Work Package 2:
Impacts, Recovery and Processes
Task 7:
Whim Moss Nitrogen Manipulation Experiment and
Open-Top-Chamber Flux Work
Dr Lucy J Sheppard, Dr Ian D Leith, Dr Alan Crossley, Matt Jones
Centre for Ecology and Hydrology, Edinburgh
51
52
Whim Moss: N Manipulation of a blanket bog ecosystem
Dr Lucy J Sheppard, Dr Ian D Leith, Dr Alan Crossley, Matt Jones
1
Summary
1.1 Whim Moss
•
Effects of ammonia gas (NH3) on N sensitive plant species are more damaging
than those of NH4+ or NO3- ions in precipitation. It is probable that
intermittent high concentrations, concealed by measurements of long-term
average (monthly) NH3 concentrations, are responsible.
•
Effects of enhanced NH3 concentrations are species specific, implying a need to
screen potentially sensitive species to identify the number of NH3 exposure
hours they can withstand at different concentrations and also to determine if
threshold concentrations exist.
•
NH3 sensitive species: The lichen, Cladonia portentosa, was the most sensitive
to high NH3 concentration. NH3 flux chamber studies also indicated high
deposition rates to this species, reflecting the large potential uptake surface.
Calluna and Sphagnum have also shown reduced cover and damage in the
field, especially when the water table was low. Interactions between N and
stress greatly increase the potential for damage. Favoured species: Crossleaved heath, Erica tetralix is expanding its cover. Erica appears to control
NH3 uptake better than Calluna. Insensitive species: The dead leaves of
cotton grass, (Eriophorum) may absorb some of the NH3, providing protection
for the new leaves. So far, we have not observed invasion by nitrophilous
grasses, as reported in studies on the dry heaths.
•
N concentrations in Sphagnum capitula (heads) showed a memory effect of
previous N exposure. Damaged Sphagnum appears not to be able to
accumulate N, as membrane integrity appears to be impaired by exposure to
NH3 concentrations > 40 µg m-3. Capitulum N concentrations at Whim Moss,
with a low N exposure history, were still almost 100% higher under the NH3
treatment than those reported by Lamers et al. (2000) for a European survey.
•
The N exposure history was apparent from the N content of the youngest
pleurocarpous moss shoots, ie. each year of N treatment raised the N content,
implying re-translocation of N. N concentrations in these mosses could
provide useful indicators of past N pollution climates, once they have been
calibrated.
•
The N content of ericoid foliage (Calluna or Erica) is less well coupled to N
deposition than moss foliage, especially that of the pleurocarpous moss
Hypnum jutlandicum. In this moss, N concentrations increased in proportion
to the N deposition received over time in both wet and dry treatments.
•
No differential effects of oxidised versus reduced N were observed in the wet
plots, except in the pleurocarpous mosses which took up more NH4+, despite
accumulated inputs of ~170 kg N ha-1 over and above ambient N. This could
53
reflect the more realistic application frequency, the washing effect of natural
precipitation and the relatively low concentrations of the applied ions, ≤ 4 mol
m-3, which represents 2-3 times cloud water and 10-15 times rainfall
concentrations at a high (600 m) altitude site in Eastern Scotland. However,
the measured increases in foliar N imply the likelihood of detrimental change
in the future.
1.2 RECOVERY in open-top chamber experiments following N
cessation
•
No regrowth was observed from existing Calluna plants affected by N
deposition > 16 kg N ha-1 y-1, because of the accelerated ageing effect.
However, there was some evidence of new Calluna propagules at higher N
inputs.
•
Previous NH3-N exposure appeared to impair reproduction or the viability of
Calluna seedlings since almost none were observed. Low doses, up to 32 kg N
ha-1 y-1, of wet deposited NH4-N did not affect propagation, enabling the
potential for some recovery of Calluna cover.
•
Moss colonisation, following the cessation of N treatment, was generally
independent of previous N treatment, except where the treatment had
influenced the extent of the ‘open’ areas or where there had been colonisation
by more aggressive competitors for light.
•
The acrocarpous moss, Dicranum scoparium, a ‘new’ species appears to be an
aggressive and strong coloniser and insensitive to previous N deposition
history.
•
A history of high NH3-N deposition encouraged invasion by nitrophilous
grasses.
1.3 Flux chamber studies
•
Canopy resistance to NH3 deposition depends on the species of bog plant, but
several species behave in a similar way. Resistance changes with respect to
NH3 concentration, leading to potential over or under estimation of N
deposition if simple models are used.
•
In the mosses Sphagnum and Hypnum (without stomata), canopy resistance
does not increase with increasing NH3, so these species form a strong sink for
NH3, making them potentially vulnerable to direct toxicity. These results
corroborate the findings from the Whim Moss field N manipulation study.
•
Canopy resistances change diurnally in species with stomata. During the day,
resistance to NH3 deposition decreases, because stomata are open enabling
more NH3 to be absorbed.
54
•
Surface wetness has important implications for NH3 deposition. Wet surfaces
have a lower canopy resistance to NH3, which can result in increased N
deposition. Bog ecosystems should therefore provide a strong NH3 sink and
may be more vulnerable than drier ecosystems to NH3 damage. Acid bog
ecosystems will also absorb more NH3 because it is an alkaline gas.
2. Policy Relevance
The work, undertaken at Whim Moss and in the open-top chambers (OTCs),
addresses 5 issues connected with source attribution and damage or recovery:
Separation of the different N forms allows:
1. Identification of N sensitive species and traits that can be monitored to
conform with the European Habitats Directives.
2. Validation of critical loads (CL) for bogs and evaluation of the role of
concentration (Critical Levels).
3. Evaluation of the potential of N impacts on wetlands (eutrophication or
acidification) to affect climate change via effects on C and N sequestration and
greenhouse gas emissions (CO2, N2O, CH4).
4. Provision of a demonstration of N impacts on a blanket bog, under near
natural exposure conditions, and at a site that has a low N history of ambient
inputs to identify species at risk and validate the UNECE agreed CL taking
into account modifiers i.e. P&K availability and environmental conditions.
5. Examination of the potential for recovery, recolonisation and invasion of bog
species in peat monoliths, previously exposed to 5 years of N inputs, in OTCs.
6. Provision of equations to estimate N deposition from NH3 concentration data
to individual species and mixtures of bog vegetation and identification of the
role of environmental influences on NH3 – N deposition processes based on
canopy resistance theory.
2.1 Background
Whim Moss (280 m asl) represents the transition between a lowland raised
ombrotrophic bog and an upland blanket mire (Calluna vulgaris – Eriophorum
vaginatum blanket mire, NVC M19a). The major European resource of these mires
with their enormous conservation value is based in the U.K., where < 6% exists in
pristine condition (Thompson et al. 1995).
There were no field studies, prior to Whim, to evaluate the impacts of N under near
natural conditions, where treatment is coupled to wind direction and speed,
temperature and rainfall, taking place throughout the year when meteorological
conditions permit. Whim is also unique in that the wet N treatment doses span the
UK deposition range at 8, 24 and 56 kg N ha-1 yr-1, over and above the background 810 kg N ha-1 yr-1 ambient inputs. The ionic concentrations are below 4 mol m-3 for
95% of the time and most importantly the N deposition history of the site is low so the
system should not have started to acclimate. The treatment solutions NaNO3 or
NH4Cl, applied in rainfall, increase the average annual precipitation by 10%. The
large plots, 13 m2 can accommodate a range of sampling strategies, are widely
separated to minimize cross contamination and well replicated, 4 plots per wet
treatment. The NH3 fumigation transect simulates emissions downwind of a 100,000
55
bird poultry unit (PPCR 2000), with NH3 concentrations being monitored along the 60
m transect.
Recovery from N deposition is being assessed in the OTC’s (Emmett et al. 2004)
where 7 bog species (Calluna vulgaris, Molinia caerulea, Deschampsia flexuosa,
Eriphorum vaginatum, Potentilla erecta, Narthecium ossifragum and Polytrichum
commune planted in 3 m2 acid peat monoliths had been previously treated with NH4Cl
or NH3. For 5 years (1999-2003), from April to October, NH4Cl was applied thrice
weekly at 4, 12, 20, 32, 68 and 132 kg N ha-1 yr-1 or gaseous NH3 continuously at
ambient (1.5), 6, 20, 45 and 90 µg m-3. Treatment stopped in October 2002 and since
then the chambers have been exposed to ambient rainfall and gases. The Calluna was
cut back to ground level in January 2004.
Resistances to NH3 deposition to bog species are being quantified using a purpose
built flux chamber for gas phase NH3, so that comparisons can be drawn with wet N
deposition. Approximately 2.5 m2 of bog vegetation mixtures or monocultures have
been exposed to varying NH3 concentrations to provide improved estimates of surface
resistances.
3 Project Update
Whim Moss: Initially the emphasis of the project was to establish a site where
impacts of reactive N on a blanket bog ecosystem under near natural conditions could
be demonstrated to stakeholders. A range of potentially N sensitive species including
the lichen Cladonia portentosa, pleurocarpous mosses Hypnum jutlandicum and
Pleurozium schreberi along with Sphagnum capillifolium, grasses including
Eriophorum spp, and ericoids (Erica tetralix, Calluna vulgaris, Vaccinium oxycoccus,
V. myrtillus and Empetrum nigrum) have been monitored. A comprehensive set of
pre-treatment foliar N, P and Mg values were collected together with plot pH.
Meteorological parameters, including water-table depth, rainfall, temperature, net and
solar radiation are measured continuously together with ambient and treatment N
inputs (Sheppard et al. 2004; Leith et al. 2004). Photographic records have been kept
of changes to healthy and damaged species. Monthly biochemical and physiological
changes were monitored over the second and third treatment seasons in the key
species C. vulgaris and S. capillifolium (Carfrae 2005). The site was also used to test
the specificity of different biomarkers (Leith et al. 2005).
Under this project changes in species cover based on permanent quadrats (3 x 0.25m2)
per wet plot and 3 per position along the ammonia transect have been assessed,
biannually for the wet plots and annually for the transect, 160 in total. The N status of
4 moss or lichen species and Calluna has been measured and this summer the extent
to which the added N has moved down the vegetation/peat profile will be examined.
Measurements are underway to validate the N saturation concept (Aber et al. 1989)
for blanket bogs. Lamers et al. (2000) predicts that once the N status of the
Sphagnum exceeds 1.2%, its natural filtering effect will fail and N will start to leak
from the system. We have already exceeded these values near the NH3 source, so we
will be following N concentrations in interstitial water in and below Sphagnum
clumps using mini-Rhizon soil solution samplers together with trace gas efflux
measurements. In addition we will look at the processing of N below-ground by
56
assessing enchytraeid worm mass and their predators, microbial activity and N
mineralization potential.
OTC Recovery: In April 2005, before the start of the second growing season with no
additional N treatment, the open-top chamber monoliths (Emmett et al. 2004) were
assessed for species cover and recruitment.
Ammonia flux Analysis Studies are continuing, and the significances of
environmental variables, diurnal cycle, temperature and surface wetness for NH3-N
deposition have been evaluated.
4 Collaboration
Data for extension of the MAGIC model. The scale of the Whim Moss experiment
and the heterogeneity of such sites with respect to topography and repeating mosaics
of vegetation preclude the sampling of all treatment plots for chemical data. Likewise
the different layers of vegetation, litter and necromass mean that any sampling for
modelling purposes must be targeted after preliminary investigations. Such
preliminary studies are underway to identify where to sample, i.e. depth from the
surface etc. Standing biomass has been assessed and key properties of the peat (pH,
loss on ignition (LOI), CN, available P, exchangeable cations) are being measured. In
addition soil water is being collected monthly, 7cm into the peat, for 12 months to
characterize the peat soil for acidity, dissolved organic carbon, NO3-, NH4+, K+, Ca2+,
Mg2+ and phosphate. These data will be supplied to Chris Evans (WP3).
5 Key Findings
5.1 Nitrate versus ammonium (wet)
Species cover: After 2 years of treatment increases or decreases in cover in the
different plots were generally small (5-10% maximum), especially when the effects
on the control were taken into account (i.e. the effect of the growing conditions at the
site). A preliminary analysis suggests only one moss, Hypnum jutlandicum
differentiated between reduced and oxidized N, declining under reduced and
expanding under oxidised N (Table 1).
Table 1 Effects of reduced versus oxidized N on change in cover (after 2 treatment
years) and foliar N concentrations, negative effects are shown in bold. Values in the
same rows with different letters are significantly different p =0.05. * The data have
not been adjusted for effects of environmental conditions as judged from the changes
in the cover of the control plots.
% cover change*
Foliar N*
Oxidised
Reduced
Oxidised
Reduced
Calluna vulgaris
+9
+10
1.26
1.32
Erica tetralix
+3
+4
1.17
1.17
Eriophorum vaginatum
-2
-2
Cladonia portentosa
0.93
0.99
-4
-3
Hypnum jutlandicum
+7
1.47a
1.69b
-3
Pleurozium schreberi
-7
-4
Sphagnum capillifolium
+1
1.48
1.51
-2
57
N status of current year foliage: Hypnum again discriminated between oxidised and
reduced, with higher N concentrations under reduced N. Given that oxidised N
appeared to stimulate growth, the lower N values in Hypnum receiving NaNO3 may
be explained by growth dilution. Visible injury: was only observed on the lichen,
Cladonia portentosa in response to N dose/concentration, not N form. Damage has
not increased with accumulating N dose. Frost hardiness: Effects of reduced versus
oxidised N were dependent on dose and time of year, generalisations are not therefore
possible.
Conclusion: After 3 years at the maximum N input, 64 kg N ha-1 yr-1, it would
appear effects of N form are species dependent and far less important than the effects
N dose/concentration. So far only the ubiquitous moss, Hypnum jutlandicum has been
negatively affected by reduced N, decreasing its cover and accumulating more N.
Sphagnum and Cladonia have not shown detrimental effects of reduced N though,
many studies, especially ones that are laboratory based in hydroponics, have found
reduced N is more toxic than oxidised N.
5.2 Wet N dose and Critical Loads
Species cover: With the exception of the highest N treatment which slightly reduced
the cover of both the pleurocarpous mosses, Hypnum and Pleurozium schreberi, no
significant changes in species cover due to dose were detected for ericoids, grasses,
lichens or Sphagnum after 2 years at 16, 32 or 64 kg N ha-1 yr-1 inputs.
Table 2 Concentrations of N (% dwt) in control plots and the percentage
enhancement in response to the wet N dose in 2003 (after 60% treatment had been
applied and 2005 (after 2 years and ~ 70% of the 3rd years treatment). Apical growing
tips or the capitulum were sampled in March. (Values followed by a letter are
significantly different (p=0.05) from the control, and differences between treatments
are indicated by different letters.
# Control
16
32
64
N % dwt
% difference
% difference
% difference
1.36
<1
-2
1
Calluna 2003
1.16
9
6
17a
2005
0.72
11
14
42a
Cladonia 2003
0.93
5
22a
27a
Sphagnum 2003
1.10
12a
45b
64c
2005
Hypnum 2003
1.10
12
19a
33a
1.32
3
23b
40c
2005
# note large inter-annual differences up to 20% even though sampling occurred at the
same time of year.
N status of the youngest foliage: Despite having received less than half of the annual
target N dose at the time of sampling, significant effects on foliar N concentrations
were recorded in the first year (Table 2) for Cladonia, Sphagnum and Hypnum but not
Calluna. The largest changes were measured in Cladonia (> 40% enhancement at 64
kg N). By 2005, Sphagnum, Hypnum and Cladonia were showing significant
enhancement also in the 32 kg N treatment. The N concentrations in young Calluna
58
shoots were by contrast much less sensitive. N concentrations in Sphagnum above
1.2% exceed the level proposed by Lamers et al. (2000) to indicate N saturation.
However, there is no evidence, so far, to suggest that Sphagnum is being adversely
affected by these high N doses. Rather, it is possible that when the N deposition is
spread throughout the year, when NH4+ and NO3- concentrations are relatively low (≤
4 mol m-3) and background N inputs are low, Sphagnum is more N tolerant than
suggested by the Dutch model (Lamers et al. 2000).
P & K additions markedly affected foliar N concentrations in the ericoids, with much
smaller effects on the mosses, which were more responsive to N alone. N responses
between the ericoids were quite different. In the absence of P & K foliar N in Calluna
barely increased with NO3-, with a slightly bigger NH4+ response, whereas in Erica
foliar N went down. P & K supplied with N however, increased N in both species, but
especially in Erica.
5.3 Dry N dose and Critical Levels
Effects along the NH3 transect have been very pronounced. The mat forming lichen
Cladonia portentosa was very sensitive to the high average NH3 concentrations, 80100 µg NH3 m-3 measured 2-8 m from the NH3 source (Sheppard et al. 2003). Visible
injury and eventual death have now (April 2005) been recorded up to 44 m from the
source, indicating the rapid spread of damage with exposure duration (average >9 µg
NH3 m-3). Although the accumulated doses in the wet N plots now match some of
those along the NH3 transect, where the Cladonia have died, no such death or even
worsening damage has been seen in these wet N plots. Therefore, it appears that the
fatal damage reflects critical levels of NH3 under the type of NH3 scenarios described
by Van der Eerden (1991). Other vulnerable species include Calluna and Sphagnum
capillifolium (Fig. 1). The cover of Erica tetralix, by contrast, has increased.
Sphagnum capillifolium varies in colour reflecting different amounts of carotenoids.
The very red variant, with more carotenoids, was less sensitive to NH3. Hypnum
jutlandicum was also sensitive to NH3, but only when conditions were hot and dry.
The warm wet summer of 2003 brought about its recovery although, as found in the
wet NH4-N treatments, cover declined within 12 m of the NH3 source (Fig.1). Uptake
of NH3-N by Hypnum, indicated by % N dwt, was correlated (R2 = 0.85) with the
mean annual NH3 exposure concentration, with uptake increasing exponentially at
higher concentrations. The relationship was less good for Sphagnum and results
suggest potential loss of N, caused by membrane damage at higher NH3
concentrations, likewise for Cladonia. Foliar K concentrations were much lower near
the source, supporting the likelihood of detrimental effects of NH3 on membrane
integrity.
N interactions with stress: The differential sensitivities of Calluna and Erica to
NH3-N were investigated with respect to cold (Calluna) and desiccation stress (both).
NH3 exposure reduced frost tolerance in Calluna, however, the level of tolerance
achieved even in NH3 treated shoots was adequate for normal winter protection. NH3
exposed Calluna, was more susceptible to winter desiccation than Erica. Drying
curves indicated Calluna closed its stomata at lower water contents than Erica making
it more vulnerable to drying out and potentially taking up more NH3. Fungal
(Botrytis) damage was also observed in Calluna, but not in Erica, with damage
strongly dependent on the level of NH3-N exposure and being absent from the wet
59
plots. Analyses of foliar N status, N and chlorophyll content all indicate that N
uptake is greater in Calluna, which may reflect the effect of NH3 on stomatal control
in Calluna. Thus the expansion of Erica under conditions of high NH3 seems likely
to reflect its lower uptake of NH3 and potentially better stomatal control. Plants with
lower N contents are less susceptible to pests and pathogens.
Table 4 N dose kg N ha-1 yr-1 required to raise foliar N content by 0.1% (1mg g-1
dwt), calculated by regression of foliar N versus treatment N dose. Values in bold
indicate that the slope of the linear response was statistically significant (p < 0.05).
Probability values for estimated responses are shown for all relationships below.
Treatment
NaNO3
NH4Cl
NH3
P values
NaNO3
NH4Cl
NH3
One year
Three years
One year
Three years
One year
Three years
Sphagnum
capillifolium
9.2
13.5
10.9
11.2
13.0
13.1
Hypnum
jutlandicum
9.5
7.3
10.5
8.4
15.8
17.0
1
3
1
3
1
3
0.32
0.035*
0.12
0.15
0.22
0.16
0.10*
0.26
0.08*
0.04*
0.04*
0.11
Cladonia
portentosa
8.7
8.1
6.7
8.4
10.6
9.1
0.93
0.25
0.04*
0.13
0.80
0.32
Analysis of the relationships between N deposition and foliar N concentrations (Table
3) suggests that N uptake from NH3 is generally less than from wet applied NH4+ or
NO3- . This observation contrasts with those from the former OTC study and flags a
problem when trying to extrapolate from controlled pollutant exposures to the field.
The OTC study involved continuous exposure to constant NH3 concentrations. At
Whim Moss, as in situations downwind of agricultural sources, exposure to NH3
occurs at most 14 % of the time, and usually ≈ 5%. NH3 concentrations during
exposure are very much higher than average time integrated measurements. The
calculated NH3 deposition, even using concentration – dependent surface resistances,
does not allow for the possibility of emission of NH3 immediately after exposure to
high concentration periods (Sutton 1993) and therefore may overestimate net N
deposition in situations with intermittent exposures to high NH3 concentrations.
60
120
pre treat
year 1
year 2
120
NH3 µg m-3
pre treat
25
20
0
0
NH
3 µg
m-3
m -3
80
% co
ver
NH3 µg
% co
ver
40
5
100
20
60
10
NH3 µg m-3
25
80
Killed in years 2 and 3 so
absent up to 44 m
15
year 2
Green Calluna vulgaris
Live Cladonia portentosa
20
year 1
30
100
60
15
40
10
64 m
32 m
16 m
12 m
8m
6m
4m
2m
20
5
0
1m
0
64 m
32 m
16 m
distance from ammonia source
12 m
8m
6m
4m
2m
1m
distance from ammonia source
120
120
year 1
35
year 2
NH3 µg m-3
Dead Hypnum jutlandicum
year 2
NH3 µg m-3
100
Green Erica tetralix
-3
ver
3
NH3
40
60
8
NH
60
15
80
10
-3
80
% co
ver
Pre-treatment no dead
Hypnum
20
% co
year 1
12
30
25
pre treat
14
100
6
40
4
10
20
5
0
0
0
64 m
32 m
16 m
12 m
8m
6m
4m
2m
20
2
1m
0
64 m
32 m
16 m
distance from ammonia source
120
pre treat
35
year 1
year 2
12 m
8m
6m
4m
2m
1m
distance from ammonia source
30
120
NH3 µg m-3
pre treat
Sphagnum capillifolium
100
25
80
20
60
100
-3
80
60
40
10
40
20
5
20
0
0
NH
3
15
NH
3
15
NH3 µg m-3
% co
ver
-3
% cov
er
20
year 2
Green Eriophorum
vaginatum
30
25
year 1
10
5
0
64 m
32 m
16 m
12 m
8m
6m
4m
2m
1m
0
64 m
32 m
16 m
12 m
8m
6m
4m
2m
1m
distance from ammonia source
distance from ammonia source
Figure 1 Cover values (%) for live Cladonia, dead Hypnum, live Sphagnum, dead
Calluna, live Erica and Eriophorum at different distances from the ammonia release
pipe pre-treatment and after one or two years of treatment. The red dashed line
indicates the mean annual NH3 concentration (µg m-3) at the different distances.
5.4 Recovery in OTCs
Calluna had already gone from the 90 µg m-3 NH3 treatment, within the treatment
period, (Fig. 2). Polytrichum commune too, had disappeared from all treatments early
in 1999. The 2005 survey showed it has reappeared, but only in the control treatment.
All the other species:D. flexuosa, E. vaginatum, Molinia caerulea, Narthecium
ossifragum, Polytrichum commune and Potentilla erecta were present throughout the
study period (1999-2003) and except for Calluna are currently to be found in all
treatments. Calluna, the dominant species throughout the study period, has regrown
in treatments receiving < 32 kg N ha-1 yr-1 or exposed to < 6 µg NH3 m-3 dry N
treatments but has failed to re-establish at higher N inputs (Fig. 2d). The Calluna
plants in the higher N treatments were structurally older, their stems woodier and with
a denser canopy, features of accelerated senescence. Accelerated ageing probably
explains the inability of the woodier stems to sprout new shoots, a common
consequence of high N. Eriophorum also recovered poorly when wet and dry N
inputs exceeded 16 kg N ha-1 yr-1. There was a dramatic increase in the moss,
Dicranum scoparium a ‘new’ species, on the exposed peat, especially above 16 kg N
ha-1 yr-1 wet, but not dry treatments. Molinia caerulea % cover increased, almost
linearly, in response to N dose, wet or dry, with the opening up of the canopy (Fig. 3).
61
Deschampsia likewise appears to be benefiting from the ‘openness’ but only in the
dry N treatments. These dramatic differences in cover, contrasting between Calluna,
grasses and Dicranum colonization in the control and highest N chambers are
illustrated in Figure 3a, c & d. A total of 16 new species were recorded in the wet and
dry N treatments (Table 4). Sedges preferred the higher dry N treatments while the
mosses favoured the lower dry N treatments and were more sporadic in the wet N
deposition treatments.
Table 4 ‘New’ species recorded in the wet and dry N deposition treatments. Coloured
square indicates species presence.
Colonising species
Wet N deposition
(kg N ha-1 y-1)
8
32
64
128
Grasses, Sedges & Rushes
Agrostis tenuis
Carex nigra
Eriophorum angustifolium
Holcus lanatus
Luzula multiflora
Mosses
Aulacomnium palustre
Dicranum scoparium
Hylocomium splendens
Hypnum jutlandicum
Pleurozium schreberi
Rytidiadelphus squarrous
Spagnum capillifolium
Sphagnum papillosum
Vascular plants
Epilobium palustre
Erica tetralix
Sagina procumbens
Vaccinium myrtillus
.
62
Dry N deposition
(µg m-3)
Control
6
21
45
90
a)
Control plot April 2005
b) Pre-harvest dry N treatment
(90 ug NH3 m-3) Sept. 2003
c) Dry N treatment (90 ug NH3 m-3)
April 2005
d) Wet N dep. treatment (132 kg N ha-1 yr-1)
April 2005
Figure 2. Photographs of N treatment plots in the N deposition study prior to harvest
in 2003 ( b) and after two years non N treatment.
Young Calluna vulgaris
Dicranum scoparium
Eriophorum vaginatum
Molinia caerulea
50
Young Calluna vulgaris
60
% cover (May 2005)
% Cover (May 2005)
40
70
30
20
10
Dicranum scoparium
Eriophorum vaginatum
50
Molinia caerulea
40
30
20
10
0
0
0
0
20
40
60
80
100
120
140
-1
160
20
40
60
80
100
120
140
-1
160
-1
Previous dry N deposition treatments (kg N ha y )
-1
Previous wet N deposition treatments (kg N ha y )
Figure 3. % cover for 4 species in the wet and dry N treatments after 2 years recovery
in the absence of additional N
63
5.5 Refining Cuticular Resistance to NH3
A single cuticular resistance value was found to be inadequate to explain deposition to
different bog species because of differences in characteristics such as surface area,
presence of stomata, and surface wetness properties.
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