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An application and review of the critical load concept to the soils of northern England
Environmental Pollution 77 (1992) 205-210
An application and review of the critical load concept to the soils of northern England Simon J. Langan* & Mike Hornung Institute of Terrestrial Ecology, Merlewood Research Station, Grange-Over-Sands, Cumbria, UK, LAl l 6JU
In common with other member states of UN-ECE, maps of critical loads of transboundary air pollutants are to be produced in the UK for different receptor (waters, soils and vegetation) types. These maps will be used as a tool for assessing different deposition scenarios with proposed pollution abatement strategies. This paper presents the methodology, results and a discussion of the principles used in applying critical loads of sulphur as a pilot study for soils in northern England. For the study area, critical load classes for soils vary with geology, drift cover and slope/elevation. The area of soils in which the critical load is exceeded varies significantly according to the type of deposition data utilised. INTRODUCTION
compounds that will not cause chemical changes in the soil which will lead to long-term harmful effects on the structure and function of the ecosystem' (Nilsson & Grennfelt, 1988). Sverdrup et aL (1990) have suggested that the critical chemical value(s) for soils could be set with reference to one, or a combination of, the following indicators: soil or soil solution pH, acid-neutralising capacity, soil-solution aluminium, AI/Ca molar ratio, base saturation, nutrient concentrations, nutrient ratios. Over the medium to long term, the rate or potential rate of release of base cations by mineral weathering is a controlling factor for a number of these indicators. Weathering rates will be influenced by mineralogy, texture and hydrological properties of the soils and by the amount and chemistry of atmospheric inputs. Several methods have been proposed for the calculation of critical loads for soils including a biogeochemical steady-state calculation and the use of mathematical models such as M A G I C (Cosby et al., 1985), P R O F I L E (Warfvinge & Sverdrup, 1991) or RAINS (Kamari, 1986). However, the input data required for these approaches are rarely available for enough sites to allow regional critical load maps of soils to be produced. In this study the authors have explored the possibility of allocating soil mapping units on existing regional soil maps, for the north of England, to sensitivity classes. These classes are defined in terms of sulphur, and use available information on the mineralogy of the soil mapping units. The authors have then used the resultant critical load map with available deposition data to identify areas where the critical load is currently exceeded (exceedance maps). The impact of using total inputs, as opposed to bulk deposition, on the area of exceedance is also examined.
The concept of critical loads has gained increasing attention in recent years as a method for the development and implementation of control strategies for transboundary air pollutants. The term critical load has been defined in environmental science as, 'a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge' (Nilsson & Grennfelt, 1988). In the present context 'pollutants' refers to sulphur and nitrogen compounds. The term 'exposure' is taken as a deposition experienced on an area basis. Effects may be chemical changes in soils and waters which give rise to further direct or indirect effects on organisms. Significant harmful effects may arise from shortterm or long-term deposition levels. Sensitive elements can be part or whole of an ecosystem. Ideally, the effects are evaluated with respect to the response of one, or a group of, organisms. Thus, in surface waters, the brown trout (Salmo trutta L) has been used and critical loads determined to ensure the survival of viable populations. The critical load is then calculated using one or a group of chemical indicators for which there is information on the response of the target organisms (Bull, 1991). The critical load for acidic deposition to soils has been defined as, 'the highest deposition of acidifying * Present Address: The Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen UK, AB9 2QJ. Environ. Pollut. 0269-7491/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain 205
Simon J. Langan, Mike Hornung
206 METHODOLOGY
The soil sensitivity classes used in this study were those agreed at a workshop on critical loads held in Skokloster, Sweden, in 1988 (Nilsson & Grennfelt, 1988). Five classes were defined and the boundaries between the classes were set in terms of critical loads of S (or H ÷) km-2 year-~. As noted above, the mineralogy of soils is a major factor controlling the critical load of soils and the sensitivity classes were also characterised in terms of the dominant weatherable minerals occurring in the soils classified (Table 1). The system also incorporated a series of secondary conditions or modifiers which could change the classification of a soil based solely on mineralogy (Table 2). The soil data base used in the study was the 1 : 250 000 soil map of northern England (Soil Survey, 1983), the accompanying bulletin (Jarvis et al., 1984) and relevant memoirs and records published by the Soil Survey of England and Wales. The units on this map are associations which comprise a group of soil series. A transect was used stretching from the west to the east coast between northings (460-560). This transect provided a wide range of soil types from those of a calcareous nature on the east coast of Yorkshire to the acidic soils of upland Cumbria. For this area of the UK, solid geology maps are also available at the 1 : 250 000 scale. Two areas within the transect were selected for the detailed study, covering the English Lake District and an area of North Yorkshire on the eastern side of the country. Deposition data were provided on a 20 × 20 km2 grid from model-generated estimates of dry, seederfeeder enhanced wet, occult and the sum of these, the total deposition (UKRGAR, 1990; Fowler et al., 1991). The modelled estimates were calculated from pollutant levels monitored during 1986-88. Table 1. Mineralogical and petrological classification of soil
Class
Mineral controlling weathering
5
Quartz Potassium feldspar Muscovite Plagioclaise Biotite (<5%) Biotite Amphibole (<5%)
4
3
2
1
Pyroxene Epidote Olivine (<5%) Carbonates
Nominal parent rock
Critical load (kg S ha-1 year-l)
Granite Quartzite
<3
Granite Gneiss
3-8
Granodiorite Greywaeke Schist Gabbro Gabbro
Table 2. Conditions influencing critical loads to soils
8-16
16-32
Basalt
Limestone Marlstone
The map units, on the soil map of northern England, which fell within the study transect were allocated to one of the five sensitivity classes on the basis of the mineralogy of the dominant soil series within the mapping unit. Mineralogical information was obtained from the relevant soil bulletins and memoirs, and from data held at the Institute of Terrestrial Ecology. In addition, areas which were unsurveyed (largely urban areas) and soils which have been disturbed (mine tailings, etc.) were mapped separately. Peat soils cannot be allocated to a sensitivity class on the basis of mineralogy. Also, Cresser, M. (1991, pers. comm.) has indicated that organic peat soils respond to an effective pH rather than a total sulphur deposition. Cresser is presently applying such an approach to organic soils for the UK; consequently, these soils have been omitted in the present work. In this study, the modifiers (Table 2) of soil texture, depth and drainage have been considered in the assignment of soil series to critical load classes. The modifiers of elevation and precipitation have been incorporated into the estimates of atmospheric inputs (for the generation of exceedance maps). At present, there is not sufficient information on soil sulphate adsorption capacity to incorporate into the map and it has therefore been omitted. To allow the role of land-use, where a soil association is dominated by agriculture, the critical load has been decreased by a class. Similarly, where an area of a given soil association is dominated by coniferous plantations, the critical load has been increased by one class. Exceedance maps for the two study areas have been produced by taking the wet-deposition data for each 20 km2 grid and mapping the extent of the exceedance within that grid square. A second set of exceedance maps was produced with the same classification of soil associations but input loading was increased by considering total deposition inputs. The critical load values for each of the soil sensitivity classes used are shown in column 4 of Table 1. With this data, maps which show areas where deposited S is within the range of critical load exceedance (plotted as T) have been separated from areas in which
>32
Factor Precipitation Vegetation Elevation/slope Soil texture Soil drainage Soil/till depth Soil sulphate adsorption capacity Base cation deposition
Decrease
Increase
High Coniferous High Coarse-sandy Free Shallow Low
Low Deciduous Low Fine Confined Thick High
Low
High
Critical load concept to soils
:i :
207
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)?"2:)))73 '?±
II
•
CLASS 5
CLASS 2
•
CLASS 4
CLASS T
I
CLASS 3
~)
I
URBAN
PEAT
I0 km
(a)
"
I
CLASS S
Q
CLASS 2
T
I I
CLASS4 CLASS3
Q ~
CLASS I
PEAT
~
URBAN IO km
(a)
(b)
(b)
Fig. 1. The distribution of Skokloster critical load classes for soils in NW England without (a) and with (b) the modifier of land-use.
Fig. 2. The distribution of Skokloster critical load classes for soils in NE England without (a) and with (b) the modifier of land-use.
208
Simon J. Langan, Mike Hornung
the uppermost class value is exceeded (plotted as E) and the lower class value is not exceeded (plotted as N).
upland areas, since these soils form on the hard, slowly weathered bedrock. These uplands receive enhanced amounts of precipitation through orographic uplift, and seeder-feeder processes; consequently, the atmospheric loading to these areas is also high.
RESULTS Figures 1 and 2 show the distribution of the critical load classes for soils in the two study areas, with and without considering the influence of land-use. Figure 1 suggests that the Lake District is dominated by soils with low critical loads (high sensitivity). The classes are spatially dispersed and their scattered pattern corresponds to major topographical features of the area. Thin, poorly developed ranker soils, derived from greywackes, occur on the peaks with a low critical load; alluvial and gleyed soils in the valleys have higher critical loads. The influence of land-use does little to modify the critical load in these moorland and heath areas. Agriculture in the south and north of the map effectively increases the critical loads of the soils in these areas. Figure 2 shows the distribution of critical load classes in northeast England. Soils with higher critical loads dominate. This reflects the widespread occurrence of deeper soils derived from glacial till with a high clay content and easily weatherable minerals on a more gentle topography. The western edge of the study area shows scattered areas of soils with lower critical loads, associated with the coarse, sandy parent materials of the North York moors. The influence of relatively intensive agriculture across much of the area has the effect of increasing the critical load. Wet deposition alone is sufficient to exceed the critical loads of many Lake District soils (Fig. 3(a)). Using total deposition (Fig. 3(b)), critical loads of soils flanking the uplands to the north and southeast are also exceeded (i.e. are brought within the range between upper and lower values of critical load classes in Table 2, column 4). The apparent 'expansion' in the area exceeded by considering the additional deposition is confined to the southwest area. Figure 4(a) shows the areal extent of exceedance to be small and restricted to the upland soils of the North York moors. Comparison of the wet only with total deposition exceedance map (Fig 4(b)) suggests that the additional input from dry, occult and enhanced wet deposition is sufficient to bring a considerable area of the soils within the range of exceedance. The mapped results of soil critical loads show that the full range of classes occur within northern England, soils with the lowest critical loads occurring in the Lake District and Cumbria, and those with generally higher critical loads in East Yorkshire. In general, the soils follow the dominant mineralogy of the underlying geology or drift. The exceedance maps illustrate the implicit correlation between the most sensitive soils and
DISCUSSION The use of an existing soil data base has facilitated the allocation of soil map units to the Skokloster sensitivity classes. The success of the approach described has consequently resulted in the adoption of the method to produce first draft critical-load maps for the whole of the UK. In northern England, there are two distinct outcrops where critical loads are exceeded (centred on the study areas included in this paper): the thin and base deficient soils of the Lake District and the coarse sandy soils of the North Yorkshire moors. Severe reductions in emissions would be required to reduce deposition levels to below the critical loads of the soils in these areas. Conversely, for most of those soils where the critical load has not been reached, it is doubtful that these areas will, on the whole, show any effects of acidification. The work has also illustrated the need to consider the modifying influence of land management, particularly agriculture. This raises the question of calculating critical loads for soils under a variety of land uses in addition to forestry as indicated in the original modifying list. Exceedance maps produced by overlaying the soils critical load map with various components of atmospheric deposition have shown the importance and necessity of considering total deposition inputs. In the next stage of the work, validation of the critical loads assigned to soil units on the basis of the sensitivity classes will be undertaken. Validation will be focused particularly on comparison of the results presented with results from the application of both steadystate and dynamic models at sites with sufficient environmental data. Further validation will be provided by the measurement of the chemical indicators in areas indicating exceedance and not exceeded in the present study.
CONCLUSIONS The application of critical load sensitivity classes to a variety of soil types occurring over a large geographical area has been possible using existing data sets. The exercise has illustrated the likely spatial variations in soil sensitivity to atmospherically derived sulphur, and regions where the threshold to such inputs has been, or is in the process of being, exceeded. With preliminary
Critical load concept to soils
209
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~ I
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O TRANSITIONAL
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~
~
~
~
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~
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EXCEEDED ~
~~ ' _-'~ URBAN
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NOTEXCEEDED
Io km
(a)
(a)
iiiii
(b)
(b)
Fig. 3. The distribution of critical load exceedance for soils in NW England calculated for (a) wet deposition only and (b) total deposition.
Fig. 4. The distribution of critical load exceedance for soils in NE England calculated for (a) wet deposition only and (b) total deposition.
210
Simon J. Langan, Mike Hornung
national maps now well in progress there is a need to embark on a thorough validation of the application of critical loads to the ecosystems most at risk.
ACKNOWLEDGEMENT This work was carried out with funding from the Department o f the Environment.
REFERENCES
Bull, K. R. (1991). The critical loads/levels approach to gaseous pollutant emission control. Environ. Poll., 69, 105-230. Cosby, B. J., Wright, R. F., Hornberger, G. M. & Galloway, J. N. (1985). Modelling the effects of acid deposition: Assessment of a lumped parameter model for soil water and stream water chemistry, Water Resources Res., 21, 51-63. Fowler, D., Smith, R. I., Weston, K. J., Choularton, T. W. & Dore, A. (1991). The orographic enhancement of wet deposition in the United Kingdom. Nature, (in preparation). Jarvis, R. A., Bendelow, V. C., Bradley, R. I., Carroll, D. M.,
Kilgour, I. N. L. & King, S. J. (1984). Soils and Their Use in Northern England. Soil Survey of England and Wales, Harpenden, UK. Kamari, J. (1986). Critical deposition limits for surface waters assessed by a process-oriented model. In Critical Loads for Nitrogen and Sulphur, Ed. J. Nilsson. Nordisk Ministerad, Koberhaven. Nilsson, J. & Grennfelt, P. (ed.) (1988). Critical loads for sulphur and nitrogen. UN-ECE/Nordic Council workshop report, Skokloster, Sweden. Nordic Council of Ministers NORD 1988:97. Soil Survey of England and Wales (1983). Soils in Northern England, sheet 5, Scale 1 : 250 000 Soil Survey of England and Wales, Harpenden, UK. Sverdrup, H. U., de Vries, W. & Henriksen, A. (1990). Mapping critical loads--Guidance to criteria, methods and examples for mapping critical loads and areas where they have been exceeded. Task force on mapping, with the assistance of the United Nations Economic Commission for Europe, Bad Harzburg, FRG, 124 pp. UK Review Group on Acid Rain (UKRGAR) (1990). Acid deposition in the United Kingdom 1986--1988. Third report for the Department of the Environment, London, HMSO, 124 pp. Warvinge, P. & Sverdrup, H. U. (1992). Calculating critical loads of acid deposition with PROFILE--A steady state soil chemistry model. Water, Air, Soil, Pollut.
An application and review of the critical load concept to the soils of northern England Simon J. Langan* & Mike Hornung Institute of Terrestrial Ecology, Merlewood Research Station, Grange-Over-Sands, Cumbria, UK, LAl l 6JU
In common with other member states of UN-ECE, maps of critical loads of transboundary air pollutants are to be produced in the UK for different receptor (waters, soils and vegetation) types. These maps will be used as a tool for assessing different deposition scenarios with proposed pollution abatement strategies. This paper presents the methodology, results and a discussion of the principles used in applying critical loads of sulphur as a pilot study for soils in northern England. For the study area, critical load classes for soils vary with geology, drift cover and slope/elevation. The area of soils in which the critical load is exceeded varies significantly according to the type of deposition data utilised. INTRODUCTION
compounds that will not cause chemical changes in the soil which will lead to long-term harmful effects on the structure and function of the ecosystem' (Nilsson & Grennfelt, 1988). Sverdrup et aL (1990) have suggested that the critical chemical value(s) for soils could be set with reference to one, or a combination of, the following indicators: soil or soil solution pH, acid-neutralising capacity, soil-solution aluminium, AI/Ca molar ratio, base saturation, nutrient concentrations, nutrient ratios. Over the medium to long term, the rate or potential rate of release of base cations by mineral weathering is a controlling factor for a number of these indicators. Weathering rates will be influenced by mineralogy, texture and hydrological properties of the soils and by the amount and chemistry of atmospheric inputs. Several methods have been proposed for the calculation of critical loads for soils including a biogeochemical steady-state calculation and the use of mathematical models such as M A G I C (Cosby et al., 1985), P R O F I L E (Warfvinge & Sverdrup, 1991) or RAINS (Kamari, 1986). However, the input data required for these approaches are rarely available for enough sites to allow regional critical load maps of soils to be produced. In this study the authors have explored the possibility of allocating soil mapping units on existing regional soil maps, for the north of England, to sensitivity classes. These classes are defined in terms of sulphur, and use available information on the mineralogy of the soil mapping units. The authors have then used the resultant critical load map with available deposition data to identify areas where the critical load is currently exceeded (exceedance maps). The impact of using total inputs, as opposed to bulk deposition, on the area of exceedance is also examined.
The concept of critical loads has gained increasing attention in recent years as a method for the development and implementation of control strategies for transboundary air pollutants. The term critical load has been defined in environmental science as, 'a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge' (Nilsson & Grennfelt, 1988). In the present context 'pollutants' refers to sulphur and nitrogen compounds. The term 'exposure' is taken as a deposition experienced on an area basis. Effects may be chemical changes in soils and waters which give rise to further direct or indirect effects on organisms. Significant harmful effects may arise from shortterm or long-term deposition levels. Sensitive elements can be part or whole of an ecosystem. Ideally, the effects are evaluated with respect to the response of one, or a group of, organisms. Thus, in surface waters, the brown trout (Salmo trutta L) has been used and critical loads determined to ensure the survival of viable populations. The critical load is then calculated using one or a group of chemical indicators for which there is information on the response of the target organisms (Bull, 1991). The critical load for acidic deposition to soils has been defined as, 'the highest deposition of acidifying * Present Address: The Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen UK, AB9 2QJ. Environ. Pollut. 0269-7491/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain 205
Simon J. Langan, Mike Hornung
206 METHODOLOGY
The soil sensitivity classes used in this study were those agreed at a workshop on critical loads held in Skokloster, Sweden, in 1988 (Nilsson & Grennfelt, 1988). Five classes were defined and the boundaries between the classes were set in terms of critical loads of S (or H ÷) km-2 year-~. As noted above, the mineralogy of soils is a major factor controlling the critical load of soils and the sensitivity classes were also characterised in terms of the dominant weatherable minerals occurring in the soils classified (Table 1). The system also incorporated a series of secondary conditions or modifiers which could change the classification of a soil based solely on mineralogy (Table 2). The soil data base used in the study was the 1 : 250 000 soil map of northern England (Soil Survey, 1983), the accompanying bulletin (Jarvis et al., 1984) and relevant memoirs and records published by the Soil Survey of England and Wales. The units on this map are associations which comprise a group of soil series. A transect was used stretching from the west to the east coast between northings (460-560). This transect provided a wide range of soil types from those of a calcareous nature on the east coast of Yorkshire to the acidic soils of upland Cumbria. For this area of the UK, solid geology maps are also available at the 1 : 250 000 scale. Two areas within the transect were selected for the detailed study, covering the English Lake District and an area of North Yorkshire on the eastern side of the country. Deposition data were provided on a 20 × 20 km2 grid from model-generated estimates of dry, seederfeeder enhanced wet, occult and the sum of these, the total deposition (UKRGAR, 1990; Fowler et al., 1991). The modelled estimates were calculated from pollutant levels monitored during 1986-88. Table 1. Mineralogical and petrological classification of soil
Class
Mineral controlling weathering
5
Quartz Potassium feldspar Muscovite Plagioclaise Biotite (<5%) Biotite Amphibole (<5%)
4
3
2
1
Pyroxene Epidote Olivine (<5%) Carbonates
Nominal parent rock
Critical load (kg S ha-1 year-l)
Granite Quartzite
<3
Granite Gneiss
3-8
Granodiorite Greywaeke Schist Gabbro Gabbro
Table 2. Conditions influencing critical loads to soils
8-16
16-32
Basalt
Limestone Marlstone
The map units, on the soil map of northern England, which fell within the study transect were allocated to one of the five sensitivity classes on the basis of the mineralogy of the dominant soil series within the mapping unit. Mineralogical information was obtained from the relevant soil bulletins and memoirs, and from data held at the Institute of Terrestrial Ecology. In addition, areas which were unsurveyed (largely urban areas) and soils which have been disturbed (mine tailings, etc.) were mapped separately. Peat soils cannot be allocated to a sensitivity class on the basis of mineralogy. Also, Cresser, M. (1991, pers. comm.) has indicated that organic peat soils respond to an effective pH rather than a total sulphur deposition. Cresser is presently applying such an approach to organic soils for the UK; consequently, these soils have been omitted in the present work. In this study, the modifiers (Table 2) of soil texture, depth and drainage have been considered in the assignment of soil series to critical load classes. The modifiers of elevation and precipitation have been incorporated into the estimates of atmospheric inputs (for the generation of exceedance maps). At present, there is not sufficient information on soil sulphate adsorption capacity to incorporate into the map and it has therefore been omitted. To allow the role of land-use, where a soil association is dominated by agriculture, the critical load has been decreased by a class. Similarly, where an area of a given soil association is dominated by coniferous plantations, the critical load has been increased by one class. Exceedance maps for the two study areas have been produced by taking the wet-deposition data for each 20 km2 grid and mapping the extent of the exceedance within that grid square. A second set of exceedance maps was produced with the same classification of soil associations but input loading was increased by considering total deposition inputs. The critical load values for each of the soil sensitivity classes used are shown in column 4 of Table 1. With this data, maps which show areas where deposited S is within the range of critical load exceedance (plotted as T) have been separated from areas in which
>32
Factor Precipitation Vegetation Elevation/slope Soil texture Soil drainage Soil/till depth Soil sulphate adsorption capacity Base cation deposition
Decrease
Increase
High Coniferous High Coarse-sandy Free Shallow Low
Low Deciduous Low Fine Confined Thick High
Low
High
Critical load concept to soils
:i :
207
c
)?"2:)))73 '?±
II
•
CLASS 5
CLASS 2
•
CLASS 4
CLASS T
I
CLASS 3
~)
I
URBAN
PEAT
I0 km
(a)
"
I
CLASS S
Q
CLASS 2
T
I I
CLASS4 CLASS3
Q ~
CLASS I
PEAT
~
URBAN IO km
(a)
(b)
(b)
Fig. 1. The distribution of Skokloster critical load classes for soils in NW England without (a) and with (b) the modifier of land-use.
Fig. 2. The distribution of Skokloster critical load classes for soils in NE England without (a) and with (b) the modifier of land-use.
208
Simon J. Langan, Mike Hornung
the uppermost class value is exceeded (plotted as E) and the lower class value is not exceeded (plotted as N).
upland areas, since these soils form on the hard, slowly weathered bedrock. These uplands receive enhanced amounts of precipitation through orographic uplift, and seeder-feeder processes; consequently, the atmospheric loading to these areas is also high.
RESULTS Figures 1 and 2 show the distribution of the critical load classes for soils in the two study areas, with and without considering the influence of land-use. Figure 1 suggests that the Lake District is dominated by soils with low critical loads (high sensitivity). The classes are spatially dispersed and their scattered pattern corresponds to major topographical features of the area. Thin, poorly developed ranker soils, derived from greywackes, occur on the peaks with a low critical load; alluvial and gleyed soils in the valleys have higher critical loads. The influence of land-use does little to modify the critical load in these moorland and heath areas. Agriculture in the south and north of the map effectively increases the critical loads of the soils in these areas. Figure 2 shows the distribution of critical load classes in northeast England. Soils with higher critical loads dominate. This reflects the widespread occurrence of deeper soils derived from glacial till with a high clay content and easily weatherable minerals on a more gentle topography. The western edge of the study area shows scattered areas of soils with lower critical loads, associated with the coarse, sandy parent materials of the North York moors. The influence of relatively intensive agriculture across much of the area has the effect of increasing the critical load. Wet deposition alone is sufficient to exceed the critical loads of many Lake District soils (Fig. 3(a)). Using total deposition (Fig. 3(b)), critical loads of soils flanking the uplands to the north and southeast are also exceeded (i.e. are brought within the range between upper and lower values of critical load classes in Table 2, column 4). The apparent 'expansion' in the area exceeded by considering the additional deposition is confined to the southwest area. Figure 4(a) shows the areal extent of exceedance to be small and restricted to the upland soils of the North York moors. Comparison of the wet only with total deposition exceedance map (Fig 4(b)) suggests that the additional input from dry, occult and enhanced wet deposition is sufficient to bring a considerable area of the soils within the range of exceedance. The mapped results of soil critical loads show that the full range of classes occur within northern England, soils with the lowest critical loads occurring in the Lake District and Cumbria, and those with generally higher critical loads in East Yorkshire. In general, the soils follow the dominant mineralogy of the underlying geology or drift. The exceedance maps illustrate the implicit correlation between the most sensitive soils and
DISCUSSION The use of an existing soil data base has facilitated the allocation of soil map units to the Skokloster sensitivity classes. The success of the approach described has consequently resulted in the adoption of the method to produce first draft critical-load maps for the whole of the UK. In northern England, there are two distinct outcrops where critical loads are exceeded (centred on the study areas included in this paper): the thin and base deficient soils of the Lake District and the coarse sandy soils of the North Yorkshire moors. Severe reductions in emissions would be required to reduce deposition levels to below the critical loads of the soils in these areas. Conversely, for most of those soils where the critical load has not been reached, it is doubtful that these areas will, on the whole, show any effects of acidification. The work has also illustrated the need to consider the modifying influence of land management, particularly agriculture. This raises the question of calculating critical loads for soils under a variety of land uses in addition to forestry as indicated in the original modifying list. Exceedance maps produced by overlaying the soils critical load map with various components of atmospheric deposition have shown the importance and necessity of considering total deposition inputs. In the next stage of the work, validation of the critical loads assigned to soil units on the basis of the sensitivity classes will be undertaken. Validation will be focused particularly on comparison of the results presented with results from the application of both steadystate and dynamic models at sites with sufficient environmental data. Further validation will be provided by the measurement of the chemical indicators in areas indicating exceedance and not exceeded in the present study.
CONCLUSIONS The application of critical load sensitivity classes to a variety of soil types occurring over a large geographical area has been possible using existing data sets. The exercise has illustrated the likely spatial variations in soil sensitivity to atmospherically derived sulphur, and regions where the threshold to such inputs has been, or is in the process of being, exceeded. With preliminary
Critical load concept to soils
209
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URBAN NOTEXCEEDED
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~
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EXCEEDED ~
~~ ' _-'~ URBAN
a~ TRANSITIONAL
NOTEXCEEDED
Io km
(a)
(a)
iiiii
(b)
(b)
Fig. 3. The distribution of critical load exceedance for soils in NW England calculated for (a) wet deposition only and (b) total deposition.
Fig. 4. The distribution of critical load exceedance for soils in NE England calculated for (a) wet deposition only and (b) total deposition.
210
Simon J. Langan, Mike Hornung
national maps now well in progress there is a need to embark on a thorough validation of the application of critical loads to the ecosystems most at risk.
ACKNOWLEDGEMENT This work was carried out with funding from the Department o f the Environment.
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