Report to TFIAM, May 2003
Strategies to reduce deposition of nitrogen from agricultural sources
on sensitive ecosystems: spatial considerations
H M ApSimon, T Loh, T Oxley, A Grossinho
Department of Environmental Science and Technology, Imperial College London
Introduction
Concerns about the contribution of nitrogen deposition to ecosystem change through
processes of acidification and eutrophication have led to international action to
control emissions. In this context European emissions of reduced nitrogen in the form
of ammonia are comparable with those of oxidised nitrogen from stationary and
mobile sources combined. A large proportion of the reduced nitrogen comes from
agricultural sources particularly animal wastes. Under the Convention on Long-Range
Transboundary Air Pollution of the UN Economic Commission for Europe (the
Gothenburg protocol), and subsequent EC directives, national emission ceilings for
each member country have been set on pollutants contributing to transboundary
acidifcation, eutrophication and formation of excess ozone. These ceilings are to be
attained by 2010, and include ammonia emissions, where for the UK the ceiling set is
297 kilotonnes per year of ammonia as NH3 (equivalent to 244 kT of nitrogen in
reduced form).This represents a 19 % reduction relative to NH3 emissions in 1990,
compared with much larger reductions of 59% and 85% respectively for NOx and
SO2. It is however much harder to reduce ammonia emissions than SO2 and NOx
where very efficient “end-of-pipe” control technologies are available.
The subject of this paper is to raise alternative ways of controlling the deposition of
ammoniacal nitrogen in areas that are sensitive to such deposition, that can
complement direct control of emissions. This involves spatial separation of emissions
and sensitive areas, hence diluting the concentrations and deposition where damage
can occur. This is analogous to the effect of tall stacks in reducing ground level
concentrations from major combustion sources to complement measures to control the
quantities emitted.
These ideas will be illustrated in the context of the UK, but should also be applicable
in other countries. Thus the current analysis of the UK situation will be followed by
case studies of a more generic nature involving different ecosystem configurations
relative to area and point sources of ammonia. Simple modelling studies will illustrate
how low emission strips round such ecosystems can limit the nitrogen deposition
upon them. Finally the value of combining such measures to separate source areas and
sensitive ecosystems with direct emission abatement measures will be explored, and
suggestions made for further investigation.
Current analysis of the situation in the UK
The UK in parallel with other countries is developing strategies for attainment of
these emissions ceilings. The far shorter range of ammonia emissions and higher local
deposition of reduced nitrogen as compared with oxidised nitrogen means that for
protection of UK ecosystems control of reduced nitrogen deposition is particularly
important. Thus based on 1996-1997 UK emissions of N in oxidised form were 615
kT compared with 178 kT deposited in the UK, much of this originating from outside
the UK. By comparison emission of reduced N as ammonia was 282 kT, of which 236
kT is from agriculture; with 209kT of reduced N deposited, most of which originated
in the UK (NEGTAP,2001). Moreover ammonia emissions are widespread and
patchy, and can occur at high densities in rural areas close to the sensitive natural
ecosystems at risk. The aim is therefore to develop strategies that are cost-effective in
controlling emissions in order to achieve the ceilings, but at the same time maximise
protection of UK ecosystems.
The criteria for protection are based on the definition of critical loads as the maximum
annual levels of deposition sustainable in the long term without adverse effects. A
great deal of effort has been put into quantifying and mapping these critical loads for
different types of ecosystem. For acidification this involves a combination of sulphur
and nitrogen deposition., whereas for eutrophication it is only the nutrient nitrogen
deposition from both NOx and NH3 emissions combined that is of importance.
Detailed critical load maps have been compiled for the UK on a 1 x1 km grid by CEH
Monkswood (Hall et al 2003). Comparing maps of deposition with the critical load
data indicates where there is exceedance, and hence where ecosystems may be at risk.
Figure 1a illustrates the situation for 1996 emissions with respect to effects of
eutrophication in the UK.
Modelling studies are now being used to investigate potential strategies to reduce such
exceedance, and improve ecosystem protection, that are also cost effective. These
include integrated assessment modelling to bring together data on emissions,
atmospheric transport, critical load data, and the potential measures to reduce
emissions with their associated costs. Thus the UKIAM model is being developed at
a UK scale (distinguishing emissions from different counties and using a 5x5 km grid
for concentrations and deposition) in an analogous way to the ASAM and RAINS
models used during development of the Gothenburg protocol to investigate coteffective strategies to reduce transboundary air pollution on a European scale. The
emissions from agriculture have been compiled by a working group on emissions
(Misselbrook et al 2000, Dragositis 1998). These account for 84% of the ammonia
emissions in the UK; with the remaining non-agricultural emissions arising from such
sources as horses (classified as non-agricultural animals), vehicle exhausts, fertiliser
production and other industrial processes, landfill and sewage sludge disposal, and
mapped accordingly.
Modelling of atmospheric transport is based on the FRAME model of CEH Edinburgh
(Singles et al 1998), simulating the transport of columns of air across the UK, picking
up emissions of ammonia and other pollutants, and simulating the processes of
vertical diffusion and chemical transformation to ammonium particulate matter, and
dry and wet deposition. This has been used to derive source-receptor matrices for use
in the UKIAM model for rapid assessment of different emission reduction measures,
and selection of abatement strategies that reduce ecosystem exceedance of critical
loads as much as possible with increasing levels of expenditure. Typically such
analysis picks out measures which make a large improvement at low cost, and
measures which do little to improve the situation or are of very high cost.
For ammonia the potential abatement measures involve reduction of emissions from
animal wastes from animal housing, subsequent storage, and eventual spreading.
Avoiding use of urea as a fertiliser can also be effective. On the European scale these
were investigated in the MARACCAS project (Cowell and ApSimon 1998), using
estimates of the efficiency, applicability depending on agricultural practices and
conditions, and the costs of each measure. Cost curves were compiled for each
country summarising the emission reductions possible in order of increasing cost per
ton abated. In the UK the NARSES project is investigating potential reductions in a
similar manner but addressing the spatial variability, using GIS to assemble detailed
data relating to emissions and agricultural conditions in order to assess the
geographical variation in the applicability of measures and their capacity to reduce
emissions and associated costs. This is just the information required on ammonia by
UKIAM to consider combined strategies for abatement of ammonia, oxides of
nitrogen, and sulphur dioxide in the UK
Unfortunately initial assessments have indicated that approximate simulation of the
maximum feasible reduction of UK ammonia emissions, giving a total reduction of
the order of 15 to 20% , still leaves a large proportion of UK ecosystems with
exceedance of the critical loads (e,g, see figure 1b). This exceedance is not removed
by simultaneous scaling of NOx emissions to achieve the Gothenburg protocol
ceiling. However such assessment is based on averaging emissions and deposition
spatially within each 5x5 km grid cell. In practice both will be very patchy and nonuniform. Hence this modelling approach may either significantly overestimate or
underestimate exceedance of critical loads. This problem of uncertainty due to spatial
resolution is addressed in the sections below.
In order to investigate deposition patterns over shorter distance scales we have used
the LADD model developed jointly by Imperial College and CEH Edinburgh. Like
the FRAME model this is a Lagrangian model assuming columns of air advected
across the area of interest, picking up emissions on the way and simulating vertical
diffusion, chemical transformation and deposition processes (Hill 1998). It has been
applied successfully at a farm scale in a landscape with major poultry units, and used
to illustrate the spatial variability in deposition in typical situations (e.g.Dragositis et
al 2002)
Case studies of a receptor oriented approach
We shall now illustrate a simple source-apportionment approach for deposition of
nitrogen and consider this relative to a typical critical load for protection from
eutrophication. This will illustrate the importance of the local emissions within the
same grid square as the ecosystem. We can then consider different simplified
situations to show how deposition on an ecosystem varies between different
situations. Overall the results suggest that buffer zones between emissions and
sensitive ecosystems could help to reduce exceedance and hence decrease the
proportion at risk
Critical loads for eutrophication range from around 5 to 35 kg N ha-1yr-1. For
example changes in ground flora and mycorrihzae have been observed in acidic
coniferous forests with deposition between 7 and 20 kg N ha-1yr-1. Similarly for
lowland dry heaths transition from heather to coarse grass can occur at deposition
between 15 and 20 kg N ha-1yr-1 (e,g, NEGTAP report 2001, chapter 7)1. For
1
Note that some critical loads have been revised downwards following critical review in the UN ECE
programme under the Convention on Long-Range Transboundary Air Polution.
illustrative purposes therefore we shall consider an idealised ecosystem with a critical
load of 15 (+/- 50%) kg N ha-1yr-1.
Now let us consider the deposition averaged over a 5x5 km grid square, and the
relative contributions from different surrounding areas. Thus imagine a 5x5 km grid
square embedded in a larger 25 x 25 km grid square with background concentrations
imported into this larger square from the rest of the UK and overseas. Let us simplify
the wind-rose for the moment and assume the wind strength and direction are
isotropic.
The concentrations of reduced N arriving from beyond the larger grid square are
likely to be of the order of 1 g.m3. Assuming typical rates of deposition this would
typically yield an average of about 3kg of N per hectare per year to sensitive
ecosystems such as heathland. Even if this was twice as large, and there was some
additional wet deposition of ammonium aerosol, this imported contribution would still
be much less than the critical load, implying that there needs to be a significant
contribution from more local emissions to cause exceedance.
Now let us consider the contribution of the 25x25km grid square to the 5x5 km grid
square embedded in it. Suppose we assume a rather high average emissions density
for such an area, based on inspection of the mapped emissions over the UK, of 25 kg
of reduced N/ha/yr. With such an emission density distributed uniformly over the
25x25 km square outside the 5x5 km smaller grid square at the centre, the LADD
model indicates a deposition rate of 4.5 kg N/ha/yr at the centre.
However if we now assume the same emission density within the central 5x5 km grid
square, the corresponding contribution to deposition at the centre is 13.1 kg N/ha/yr.
This contribution is almost twice as large as the combined contribution from the
surrounding large square and longer range, and on its own is comparable with the
critical load- see table.
Source area
Transboundary and national->
conc ~1g.m-3 on boundary of 25x25
km area
Emission of 25 kg of N/ha/yr over
25x25 km area outside 5x5 km square
in centre
Emission of 25 kg of N/ha/yr over 5x5
km area in centre
N deposition
at centre
3 kg N/ha/y
4.5
13.1
Different configurations within the central 5x5 km grid square
The analysis illustrates the dominant contribution of local emissions within a 5x 5 km
area containing a sensitive ecosystem or SSSI to the deposition in situations leading to
exceedance of critical loads (in this case about two thirds of the total reduced N
deposition, but in other cases it could be an even higher proportion). Without such a
major local contribution it is difficult to see how such exceedance would occur except
in a few particularly sensitive ecosystems with lower tolerance. It is therefore
important to consider carefully how such local deposition is assessed, illustrated
below by different case studies for the local 5x 5 km grid square. We have already
referred to the very uneven distribution that can occur within such a grid square down
to the field scale, as illustrated in Dragositis et al, 1998.
In the analysis above it has been assumed that the emissions are
homogeneously mixed and spread across the whole grid square, This is as though the
ecosystem area is embedded as tiny pockets with the agricultural emissions
distributed round them uniformly right up to their edge. However this may be far from
the true situation, and there are several different possible situations. For example,
from an ecosystem perpsective we can have
i) a very small ecosystem area as a small patch within an agricultural area,
considered below as if located as a small area of order 100x100 metres at the centre of
the square with area emissions surrounding it- case A
ii) alternatively the ecosystem area may be large, bordering on an agricultural
area. This can be illustrated by considering a strip along the side of the square
downwind of emissions over the rest of the square- case B
iii) the opposite of case A is a small area with intensive agriculture or major
poultry or pig units embedded within a sensitive ecosystem area- case C
We will now consider these different possibilities, and variations upon them.
Case A- small ecosystem enclosed in agricultural area
If calculated as in table 1, assuming a uniform emission density of 25 kg N per
hectare per year over the hole 5x5 km grid area and a small ecosystem of the order of
100 by 100 metres at the centre then the estimated deposition to this ecosystem using
the LADD model is 13.1 kg of N per hectare per year. (NB an isotropic wind rose has
been assumed for simplicity).
Now let us consider the effect of eliminating emissions from a small strip, just
50 metres wide surrounding the ecosystem. This reduces the deposition on the
ecosystem area by 17% to 10.9 kg of N per year, which is of the same order as the
maximum feasible reduction in UK emissions using all available measures. A further
calculation was made in which emissions were set to zero in a broader strip 150
metres wide surrounding the ecosystem. This makes a greater reduction of 34% in the
deposition on the central ecosystem, to 8.6 kg of N per hectare per year. This is a
substantial reduction.
By contrast consider an ambitious level of 30% reduction in emissions by
applying abatement measures to reduce emission from land spreading etc. This would
reduce the deposition on the central ecosystems correspondingly by 30% to 9.2 kg of
N per hectare per year. Note that in many areas overall emission reductions of this
magnitude would not be feasible depending on the farming activities and applicability
of measures. An alternative scenario is to reduce emissions locally by this amount in a
region round the ecosystem. For example if emissions were reduced by 30% within
500 metres of the ecosystem area, then this gives a comparable reduction of 17% to
having zero emissions in a narrower strip just 50 metres wide. This might be achieved
by land-use patterns with for example grazing or lower emitting areas close to the
sensitive ecosystem areas, or by limiting the amounts of manure that can be applied to
arable land in such areas.
Case B- large ecosystem area bordering on square
By contrast let us consider a larger ecosystem area adjacent to farming land, which
can be illustrated by assuming that the ecosystem area overlaps down one side of the
5x 5 km square in a band 500 metres wide. We can assume the same emission density
of 25 kg N per hectare per year as in the previous case over the 90% of the gird area
that is agricultural. If the prevailing wind is more from the direction of the agricultural
area towards the ecosystem area then deposition will be higher than that calculated
using an isotropic wind rose. So calculations for this situation have been calculated
both for an isotropic wind-rose and with a more typical wind-rose for the UK with
more frequent winds from the south-west.
As the base case the maximum deposition on the ecosystem area assuming that
the agricultural emissions are uniform over the rest of the grid-square right up to the
edge of the ecosystem was calculated to be 7.1 kg N ha-1.y-1 for the isotropic wind
rose, and 9.3 kg N for the wind-rose more typical of UK conditions. If now emissions
are removed from a 50 metre wide strip along the border of the ecosystem, then the
deposition is reduced to 5.6 and 7.5 kg of N respectively- or by approximately 20%
for both. As in the previous Case A, we can compare with an ambitious reduction in
emission by 30% over the whole square by applying abatement measures, giving a
corresponding 30% reduction in the resulting deposition to 5.0 and 6.5 kg of N ha-1y-1.
If such a 30% reduction is limited to a band 500 metres wide along the perimeter of
the ecosystem area, then this yields a corresponding reduction of around 19% to 5.7
and 7.6 kg of N ha-1y-1 for the isotropic and anisotropic wind-roses respectively.
Case C Farm area enclosed within a large ecosystem area
We can now consider the case where some intensive agricultural activities are
embedded in a large sensitive ecosystem area, which will be illustrated by a farm area
1x 1 km in extent at the centre of a 5x5 km grid of which the remainder is heathland
or equivalent natural ecosystem area. It will be assumed that the agricultural area is
intensively farmed with, for example, high rates of manure/slurry application, giving
en emission density of 50 kg of N ha-1y-1. This is twice that of the previous cases but
over a much smaller area.
If we again start by assuming that the emissions are uniform over the whole
area surrounding the farm land, then the highest deposition over the surrounding
ecosystem area again differs in magnitude and location when an assumed isotropic
wind rose is replaced by the wind-rose more characteristic of the UK. The LADD
model yields values of 8.4 and 11.2 kg of N ha-1.y-1 respectively for these two
situations.
If now emissions are set to zero along a 50 metre wide strip round the edge of
the farm area, that is next to the ecosystem boundary, then there is a substantial
reduction of 37% assuming the wind is isotropic, and 34% using the UK wind rose,
giving reduced deposition values of 5.3 and 7.5 kg of N ha-1.y-1. Part of the reduction
is due to a change in the total emissions of 18%, because in this situation the area over
which the emissions are cut out is this much larger proportion of the total emitting
area. By contrast if emissions were reduced uniformly by the larger amount of 30%
the reduction in emission would still be slightly less than for the perimeter strip
situation – that is deposition values of 5.9 and 7.9 kg of N ha-1y-1.
However an alternative situation is that of a major very concentrated source
located somewhere within the pocket of farmland, Thus suppose that instead of a
dispersed source, there is a poultry unit generating around 6.3 tons of ammoniacal N
per year embedded in a 1x1 km farm area generating more moderate area emissions of
25 kg of N ha-1.y-1 as in cases A and B. If the total resulting emission of 8.8 tons was
treated as if distributed uniformly over the 1x1 km farm area, the calculated maximum
deposition at the edge of the ecosystem becomes 14.8 and 19.7 kg N ha-1y-1 with the
uniform and more typical UK wind-rose respectively. By contrast consider the
situation if 6.3 tons of the emission is treated as a concentrated source with
dimensions more typical of a large poultry unit, but located 100 metres from the north
and east farmland boundaries. This gives much higher values for localised deposition
at the edge of the ecosystem of 18.3 and 28.1 kg of N.ha-1.y-1 for the 2 wind rose
assumptions. Moving the poultry unit further away so that it is 300 metres from the
north and east boundaries more than halves this deposition to 9.0 and 13.3 kg of N
ha-1.y-1 respectively. Finally putting the poultry unit at the centre of the farmland, 500
metres from each boundary, gives the very much lower values of 6.0 and 7.7 kg of N
ha-1.y-1. This illustrates how critically the occurrence of deposition in excess of critical
loads within the ecosystem area will be very dependent on the separation distance
from major pig/poultry unit sources. In the situations considered here the deposition
could be higher or less than the valued calculated if the emission was evenly spread
over the farmland area. But the implication here is that deposition and exceedance
decrease sharply with relocation of a concentrated source by a few hundred metres.
Table 2: Deposition to ecosystem (maximum) due to emissions in 5x5 km grid square
Isotropic
UK windwind
rose
Case A: small ecosystem at centre of agricultural
area
i)Uniform emission density 25 kg N/ha/yr over grid
13.1
kg N/ ha/y
% figures in
ii) zero emission within 50 metres of ecosystem
10.9 (17%)
italics are
iii) zero emission within 150 metres
8.6 (34%)
percentage
iv) 30% reduction in emissions
9.2 (30%)
reductions
v) emissions 30% less within 500 metres
10.9 (17%)
Case B: large ecosystem bordering on agricultural
area
i) uniform emission density 25 kg N/ha/y except over
7.1
9.3
500 m wide band down side of square
kg N/ha/y
ii) zero emissions in strip within 50 metres
5.6 (21%)
7.5 (19%)
iii) uniform 30% reduction in emissions
5.0 (30%)
6.5 (30%)
iv) 30% reduction within 500 metres
5.7 (20%)
7.6 (18%)
Case C: farm area enclosed in large ecosystem area
i) uniform emission 50 kg N/ha/y over 1x 1 km farm
8.4
11.2
ii) zero emissions 50 m strip round perimeter
5.3 (37%)
7.5 (34)
iii) 30% reduction across area
5.9 (30%)
7.9 (30%)
iv) uniform. emission =25 kg N/ha/y +6.3 tons spread
out over 1x1 km farm area
v) move source 6.3 t/y to 100 m from N and E edge
vi) move source 6.3 t/y to 300 m from N and E edges
vi) move source 6,3 t/y to 500 m from N and E edges
i.e. to centre of 1 x 1 km farm area
14.8
19.7
18.3
9.0
6.0
28.1
13.3
7.7
Discussion
For ease of comparison the illustrative cases assumed above are summarised in table
2. The general conclusion is that avoiding or reducing emissions close to the
perimeter of sensitive ecosystem areas can effectively reduce the deposition of
reduced N on them by as much as the application of stringent abatement measures to
reduce emissions over the whole of the local 5x5 km grid square. A combination of
both spatial separation and abatement measures could yield an even larger reduction.
Preliminary assessments have suggested that assuming spatial separation can make a
substantial improvement in ecosystem exceedance with respect to nutrient nitrogen,
and it is expected that it will also improve the corresponding exceedance maps based
on critical loads for acidification.
It is also important to recognise the large potential errors attached to estimates of
exceedance of critical loads when smoothing out the spatial variation in deposition
over grid squares. In the UK case discussed above the 5x5km grid resolution is much
finer than that used in European scale integrated assessment where 50 by 50 km is the
highest resolution available (work for the Gothenburg protocol was based on even
larger grids ~150 x150 km). It may be useful to explore this by comparing exceedance
estimates based on finer national scale grids with a corresponding smoothed out
deposition field with the coarser European scale resolution.
Conclusions and further work
The above analysis suggests that avoiding emissions close to sensitive ecosystems,
even along a narrow band of width 50 metres along the perimeter, can improve the
protection of ecosystems and help to reduce exceedance and associated effects.
Simple cases have been used to illustrate situations for small and large ecosystem
areas. These are somewhat idealise but indicate the deviation from traditional
modelling assumptions.
The potential for exploiting reduction of deposition to sensitive ecosystem areas by
restricting emissions close to their perimeters should be investigated further. In
modelling deposition and exceedance, the effect of alternative assumptions about
contributions from within the local grid square also needs to be addressed.
Acknowledgements
This work has been funded by the UK Department of Environment, Food and Rural
Affairs. We should also like to thank CEH Edinburgh for provision of data from the
FRAME model, and CEH Monkswood for the compilation and supply of critical load
maps and assistance in calculation of exceedance.
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