
with the vegetation N pool. Although not significant at the and above ground biomass (Fig. 2 a) once the effects of pH had
95% level, sward height appeared to increase with N inputs to been removed, and a positive increase in the vegetation N pool,
a maximum of roughly 80 cm (Fig. 1 b). There were no signifi- although this was not significant at the 95% level (Table 2).
cant relationships between N inputs and any soil parameters. There was also a significant negative relationship between N
C : N ratios were not calculated for these habitats due to the inputs and species richness (Fig. 2 b). N inputs were negatively
Nitrogen Deposition on Dunes Plant Biology 6 (2004) 601
3000 120
Above ground biomass (g/m )
2
2500 100
Sward height (cm)
2000 80
1500 60
1000 40
500 20
0 0
0 10 20 30 40 0 10 20 30 40
a Calculated nitrogen deposition b Calculated nitrogen deposition
(kg N ha–1y–1) (kg N ha –1y–1)
Fig. 1 Data from the mobile and semi-fixed dunes in relation to N in- (cm). Filled diamonds represent calcareous sites, open squares repre-
puts showing (a) above ground biomass (gm±2) and (b) sward height sent acidic sites. Bars show 1 s.e.
Table 3 Site mean values of percentage cover of the four most com- Ellenberg N numbers and N : P ratios
mon species, split by habitat type. For description of layout, refer to
legend in Table 2 Using regression to allow for the effects of pH, and including
the full data set of all 156 quadrats and associated soil and veg-
Mobile and Fixed dunes Dune slacks etation samples, the mean Ellenberg N numbers were nega-
semi-fixed and dune
tively related to soil available N per gram organic matter in
dunes grasslands
the mobile dunes (R2 = 20.2 %, p = 0.039), and positively related
Ammophila arenaria 42.4 (+) 8.8 not analysed to soil %N (R2 = 36.2 %, p = 0.016) and to soil organic matter
(31.3 ± 55.5) (0 ± 30.1) (R2 = 36.3 %, p = 0.022) in the fixed dunes. Ellenberg N numbers
did not relate significantly to any of the soil and vegetation
Carex arenaria 2.7 # 4.8 + 2.3 +
parameters in the dune slacks. Despite the significant relation-
(0 ± 8.8) (0.3 ± 25.4) (0 ± 7.3)
ships of atmospheric N deposition with various soil and vege-
Festuca rubra 14.5 # 17.9 4.4 tation parameters in the mobile and fixed dunes, the mean
(5.3 ± 26.7) (0 ± 41.7) (0 ± 14.0) Ellenberg N number per site was not significantly related to
Hypochaeris radicata # 3.2 # 4.7 0.3 ++ atmospheric N deposition in any of the habitats, or across the
(0.3 ± 9.3) (0.3 ± 17.5) (0 ± 1.0) data set as a whole. Neither abundance-weighting nor inclu-
sion of the Siebel bryophyte numbers improved the relation-
ship of mean Ellenberg N with atmospheric N deposition. N : P
ratios of total above ground vegetation in the fixed dune grass-
lands ranged from 5.0 to 15.6 indicating borderline P limitation
related to soil available N (Fig. 2 c) and positively related to soil in some samples. However, ratios varied strongly both within
C : N ratio (Fig. 2 d). However, neither the total N in the soil nor sites and between sites. There was no significant relationship
the available N content of the organic matter showed any rela- between soil available P and N : P ratios in the vegetation, and
tionship to N inputs. Mean Ellenberg N values in the fixed there was no relationship between site mean N : P ratios and N
dunes were not related to N inputs. Carex arenaria showed a deposition.
significant positive response to N inputs and was the only spe-
cies in this habitat to do so (Table 3).
Discussion
Dune slacks Mobile and semi-fixed dunes
There were no significant relationships of N inputs with any of Results suggest that there is a relationship between N deposi-
the soil or vegetation parameters in the dune slacks (Table 2). tion and biomass in the mobile dunes, largely accounted for by
Mean Ellenberg N values in the dune slacks were not related to an increase in sward height and an increase in cover of the
N inputs. However, percentage cover of both C. arenaria and dominant species A. arenaria. Studies aimed at improving
Hypochaeris radicata showed a significant positive response dune stabilisation showed that the classic European dune-
to N inputs (Table 3). binding species (A. arenaria, Elytrigia juncea and Leymus are-
narius) all responded to fertiliser applications (Adriani and Ter-
windt, 1974; Greipsson and Davy, 1997), as did a study on Am-
602 Plant Biology 6 (2004) M. L. M. Jones et al.
3000 35
Above ground biomass (g/m )
2
30
2500
25
Species richness
2000
20
1500
15
1000 10
500 5
0 0
0 10 20 30 40 0 10 20 30 40
a Calculated nitrogen deposition b Calculated nitrogen deposition
(kg N ha –1y–1) (kg N ha–1y –1)
2 50
Available N (m g/100 g dry soil)
1.8 45
1.6 40
1.4 35
Soil C:N r atio
1.2 30
1 25
0.8 20
0.6 15
0.4 10
0.2 5
0 0
0 10 20 30 40 0 10 20 30 40
c Calculated nitrogen deposition d Calculated nitrogen deposition
(kg N ha –1 y –1 ) (kg N ha –1 y –1)
Fig. 2 Data from the fixed dunes and dune grasslands in relation to N diamonds represent calcareous sites, open squares represent acidic
inputs showing (a) above ground biomass (gm±2), (b) species richness, sites, triangles show soils from the Outer Hebrides for comparison
(c) soil available N (mg 100 g dry soil±1), and (d) soil C : N ratio. Filled (not included in the analysis). Bars show 1 s.e.
mophila breviligulata in Canada (Boudreau and Houle, 2001), Fixed dune grasslands
although a Canadian study on foredune vegetation showed no
increase in plant growth following N and P additions (Houle, The negative relationship of N inputs with available N is sur-
1997). Pot studies have also shown that A. arenaria increases prising. A plausible hypothesis is that atmospheric deposition
tillering and leaf length with increased nitrogen supply (Pavlik, is reducing the activity of nitrogen fixing organisms and there-
1983). The long-term implications of this increased biomass fore nitrate production. Inputs from N-fixation by cyanobacte-
may therefore be to accelerate rates of organic matter accumu- ria on damp sand in dune grassland have been estimated at
lation and consequently soil development. This will lead to a 25 kg N ha±1 yr±1 (Stewart, 1967), although extrapolation of
reduction in the area of the mobile components of these dune data from Johnson (Johnson, 1979) suggests that inputs from
systems, and more rapid stabilisation of an ecosystem whose legumes in dune grasslands may only be in the order of 5 kg N
long-term existence is dependent on the dynamics of wind- ha±1 yr±1. Nevertheless, these amounts are equivalent to inputs
blown sand. A further consequence of more rapid nitrogen ac- from atmospheric deposition and represent a significant part
cumulation in the soil may be to alter the course, or even the of the N budget. An alternative hypothesis may be that the
endpoint, of the usual dune succession. increased plant growth is leading to depletion of available N
in the soil. The soil C : N ratios show a similar range to those
reported by Kooijman and Besse (2002), although the positive
relationship with N inputs is again surprising. A similar in-
Nitrogen Deposition on Dunes Plant Biology 6 (2004) 603
crease in C : N ratios has been shown in some forest soils likely that P limitation has dramatically influenced the ob-
(Billett et al., 1990). While these relationships appear counter- served relationships with N deposition.
intuitive, they are reinforced by the soil data from relatively
pristine air-quality sites in the Outer Hebrides. Berendse The impacts of N deposition have been mentioned by several
(1998) suggests that during the course of succession, as more authors in relation to dunes in north west Europe (Dopheide
woody species appear, the litter contains more phenols and or- and Verstraten, 1995; Ketner-Oostra, 2001; Ovesen, 2001; van
ganic compounds and decomposition rates slow-down. How- der Laan, 1985) but these impacts have rarely been quantified,
ever, there was no obvious shift towards more woody species with the exception of Dopheide and Verstraten (1995). This is
at the high N sites. Increased N deposition can also result in therefore the first published systematic survey which identi-
slower long-term litter decomposition in N-saturated forest fies relationships between N inputs and the soil and vegetation
systems as the C-supply becomes limiting for microbial degra- of dune systems.
dation (Berg et al., 1998). There is clearly further research
needed to elucidate the mechanisms behind the observed rela- Critical Loads
tionship of N with C : N ratios.
This survey has shown relationships of a number of param-
Dune slacks eters with N inputs which include: increased biomass in the
mobile and fixed dunes; increased cover of certain species in
The lack of significant responses of biomass and soil param- the fixed dunes and dune slacks; reduced N availability and in-
eters in the dune slacks is surprising, since Willis (1963) creasing C : N ratio in the fixed dunes. While this survey only
showed that dune slack vegetation at Braunton Burrows re- shows an association and not causality, on the basis of these
sponded to N applications. However, the increases in cover of results it appears likely that the sites with higher N deposition
C. arenaria and H. radicata show that individual species may are exceeding the critical load for nutrient nitrogen for sand
be responding to increased N deposition, even if major vegeta- dunes. In many of the variables where significant relationships
tion changes are not evident across the N deposition gradient. with N inputs were observed, a threshold at around 15 kg N
This may, in part, be due to the absence of dune slacks at two of ha±1 yr±1 seemed to differentiate between the response of the
the high N sites. cluster of sites experiencing low N inputs and the responses
of those sites experiencing higher loads of N deposition. This
Ellenberg indicator values and N : P ratios reinforces findings by Dutch authors using a mesocosm ap-
proach (Tomassen et al., 1999). Therefore, acknowledging the
Ellenberg N values in this study did not reveal changes in the precautionary principle behind the critical load concept, and
vegetation in response to the N deposition gradient. As in other the need to account for different base status and degree of
studies (Hill and Carey, 1997; Melman et al., 1988; Pitcairn et management at sites (Achermann and Bobbink, 2003), a pre-
al., 2003), neither abundance-weighting nor inclusion of the liminary critical load range of 10 ± 20 kg N ha±1 yr±1 is proposed
Siebel indicators for bryophytes improved the relationship for mobile and fixed sand dune systems. There are not enough
with N deposition. Due to the variety of species and vegetation data to suggest a critical load for humid dune slacks. This sug-
communities encountered across the survey, it was hoped that gested critical load is an important first step towards providing
Ellenberg N values would be able to provide a unifying ap- protection of a valuable European habitat which, until now,
proach to analysing potential N responses. However, this ap- has not been included in the critical loads manual (Achermann
proach was not successful. As identified by Hill and Carey and Bobbink, 2003). However, further studies are necessary to
(1997) and Schaffers and Sykora (2000), Ellenberg N seems to validate the suggested critical loads for dry dune habitats and,
be a general indicator of soil fertility rather than specifically a in particular, to examine potential effects of N in dune slacks.
nitrogen index. Secondly, in habitats such as the mobile dunes,
the available species pool is restricted to those species capable In conclusion, the results of this field survey suggest that at-
of surviving the harsh physical conditions of water stress, sal- mospheric N deposition is having an effect on soil and vegeta-
inity, and disturbance, thus limiting potential changes to com- tion in some sand dune sites in England and Wales. However,
munity composition in response to N inputs. Despite their use- there does not appear to be one single indicator of excess N
fulness in detecting change over time in other dune systems deposition which is common to all sites, and different varia-
(van der Maarel et al., 1985) and eutrophication effects in for- bles respond in different dune habitats. Integrated indices
ests (Diekmann and DuprØ, 1997), they were of limited use in such as Ellenberg N values have not proved to be of use in this
this study, and a recent UK study along a local gradient of NH3 survey. There is considerable work required to improve our un-
deposition (Pitcairn et al., 2003) also showed no significant re- derstanding of the long term consequences of enhanced N
lationship with Ellenberg N values. deposition in the dry dunes and in dune slacks. There is also a
need to build on the results of this survey to develop reliable
Phosphorus availability may limit community responses to N indicators of excess N deposition in dune systems.
deposition and N : P ratios can be used to determine which
element is more limiting to plant growth, with ratios above Acknowledgements
16 indicating P limitation (Koerselman and Meuleman, 1996).
In this study the N : P ratios did not suggest greater P limitation This survey was co-funded by Countryside Council for Wales
with increasing N deposition, and observed vegetation re- (CCW) and English Nature (EN) and thanks are due to staff in
sponses were reasonably consistent across the N deposition these agencies for providing information and acting as central
gradient. The two sites with the highest N deposition had rela- contacts. Thanks are also due to wardens and staff at the study
tively high available P levels in the soil which may have con- sites who installed and collected the diffusion tubes. Thanks
tributed to these sites remaining N limited. Therefore it is un- go to colleagues at CEH Banchory for collecting soil sam-
604 Plant Biology 6 (2004) M. L. M. Jones et al.
ples from the Hebrides. Advice on interpretation of the NO2 Jones, M. L. M., Hayes, F., Brittain, S. A., Haria, S., Williams, P. D.,
and NH3 concentration data was helpfully provided by Mark Ashenden, T. W., Norris, D. A., and Reynolds, B. (2002 a) Changing
Sutton, Sim Tang and Mark Theobald at CEH Edinburgh. nutrient budgets of sand dunes: Consequences for the nature con-
servation interest and dune management. 2. Field survey. Contract
Report September 2002. CCW Contract No: FC 73-01-347. CEH
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