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Sulphate adsorption capacity and pH of upland podzolic soils in Scotland: Effects of parent material, texture and precipitation chemistry
Applied Geochemistry, Vol. 9, pp. 127-139. 1994
Pergamon
Elsevier Science Ltd Printed in Great Britain. All rights reserved 0883-2927/94 $6.00+ 0.00
Sulphate adsorption capacity and pH of upland podzolic soils in Scotland: Effects of parent material, texture and precipitation chemistry D. BARTON, D. HOPE, M. F. BILLETF a n d M. S. CRESSER Department of Plant and Soil Science, University of Aberdeen, Meston Building, Aberdeen AB9 2UE, U.K. (Received 13 April 1993; accepted in revised form 20 September 1993) A b s t r a c t - - A regional survey of podzol B horizons has been carried out to investigate the effects of parent
material and soil texture on sulphate (SO~4-) adsorption capacity and pH in Scottish soils. Sulphate adsorption was measured on equilibration of the soils with 10 and 100 mg l- l SOl- solutions. The results showed that soil texture and soil parent material had a significant effect on SO~4 adsorption. Significant correlations were found between sulphur (S) deposition loads and SOl- adsorption, and between precipitation pH and soil pH, but not between total hydrogen ion (H+ ) load and soil pH, even on sensitive soils. Relationships between the chemical composition of atmospheric deposition and soil pH could bc marginally improved if the possible amelioration of acidification by base cation inputs, especially on sensitive soils, was taken into account. INTRODUCTION
S O ] - adsorption and retention is, to some extent, dependent upon the concentration of these hydrous THEREIS still considerable uncertainty concerning the oxides in soils (JOHNSON and TODD, 1983). It might precise quantitative effects of acidic deposition on also be expected to be affected by their total surface freshwater and soil acidification. In part this is be- area. Sulphate adsorption is further influenced by the cause internal proton production, sometimes intensi- concentration of SO 2- in the incoming solution relafied by changes in land-use, is a major contributor to tive to the concentration at which the soil had long-term acidification (McFEE et al., 1977; KRUG previously equilibrated (JOHNSON and Tooo, 1983). and FRINK, 1983; TABATABAI, 1984; PETERSON, 1986). Organic matter is known to inhibit SO]4- adsorption External loads of acidity from anthropogenic by blocking adsorption sites (JOHNSON et al., 1980; sources, superimposed on the natural stores of JOHNSON and TODD, 1983). acidity, can give rise to major changes in soil solution Soil mineralogy is a critical factor in regulating the and surface water acidity and alkalinity (VAN BREE- release of base cations to balance leaching losses and MAN et al., 1984; REUSS et al., 1987). They also, also in controlling the Fe and AI hydrous oxide thereby, contribute to further soil acidification in the content of soils. In highly weathered soils, SO~4long term. Sulphate is a major component of atmos- adsorption is thought to be most important within pheric deposition and HENRIKSON (1979) suggested soils developed from base-rich parent materials that S deposition was a major contributor to surface having abundant free Fe and A1 hydrous oxides and water acidification. In the absence of this mobile less important on those developed on more acidic anion, the store of acid derived from natural soil parent materials (Sco~, 1976). processes seldom appears to result in the acidification Critical loads of acidic and S deposition have been of freshwaters to the extent that hydrogen ion and set for soils derived from various geological materials aluminium ion concentrations are mobilized to levels and proposed sensitivity classes have been estabtoxic to fish (REuss et al., 1987). Adsorption reac- lished on the basis of mineralogy and its control on tions within the soil profile can regulate concen- weathering rates (NILSSON and GRENNFELT, 1988). trations of aqueous S O l - , and SO~4- adsorption can These form the basis of maps, showing sensitivity to prevent the transport of base cations and of H + and acidification, which have now been produced for A13+ from soils (JOHNSONet al., 1979). Adsorption of Scottish soils (WILSON et al., 1988). The mapping SO 2- can also result in the release of O H - into the approach depends upon soil association, based on the soil solution (CHAOetal., 1965; RAJAN, 1978; PARFITT soil parent material, being a key factor which can be and SMART, 1978) and induce changes in surface used to identify vulnerable soils. The maps take into charge causing an increase in cation exchange capac- account other interacting factors including soil type, ity (RAJAN, 1978; WIKLANDER,1980). vegetation, landform and land-use, which may Sulphate adsorption is known to occur on Fe and overide the influence of parent material. However, it A1 hydrous oxide surfaces (CHAOet al., 1964; PARFITT is unclear to what extent potentially sensitive soils a n d SMART, 1978; RAJAN, 1978) and the degree of already show evidence of accelerated acidification,
t27
128
D. Barton et al.
w h e r e critical loads are e x c e e d e d , and w h e t h e r such soils will eventually b e c o m e m o r e acidic than they already are. The SO42- a d s o r p t i o n capacity o f the soils, being r e g a r d e d as a delaying m e c h a n i s m rather than a p e r m a n e n t l y ameliorative process, has not b e e n taken into consideration generally w h e n setting critical loads. This study set out to investigate the acidification and S O l - a d s o r p t i o n status of Scottish upland soils on a regional basis, in relation to p a r e n t material, soil texture and a t m o s p h e r i c pollution. It was felt that soils d e v e l o p e d from sensitive p a r e n t materials should be the first to show p H falls associated with deposition acidity because of their limited capacity to buffer acidification by mineral weathering. Statistical analysis should identify the key factors which affect S O ] - a d s o r p t i o n characteristics. T h e s e could t h e n be used in considering the a p p a r e n t soil sensitivity to p H fall as a c o n s e q u e n c e of a t m o s p h e r i c pollution.
MATERIALS AND METHODS This study was based on the 1:250,000 soil maps compiled by the Soil Survey of Scotland (SOIL SURVEYOF SCOXLANO, 1984). The individual mapping units of these maps are based upon soil type, parent material, landforms and the principal associated vegetation and land-use patterns. To facilitate studying relationships between soil chemistry and existing pollutant deposition maps for the U.K., the sampling programme was based on the Ordnance Survey 20 km square grid system used by the U.K. Review Group on Acid Rain (UKRGAR, 1990). Mean values from 1986-1988 of precipitation chemistry and pollution inputs are available for each grid square. The dominant soil association within a grid square was identified, and to minimise variability, sampling areas were selected which satisfied certain site and profile characteristics. Sampling constraints were as follows: vegetation elevation slope soil type drainage B horizon thickness
dry Atlantic heather moor 200-500 m 5--45° peaty or humic iron podzols free 15-40 cm
The soil associations sampled comprised predominantly of soils developed on glacial drifts derived from particular bedrock geologies. Within each 20 km grid square, from the land area which fitted within the above constraints, two sampling sites were selected at random. Soil profiles were exposed by either digging a soil pit or digging back into a cutting. Provided the soil profiles did not exhibit obvious anomalies, for example as a consequence of burrowing animal activity or the presence of boulders, two samples were carefully removed from the entire B horizon. From each, after individual mixing, 200-500 g sub-samples were transfered to clean labelled polythene bags for transport back to the laboratory for analysis. In some areas, grassland replaced Calluna as the dominant vegetation type. If no alternatives for that grid could be found, soils were sampled and the unsatisfactory features were recorded. Of the 20 km grid squares on mainland Scotland that were found to contain suitable sites for sampling, 86% were sampled. In all, 257 samples were collected from 132 grid squares between August and November 1991. In each 20 km grid square, the area of land within the
sampling constraints was invariably much less than 400 kn~. To gain an indication of the variability which might be encountered around each sampling site, a group of 12 appropriate moorland podzol B horizon soils was sampled from a 10 km2 site in the Glendye catchment, 40 km south west of Aberdeen. These were subjected to the same analytical procedures as the other soils in this study, pH values ran~ged from 4.08 to 4.51 (median 4.22). The amount of SO~- adsorbed by the soils on shakin~ with 100 mg 1-1 SOlranged from 0.7 to 2.32/~moles g " (median 1.24). This study gave some indication of the type of potential variability within each 20 km grid square. The soil chemical data was therefore treated as two separate sets as a means of dealing with the variability within the sample population. Thus if similar spatial trends and statistical relationships occurred in both sets of soils, then the relationships observed are less likely to be due to chance alone. Set I and Set 2 represented the 1st and 2nd soil samples taken after random selection from appropriate land within each grid square. Set 1 contained 132 soil samples and Set 2, 125 soil samples. There were fewer samples in Set 2 since duplicates were not obtained from a limited number of sites. Individual soil samples were allocated to their particular texture and parent material classes (Tables 1 and 2). Centroid percentage sand contents for each textural class were derived from the standard particle-size class triangular diagram used by the Soil Survey of England and Wales (HODGSON, 1976). Soils were allocated to seven different parent material classes. The classification was based on the elemental composition of the parent material from which the soils were derived. Values of % SiO 2 composition were obtained from a number of standard geological texts (Table 2). Estimations of SiO 2 for specific lithologies (sandstones, granites, basalts) were straightforward, whereas estimates for major regional lithological units, such as the Lewisian Gneiss, Moine Schist and Dalradian Schist, were more difficult because of the diverse lithological composition within each group. Mean SiO2 values were given when therc was more than one common rock type within each group. Soil samples were sieved and measurements of pH (0,01 M CaCI2), SO ] - adsorption capacity and moisture content were made on the <2 mm fraction. The texture of the soils was assessed by hand. All measurements were carried out on field moist soils. Soil pH measurements were obtained using a glass calomel combined electrode pair after mixing 10 g of soil with 25 ml 0.01 M CaCI 2, and leaving for l h to equilibrate. Sulphate adsorption capacity was determined using the following method. Duplicate 5 g soil samples were shaken for 6 h at room temperature with 25 ml sulphate solution (K2804). One set of duplicate soils was shaken with 100 mg 1-I, another with 10 mg 1-1 SO~4- solutions. The 10 mg 1-1 SO ] - solution relates to known soil solution concentrations whereas 100 mg I 1 SOl- is commonly used to measure SO ] - adsorption capacity (e.g. JOHNSON and HENOERSON, 1979). Previous work has identified the importance of soil and solution pH in adsorption experiments (CHAO et al., 1963; NODVINet al., 1986; HARRISONet al., 1989; GUADALIX Table 1. Per cent sand content and number of samples within each textural class No. of samples Texture Class l 2 3 4 5
Soil texture Sand Loamy sand Sandy loam Sandy silt loam Silt loam
% Sand
Set 1
Set 2
92 82 63 35 10
6 31 45 29 21
4 32 45 27 17
Sulphate and pH of upland podzolic soils in Scotland
129
Table 2. Per cent SiO 2 content and number of samples within each parent material class Parent material class
No. of samples Geology Quartzites and sandstones Granite Lewisian gneiss and Moine schists Greywackes and shales Intermediate igneous rocks Dalradian schists Basic igneous rocks
% SiO2
Set 1
Set 2
74.6 71.3 66.7 63.4 61.4 55.1 49.7
20 15 39 19 7 21 9
18 14 37 19 7 21 7
10 mg / litre equilibration .~
0.1
~
-0.1
,SetlDset2
a -o.2
i
~
i
i
i
i
1
2
4
100 mg / litre equilibration
IISet 1
3
,~ .2
~
3
Textural Class
10 m ~ u i l i b r a t i o n 0.I
PETrlJOHN(1975) Cox et al. (1979) C~IG (1983) PETrlJOHN(1975) Cox et al. (1979) MUELLERand SAXENA(1977) Cox et al. (1979)
100 mg/litre equilibration
Textural Class .~
Reference
"T
2 -O.l
~-0.2 ~
0 1
2
3
4
5
6
Parent Material Class
'
1
'
2
3
4
5
6
7
Parent Material Class
F~c. 1. Mean sulphate adsorption and 95% confidence limits for each textural and parent material class. and PARDO, 1991). All the 100 mg 1-I and 10 mg 1-180~4 solutions used were accordingly adjusted to within _+0.1 units of soil pH using 0.1 M HC1. After shaking, the soils were left to equilibrate for approximately 12 h and then centrifuged, filtered (Whatman No. 1 filter paper) and the filtrate analysed for SO]4- using ion chromatography (Dionex Series 4500i instrument with lonPac AS4A column). Adsorbed SO2- was calculated from the difference in concentration between that in the initial solution and that in the final filtrate. The results are expressed as SOl- adsorbed in pmoles SO] per g oven dry soil.
RESULTS SO~- A d s o r p t i o n T e x t u r e , Mean values of S O ] - adsorption and the upper or lower 95% confidence limits, determined at equilibrium S O ] - concentrations of 10 mg 1- j and 100 mg 1-~ , are shown for each textural class in Fig. 1. Analysis of the two sets of data gave comparable results and similar trends for the soils equilibrated with the 100 mg 1- j SO42- solution. Differences between the two sets of soils were observed when the
soils were shaken with 10 mg 1-1 SO]4-. The results showed that the capacity of soils to retain SO 2increased uniformly from sand (Class 1) to silt loam (Class 5) textured soils. All textural classes showed a net release of SO 2-, in the 10 mg 1-1 equilibration, except the silt loam soils (Class 5) for both Sets 1 and 2. P a r e n t material. Mean values of SO]4- adsorption calculated for each parent material class showed that both sets of data displayed similar trends for the soils equilibrated with 100 mg 1-I SO 2 (Fig. 1). In the soils shaken with 10 mg 1 i SO] the agreement of the results was not as good between Set 1 and Set 2. The 100 mg 1 J results showed a relatively uniform increase in S O ] - adsorption capacity from those soils developed from parent materials with a high silica content (quartzites and sandstones) to those with a low silica content, e.g. basic igneous rocks. Although soils developed on parent material Class 5 (intermediate igneous rocks) do not follow this overall trend, the 95% confidence limits are quite large for this group of soils. Mean S O l - adsorption values
130
D. Barton et al.
calculated for the soils shaken with 10 mg 1-1 SO 2showed that all the parent material classes, excluding Class 7 (basic igneous rocks), tended to release S042-" Regional trends and correlations. Maps showing the spatial variability in SO ] - adsorption capacity in Scotland at both 10 mg 1- l and 100 mg 1- l SO]a- are shown in Fig. 2. Although it is difficult to analyse the data visually, the maps for Set 1 and Set 2 soils equilibrated at 100 mg 1-1 identify areas of low SO]aadsorption capacity (<2/,(moles g-1 SO]4-) concentrated in eastern, northern, central and southern Scotland. Soils with the highest adsorption capacities appear to occur more commonly on the west coast of Scotland and the western central area, where S deposition loads are highest ( U K R G A R , 1990). Maps of S O ] - adsorption capacity derived from soils treated with 10 mg 1-1 SO 2- are divided into soils which adsorb and soils which release SO24- . These appear to show the same broad patterns as for the soils equilibrated with the 100 mg 1-1 solution, with large areas of soils showing a net release of SO]a- concentrated in the northern, eastern and southern parts of Scotland. To provide a more rigorous analysis of the regional data, the relationships between SO]n- adsorption capacity and sulphur deposition were tested using Spearman Rank correlation analysis (Table 3). Sulphur deposition data was supplied by the Warren Spring Laboratory ( U K R G A R , 1990). The results showed significant (P < 0.001) correlations between the amount of SO]- the soils adsorbed from the 100 mg 1-1 SO ] - solution and inputs of wet (marine and non-marine) plus dry deposited S Q~g m -2 a - l ) . A significant (P < 0.001) correlation was also observed between SO~- adsorbed with the 10 mg 1-1 treatment for Set 1 soils but not for Set 2. The relationships were such that the soils with greater capacity to adsorb SO]- were found in those areas that have greater sulphur deposition loads. These results are the opposite trend to what intuitively might be expected, and possibly suggest a coincidental (rather than causal) relationship. The distribution of parent materials (Fig. 3) is such that regions of high S deposition, especially in western central areas, often have a greater abundance of base rich parent materials. Moreover these are likely to result in finer textured soils. For example, 27 out of 36 soils derived from Class 4 (greywackes and shales) were in textural classes 4 and 5. There was a highly significant Spearman Rank correlation (P < 0.001) between parent material and textural class. There was also a significant (P < 0.05) inverse correlation between total S deposition and parent material SiO 2 content. Relationships between soil pH and SO]4- adsorption capacity were also investigated. A significant (P < 0.05) correlation, for Set 1 soils equilibrated in 100 mg 1- ~ SO ]- , was found between the capacity of the soils to adsorb SO42- and soil pH (Table 4). However, this correlation was not apparent for Set 2 soils at the
same SO]- concentration, nor for Set 1 and 2 soils equilibrated in 10 mg l-I SO]-. When the relationship was examined for each individual parent material class, using the combined data sets, significant relationships were only found for the Class 1 (r~ = 0.44, P < 0.01) and Class 2 (r s = 0.41, P < 0.05) soils at the 10 mg 1-1 SO']4- level and Class 1 (rs = 0.39, P < 0.05) soils at the 100 mg 1-1 level. Multiple linear regression analysis was used to examine the relationships among SO~4- adsorption capacity, as a dependent variable, and either texture and S deposition load or parent material and S deposition load, as separate pairs of independent variables. Since the results suggested that parent material and texture were correlated they could not be included in the same regression equation. It was found that for the 10 mg 1 1 results, the combination of both sets of variables accounted for less than 8% (coefficient of multiple determinations, R2 , <8%) of the variability on SO 2- adsorption capacity. Texture (quantified in terms of % sand content) alone gave the greatest coefficient value for the 100 mg 1- l equilibration results (R 2 = 47%) and the inclusion of S deposition load did not improve this value significantly. Parent material (% SiO2 content) accounted for less than 25% (R2 = 25%) of the SOl- adsorption capacity values measured at 100 mg 1-1 and again the inclusion of S deposition did not improve the coefficient value significantly.
Soil pH Texture. Figure 4 shows how mean soil pH varies with textural class for both sets of soils. The results suggest that there was no relationship between soil pH and soil texture. Table 3. Spearman Rank correlations (r0 of SO4 adsorption capacity versus S deposition load Set 1 Equilibrating concentration 10 mg 1-l SO4 100 mg I-I SO4
Set 2
No. of samples
r~
No. of samples
r,
129 132
0.31 *** 0.43***
122 125
0.10 0.33***
• **P < 0.001. Table 4. Spearman Rank correlations (r,) of SO4 adsorption capacity versus soil pH Set 1 Equilibrating concentration I0 mg I -) SO4 I00 mg I -i S04 *P < 0.05.
Set 2
No. of samplcs
r~
No. of samples
r~
129 132
0.08 O.19"
122 125
0.07 0.03
Sulphate and pH of upland podzolic soils in Scotland
Set 1 SO4 adsorption 10 mg/litre equnibration
13
Set 2 S04 adsorption 10 mcj/litre equilibration
S04 (urn(
S04 (umo ABC
BEE,
Set 1 S04 adsorption 100 mg/litre equilbration
S04
!
Set 2 S04 adsorption 100 mg/litre equilibration
g
SO4
m m m
m m.
FIG. 2. Regional pattern of sulphate adsorption for Set I and Set 2 soils equilibrated with 10 and 100 mg I J
so~-.
32
D. Barton et al.
Parent Material Class Distribution
PM Class Basic Igneous Rock,. Dalraclian Schists Intermediate Basic I Greywackes and Sh Lewisian Gneiss anc Granites Quartzites and San
F16.3. Regional distribution of parent material class.
iN
BELC..
ABO ~
Soil pl I
BELOW
4
4
ABO~
Soil pF
3.9
Flo. 5. Regional pattern of soil p H for Set 1 and Set 2 soils.
Set 1 Soil pH
Set 2 Soil pH
e-,
.~-
E"
© C~
C~
e'~
=_..
f,¢3
Sulphate and pH of upland podzolic soils in Scotland
135
~4.
"fll !l 1
2
3 4 Textural Class
5
1
2 3 4 5 6 Parent Material Class
7
Fl6.4. Mean pH and 95% confidence limits for each textural and parent material class. Table 5. Spearman Rank correlation (r~) of rainfall acidity with soil pH Set 1
Rainfall pH H + load
Set 2
No. of samples
rs
No. of samples
rs
132 132
0.48*** -0.07
125 125
0.38*** -0.09
***P < 0.001.
Parent material. Mean pH values were calculated for each parent material class. Results and trends were similar for Sets 1 and 2 (Fig. 4). Excluding parent material Classes 2 and 3, the mean soil pH increased from Class 1 to 7. The relatively high mean pH values found for parent material Classes 2 and 3 may be a consequence of the greater proportion of these soils being situated in relatively unpolluted areas in north and west Scotland. The soils derived from sandstones and quartzites (Class 1) had mean pH values of 3.90 and 3.96 for Set 1 and Set 2, respectively. Soils derived from basic igneous rocks (Class 7) had mean pH values of 4.40 and 4.44 for Sets 1 and 2, respectively. Regional trends and correlations. Regional variations in the pH of podzol B horizons are shown for the two sets of soils in Fig. 5. Unlike the regional SO4 adsorption maps (Fig. 2), there appears to be less variability between Set 1 and Set 2 soils. The most acidic B horizons tend to group together on the southern margin of the central valley of Scotland and also in eastern Scotland. The least acidic soils appear to be concentrated on the west coast. The relationship between volume-weighted mean rainfall pH (Warren Spring data) and soil pH was investigated statistically on a regional basis. Spearman Rank correlation analysis showed significant relationships (Set 1 and Set 2, P < 0.001) between soil pH and rainfall pH (Table 5). No significant correlation was observed between the H + deposition load (keq ha -j a-I), including both wet and dry deposition of H +, and the soil pH, in spite of the fact that such a relationship would be anticipated for sensitive soil types according to the current U.K. critical loads approach.
Multiple linear regression analysis indicated that parent material and rainfall pH accounted for less than 25% (coefficient of determination, R2, = 25%) of the variability in the soil pH results observed. Since both the nature of parent material and rainfall pH appear to influence soil pH, an examination of the relationship between soil and rainfall pH within each parent material class was carried out. Data from the two sets of soil were combined to provide sufficient data within each parent material class for statistical analysis. The graphs (Fig. 6) show that while six of the parent material classes have soils which have been sampled across a wide range of rainfall pHs (4.3-4.9), the group of soils derived from the Lewisian Gneiss and Moine Schists has been sampled from areas which cover a narrower deposition pH range (4.6 to <5). This makes interpretation of the effects of rainfall on soils developed from this parent material class difficult. Spearman Rank correlation showed significant relationships between the pH of the soil B horizons and precipitation pH for the quartzites and sandstones (Class 1) and for the granites (Class 2). The only other parent material class which gave a significant relationship was the basic igneous group (Class 7). The latter correlation was the result of a group of three high pH podzol B horizons in an area of very high base cation deposition in western Scotland. Omitting data for these sites prior to applying the statistical test resulted in no significant trend being observed. Spearman Rank correlations, between soil pH and H + ion deposition load (keq ha- 1 a - l ) , were found to be insignificant for all parent material classes except Class 7 (basic igneous rocks). No relationship was apparent for these soils, when the three high pH soils (pH values greater than 4.9) were excluded. Several of the soils with the highest pH values tended to be found on the west coast of Scotland and the Western Isles. Relatively higher base cation inputs from the atmosphere, as a consequence of the marine climate, may have an effect on soil chemistry at these locations. To investigate this, the relationship between soil pH and base cation inputs in precipitation was examined for the individual parent material classes (Fig. 7). Combined data sets were again analysed using Spearman Rank correlation. To obtain an indication of the relative importance of
136
D. Barton et al. Quartzites & S a n d s t o n e s
Lewisian Gneiss & Moine Schists 5.(
Granites
5.6
5.6
o
o
8
4.8 o o
8
B
o° o
s
4'.4
~'
416
51i
4
rs= 0.62 p < 0.001
~ 3.
o
4.8
3.
4.4
od
5A
414
416
4.8
5
I n t e r m e d i a t e I g n e o u s Rocks
o a
%
c
o
3.2
4.6
4.8
Dalradian Schists
5.6
,¢ gT.°~ ¢ o
o
4.4
is= 0.11
rs= -0.10
4.6
Basic I g n e o u s R o c k s
4.4
m o v
4
4.8
3.~
4-8f
v
8
@
rs= -0.17
4.6
o
~= 0.22
0.61
p < o.oi
4.@
o
om
•
o
3.~
'
~ o
o
~ ~e °
rs=
Greywackes & Shales
4.8
o
oq~
o~
o
4.~
o
o°
4.8
414
5
416
418
Rainfall pH
Rainfall pH
5.6 m
4.8 o
rs= 0_54 p < 0.05 3.
4'.6
414
4.8
'
Rainfall pH Fie,. 6. The relationship between rainfall pH and soil pH with respect to parent material class (& = Spearman Rank Correlation Coefficient).
5.6
Quartzites & S a n d s t o n e s
4.si°~l 5"6i=[
c
4.8 e~
o [3
3.2
8
3
Greywaekes & Shales
5.6
'i!
8
oo
rs= 0.75
F' < o.ool 3
4
5
6
7
I n t e r m e d i a t e I g n e o u s Rocks 5.6
3.
i
o
o oD
8g~ ooB D a
o
2
o
4.8
4
rs= 0.63 p < 0.001 5 6 7
4
~8
~*~
o oG
o
l
Lewisian Gneiss & Moine Schists
Granites
n
rs= 0.30 p < 0.0l
~
~
°
~
°
D a l r a d i a n Schists
5"6I ¢h
4.8
4.8
aG
O
51!
4
3.
4
1
2
3
5
6
Basic I g n e o u s R o c k s
4.8
~
Do
Is= 0.15
o
o
7
3.
l
3
4
5
6
(Na++ C~++ M~+) - H + k e q / h a / yr
o
4
Is= 0.09 2
o
~
o
rs= 0.13 7
3.2~
-i
2
3
4
5
6
(Na++ Ca2++ M~+) - H + keq / h a / y r
o
~
i
~
~
rs= 0.30
~
(N++ Ca2++ Mg+) . H + keq/ha/yr FIG. 7. The relationship between wet deposited base cations minus H + ions and soil pH with respect to Parent Material Class (& := Spearman Rank Corrclation Coefficient).
Sulphate and pH of upland podzolic soils in Scotland base cation inputs, the H ÷ ion precipitation load (keq ha-I a - l ) was subtracted from the sum of the deposited base cations, Na + , Mg2÷ and Ca 2÷ (keq ha- 1 a-l). Although deposition data for K ÷ was not available, this was not thought to be important, since inputs are negligible compared to those of the other base cations ( U K R G A R , 1990). For Class 1 (quartzites and sandstones), Class 2 (granites) and Class 3 (Lewisian Gneiss and Moine Schists), Spearman Rank correlation analysis showed significant correlations, P < 0.001, P < 0.001 and P < 0.01 respectively. Relationships were not significant for the other four soil parent material groups.
DISCUSSION
The results of this study show that parent material and soil particle size are both important factors which influence the capacity of mineral soils to retain SO]a-. Although texture and parent material are not totally independent variables, the importance of the former is clearly illustrated in Fig. 1, which shows that there is more than a 10 fold increase in SO]a- adsorption capacity from a sand to a silt loam for a soil equilibrated with 100 mg 1-I SO 2-. The concentration of secondary Fe and AI hydrous oxides in a soil is known to control SO]- retention (e.g. PARFI~ and SMART, 1978) and the high surface area:volume ratio of fine textured soils will increase the capacity for Fe and A1 hydrous oxide formation in soils. It is also interesting to note that when the soils were equilibrated with 10 mg 1-1 S O l - , a more realistic concentration in terms of the soil solution, only the fine textured soils had any capacity to retain SO ] - . BILLEXret al. (1990) also found that soils equilibrated with realistic concentrations of SO 2- also desorbed rather than adsorbed SO 2-" The importance of parent material in providing an indication of susceptibility has already been incorporated in sensitivity maps of soil acidification (WILSON et al., 1988). Sulphate adsorption, which increased from acid to basic parent materials, will provide a delaying mechanism for soil acidification. Our results suggest that the SO]4- adsorption capacity of most soils may already have been exceeded, if our 10 mg 1- l equilibration conditions are representative of field conditions. However, it is difficult to directly compare these observations with field conditions since certain aspects of the laboratory procedure are likely to influence the results. For example, shaking may increase the desorption of SO 2- as disturbance increases the amount of organic material in competition with SO 2 , for adsorption sites (VANCE and DAVID, 1992). Also, the equilibrium pH established after shaking may be altered from the initial adjusted pH value; a rise in solution pH would tend to promote SO ]- desorption (e.g. NODVtN et al., 1986). Although regional trends in SOl- adsorption capacity are more difficult to analyse, because of the AG 9:2-B
137
inherent variability in any study of this type, SO]adsorption capacity does show a number of interesting trends. These suggest that SO~4- adsorption capacity may be related to the S deposition load. However, as discussed in the results section, this may be a coincidental relationship and may actually be associated with the pattern of geographical distribution of soil parent material. The data showed that there was no relationship between soil pH and texture. Although the lowest soil pH values and lowest mean soil pH values occurred on the soils derived from sandstones and quartzites, soil pH and parent material class did not appear to be significantly related (Fig. 4). On a regional basis, there appeared to be stronger regional differences in soil pH than SO~4- adsorption capacity, with more acidic mineral soil horizons being located in the areas of Scotland with lower rainfall pH. Soil SO 2- saturation facilitates mineral soil acidification, but only the soils derived from quartzites, sandstones and granites show any obvious evidence that they have acidified, following the attainment of a SO2-saturated state. Such soils will have low critical loads and would therefore be the ones to first exhibit a fall in pH in response to excess acid deposition in a given region. The observation that the relationship between soil pH and SO42- adsorption capacity is significant for these soils suggests that they may have become saturated with respect to SO4- and have experienced additional acidification, controlled at least in part by SO 2- leaching. The graphs showing the relationship between rainfall pH and soil pH (Fig. 6) reflect the sensitivity of these soils to acid deposition effects with strong correlations for the most "sensitive" of the parent material classes, namely the sandstones and quartzites (Class 1) and the granites (Class 2). When base cation inputs in precipitation were also included, the strength of these relationships was increased slightly (Fig. 7). In spite of the fact that Ca 2+ has a much greater selectivity coefficient for the exchange sites than Na +, the best correlations for parent material group 1-3 soils were obtained when Na +, Ca 2+ and Mg2~ were all taken into account, probably reflecting the dominance of Na + and Mg 2+ in the maritime deposition of Scotland in general. While this does not establish unequivocally that acid deposition may cause a decline in soil pH, on the other hand a lack of any such relationship for Class 1 and 2 parent materials would call into question the validity of the critical loads approach. The results therefore highlight that base cation inputs from the atmosphere are important in governing which ions occupy cation exchange sites in soils, especially where base cation release by mineral weathering is low or has become small compared with atmospheric inputs. Since the salt effect, where base cations compete with H + ions for cation exchange sites, is more pronounced on acidic parent materials (WIKLANDER, 1975), those soils identified as sensitive
138
D. Barton et al.
to acidification may actually be buffered against falls to very low p H values (e.g. <4) in areas where there is a significant input of marine base cations. It is now well established for peats (SKmA and CRESSER, 1989; SMITH et al., 1993) that competition between atmospheric inputs of H + and of base cations, especially Ca 2÷, regulates peat pH, because of the negligible input of base cations from mineral weathering. It appears that mineral weathering derived inputs of base cations may also play a minor role in sensitive mineral soils. A recent study which examined SO]a- adsorption status and soil p H of forest soils across pollution gradients in Sweden, showed that regional trends in soil acidity closely followed the deposition pattern of acidity and also that soils in areas of greater deposition load were relatively more SO]a- saturated (ERIKSSON et al., 1992). These results on forest soils are in accordance with the findings of this study based on moorland soils. H o w e v e r , these general conclusions, about the relationship between acid deposition loads and soil chemistry in a regional context, must be considered along with other influential factors associated with climate. The influence of a maritime climate and marine derived inputs have been considered in this study, but gradients in rainfall, runoff and temperature that exist between the relatively cool, wet west and the relatively warm, dry east coasts of Scotland (BIRSE and DRY, 1970; DOORNKAMP et al., 1980) will also influence the natural processes of soil acidification. It must be noted that acidification of natural origins might be expected to be greater in wetter regions, although the opposite trend was found in practice. Certain topographical features were considered in the selection of sampling sites, but it was not feasible in this study to be selective for factors such as aspect and position along the slope. A recent study of soils along an altitudinal transect indicated that SO]4adsorption characteristics of individual profiles are dependent upon the position of the profile on a slope, adsorption being more likely on the lower slopes, with the composition of laterally flowing water strongly influencing soil properties (SANGERet al., 1992). Consequently, further detailed examination of the soil properties and site conditions would be required for the totally unequivocal conclusion that acid deposition had a major influence on soil acidity.
CONCLUSIONS The results suggest that soil texture and parent material have a direct effect on SO']4 adsorption and textural measurements should be incorporated in any assessment of soil sensitivity to acid deposition effects in the short term. The critical loads approach suggests that adverse effects on soil p H as a consequence of acid deposition might be expected first on
sensitive soils derived from quartzites, sandstones and granites. Relationships between rain p H and soil p H were found on these parent materials. Base cation inputs from the atmosphere appear to have a moderating influence on the effects of acid deposition, however, and also need to be considered when evaluating soil sensitivity and critical loads for soils. It is possible that, with further development, graphs such as those illustrated in Fig, 7 might prove useful for providing quantitative estimates of the probable response of a soil to changes in acid deposition. For sensitive parent material types, approximate predictions of soil p H might be feasible from a knowledge of H + ion and base cation inputs from the atmosphere. Acknowledgements--The authors would like to thank the landowners, including the Forestry Commission, for granting permission to sample on their land. Thanks also to field assistants Patrick Barton, Luke SangeL Clare Rance and Brian Steenson whose help was much appreciated at various stages of the sampling period. We acknowledge Warren Spring Laboratory for providing data on precipitation chemistry and also the Department of the Environment and the Natural Environment Research Council for providing financial support. Editorial handling: Ron Fuge.
REFERENCES
BILLETT M. F., PARKER-JERVISF., FITZPATRICKE. A. and CRESSER M. S. (1990) Forest soil chemical changes bctween 1949/50 and 1987. J. Soil Sci. 41,133-145. BIRSE E. L. and DRY F. T. (1970) Assessment of climatic conditions in Scotland. 1. Based on accumulated temperature and potential water deficit. Aberdeen: The Macaulay Institutc for Soil Research. CHAD T. T., HARWARD M. E. and FANG S. C. 11963) Cationic effects on sulphate adsorption by soils. Soil Sci. Soc. Am. Proc. 27, 35-38. CHADT. T., HARWARDM. E. and FANGS. C. (1964) Iron or aluminium coatings in relation to sulphate adsorption characteristics of soils. Soil Sci. Soc. Am. Proc. 28, 632635. CHAO T. T., HARWARD M. E. and FANG S. C. (1965) Exchange reactions between hydroxyl and sulphate ions in soils. Soil Sci. 99, 104-108. Cox K. G., BELL J. D. and PANKHURSI R. J. (1979) The Interpretation of Igneous Rocks. George Allan & Unwin. CRAIG G. Y. (ed.) (1983) Geology of Scotland. Scottish Academic Press. DOORNKAMPJ. C., GREGORYK. J. and BURN A. S. (198/)) Atlas of Drought in Britain 1975-76. Institute of British Geographers. ERIKSSON E., KARLTUN E. and LUNDMARKJ. E. (1992) Acidification of forest soils in Sweden. Ambio 21, 150154. GUADALIXM. E. and PARDOM. T. (1991) Sulphate adsorption by variable chargc soils. J. Soil Sci. 42, 6/)7-614. HARRISONR. B., JOHNSON D. W. and TODD D. E. (1989) Sulphate adsorption and dcsorption reversibility in a variety of forest soils. J. Envir. Qual. 18, 419-426. HENmCKSONA. (1979) A simple approach for identification and measuring acidification of frcshwatcrs. Nature 278, 542-545. HODGSONJ. M. (cd.) 11976) Soil Survey Field Handbook. Soil Survey of England and Walcs, Harpcnden.
Sulphate and pH of upland podzolic soils in Scotland JOHNSON D. W., COLE D. W. and GESSELS. P. (1979) Acid precipitation and soil sulphate adsorption properties in a tropical and in a temperate forest soil. Biotropica ll(l), 38-42. JOHNSON D. W. and HENDERSON G. S. (1979) Sulphate adsorption and sulphur fractions in a highly weathered soil under a mixed deciduous forest. Soil Sci. 128, 34-40. JOHNSON D. W., HORNBECKJ. W., KELLY J. M., SWANK W. q'. and TODD D. E. (1980) Regional patterns of sulphate accumulation: Relevance to ecosystem sulphur budgets. In Atmospheric Sulphur Deposition: Environmental Impact and Health Effects (eds D. S. SHRINERet al.), pp. 507-520. Ann Arbor Science, Ann Arbor. JOHNSON D. W. and TODD D. E. (1983) Relationships among iron, aluminium, carbon and sulphate in a variety of forest soils. Soil Sci. Soc. Am. J. 47, 782-799. KRUGE. C. and FRINKC. R. (1983) Acid rain on acid soil: A new perspective. Science 221,520-525. MCFEE W. W., KELLYJ. M. and BECK R. H. (1977) Acid precipitation effects on soil pH and base saturation of exchange sites. Water, Air Soil Pollut. 7,401-408. MUELtER R. F. and SAXENAS. K. (1977) Chemical Petrology, and Application to the Terrestrial Planets and Meteorites. Springer. NILSSON J. and GRENNFELT P. (1988) Critical loads for sulphur and nitrogen. Report from a workshop held at Skokloster, Sweden, 19-24 March. Miljorapport 15, Nordic Council of Ministers, Copenhagen. NODVIN S. C., DRISCOLLC. T. and LIKENSG. E. (1986) The effect of pH on sulphate adsorption by a forest soil. Soil Sci. 142, 69-75. PARFITr R. L. and SMARTR. S. C. (1978) The mechanism of sulphate adsorption on iron oxides. Soil Sci. Soc. Am. J. 42, 48-50. PETERSON L. (1986) Effects of acid deposition on soil and sensitivity of the soil to acidification. Experientia 42,340344. PE1~flJOHNE. J. (1975) Sedimentary Rocks. Harper & Row. RAJAN S. S. S. (1978) Sulphate adsorbed on hydrous alumina, ligands displaced and changes in surface charge. Soil Sci. Soc. Am. J. 42, 39-44.
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REUSSJ. O., Cosav B. J. and WRIGHTR. F. (1987) Chemical processes governing soil and water acidification. Nature 329, 29-32. SANGER L. J., BILLETT M. F. and CRESSER M. S. (1992) Changes in sulphate adsorption characteristics along an upland soil catena. Envir. Pollut., in press. ScoTr N. M. (1976) Sulphate content and sorption in Scottish soils. J. Sci. Food and Agric. 27, 367-372. SKIBAU. and CRESSERM. S. (1989) Prediction of long term effects of rainwater acidity on peat and associated drainage water chemistry in upland areas. Water Res. 23, 147782. SMITH C. M. S., CRESSER M. S. and MITCHELL R. D. J. (1993) Critical loads of acid deposition for dystrophic peat in Great Britain. Ambio 22, 22-26. SOILSURVEYOF SCOTLAND(1984) Organization and Methods of the 1:250,000 Soil Survey of Scotland. Macaulay Institute for Soil Research, Aberdeen. TABATABAIM. A. (1984) Effects of acid rain on soils. CRC Critical Reviews in Environmental Control (Chem. Rubh. Co.) 15, 65-110. UNITED KINGDOMREVIEWGROUPON ACID RAIN (1990) Acid Deposition in the United Kingdom 1986-1988. Warren Spring Laboratory, Stevenage, Herts. VAN BREEMAN N., DRISCOLLC. T. and MULDER J. (1984) Acid deposition and internal proton sources in acidification of soils and waters. Nature 307, 599-604. VANCE G. F. and DAVID M. B. (1992) Dissolved organic carbon and sulphate sorption by spodosol mineral horizons. Soil. Sci. 154, 136-144. WIKLANDER L. (1975) The role of neutral salts in the ion exchange between acid precipitation and soil. Geoderma 14, 93-105. WIKLANDERL. (1980) The sensitivity of soils to acid precipitation. In Effects of Acid Precipitation on Terrestrial Ecosystems (eds T. C. HUTCHINSONand M. HAVAS),pp. 553--568. Plenum Press. WILSON M. J., LILLY A., NOLAN A. J. and CRESSERM. S. (1988) Vulnerable soils and their distribution. In Acidification in Scotland. Symposium Proceedings. Scottish Development Departmcnt, Edinburgh 1989.
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Sulphate adsorption capacity and pH of upland podzolic soils in Scotland: Effects of parent material, texture and precipitation chemistry D. BARTON, D. HOPE, M. F. BILLETF a n d M. S. CRESSER Department of Plant and Soil Science, University of Aberdeen, Meston Building, Aberdeen AB9 2UE, U.K. (Received 13 April 1993; accepted in revised form 20 September 1993) A b s t r a c t - - A regional survey of podzol B horizons has been carried out to investigate the effects of parent
material and soil texture on sulphate (SO~4-) adsorption capacity and pH in Scottish soils. Sulphate adsorption was measured on equilibration of the soils with 10 and 100 mg l- l SOl- solutions. The results showed that soil texture and soil parent material had a significant effect on SO~4 adsorption. Significant correlations were found between sulphur (S) deposition loads and SOl- adsorption, and between precipitation pH and soil pH, but not between total hydrogen ion (H+ ) load and soil pH, even on sensitive soils. Relationships between the chemical composition of atmospheric deposition and soil pH could bc marginally improved if the possible amelioration of acidification by base cation inputs, especially on sensitive soils, was taken into account. INTRODUCTION
S O ] - adsorption and retention is, to some extent, dependent upon the concentration of these hydrous THEREIS still considerable uncertainty concerning the oxides in soils (JOHNSON and TODD, 1983). It might precise quantitative effects of acidic deposition on also be expected to be affected by their total surface freshwater and soil acidification. In part this is be- area. Sulphate adsorption is further influenced by the cause internal proton production, sometimes intensi- concentration of SO 2- in the incoming solution relafied by changes in land-use, is a major contributor to tive to the concentration at which the soil had long-term acidification (McFEE et al., 1977; KRUG previously equilibrated (JOHNSON and Tooo, 1983). and FRINK, 1983; TABATABAI, 1984; PETERSON, 1986). Organic matter is known to inhibit SO]4- adsorption External loads of acidity from anthropogenic by blocking adsorption sites (JOHNSON et al., 1980; sources, superimposed on the natural stores of JOHNSON and TODD, 1983). acidity, can give rise to major changes in soil solution Soil mineralogy is a critical factor in regulating the and surface water acidity and alkalinity (VAN BREE- release of base cations to balance leaching losses and MAN et al., 1984; REUSS et al., 1987). They also, also in controlling the Fe and AI hydrous oxide thereby, contribute to further soil acidification in the content of soils. In highly weathered soils, SO~4long term. Sulphate is a major component of atmos- adsorption is thought to be most important within pheric deposition and HENRIKSON (1979) suggested soils developed from base-rich parent materials that S deposition was a major contributor to surface having abundant free Fe and A1 hydrous oxides and water acidification. In the absence of this mobile less important on those developed on more acidic anion, the store of acid derived from natural soil parent materials (Sco~, 1976). processes seldom appears to result in the acidification Critical loads of acidic and S deposition have been of freshwaters to the extent that hydrogen ion and set for soils derived from various geological materials aluminium ion concentrations are mobilized to levels and proposed sensitivity classes have been estabtoxic to fish (REuss et al., 1987). Adsorption reac- lished on the basis of mineralogy and its control on tions within the soil profile can regulate concen- weathering rates (NILSSON and GRENNFELT, 1988). trations of aqueous S O l - , and SO~4- adsorption can These form the basis of maps, showing sensitivity to prevent the transport of base cations and of H + and acidification, which have now been produced for A13+ from soils (JOHNSONet al., 1979). Adsorption of Scottish soils (WILSON et al., 1988). The mapping SO 2- can also result in the release of O H - into the approach depends upon soil association, based on the soil solution (CHAOetal., 1965; RAJAN, 1978; PARFITT soil parent material, being a key factor which can be and SMART, 1978) and induce changes in surface used to identify vulnerable soils. The maps take into charge causing an increase in cation exchange capac- account other interacting factors including soil type, ity (RAJAN, 1978; WIKLANDER,1980). vegetation, landform and land-use, which may Sulphate adsorption is known to occur on Fe and overide the influence of parent material. However, it A1 hydrous oxide surfaces (CHAOet al., 1964; PARFITT is unclear to what extent potentially sensitive soils a n d SMART, 1978; RAJAN, 1978) and the degree of already show evidence of accelerated acidification,
t27
128
D. Barton et al.
w h e r e critical loads are e x c e e d e d , and w h e t h e r such soils will eventually b e c o m e m o r e acidic than they already are. The SO42- a d s o r p t i o n capacity o f the soils, being r e g a r d e d as a delaying m e c h a n i s m rather than a p e r m a n e n t l y ameliorative process, has not b e e n taken into consideration generally w h e n setting critical loads. This study set out to investigate the acidification and S O l - a d s o r p t i o n status of Scottish upland soils on a regional basis, in relation to p a r e n t material, soil texture and a t m o s p h e r i c pollution. It was felt that soils d e v e l o p e d from sensitive p a r e n t materials should be the first to show p H falls associated with deposition acidity because of their limited capacity to buffer acidification by mineral weathering. Statistical analysis should identify the key factors which affect S O ] - a d s o r p t i o n characteristics. T h e s e could t h e n be used in considering the a p p a r e n t soil sensitivity to p H fall as a c o n s e q u e n c e of a t m o s p h e r i c pollution.
MATERIALS AND METHODS This study was based on the 1:250,000 soil maps compiled by the Soil Survey of Scotland (SOIL SURVEYOF SCOXLANO, 1984). The individual mapping units of these maps are based upon soil type, parent material, landforms and the principal associated vegetation and land-use patterns. To facilitate studying relationships between soil chemistry and existing pollutant deposition maps for the U.K., the sampling programme was based on the Ordnance Survey 20 km square grid system used by the U.K. Review Group on Acid Rain (UKRGAR, 1990). Mean values from 1986-1988 of precipitation chemistry and pollution inputs are available for each grid square. The dominant soil association within a grid square was identified, and to minimise variability, sampling areas were selected which satisfied certain site and profile characteristics. Sampling constraints were as follows: vegetation elevation slope soil type drainage B horizon thickness
dry Atlantic heather moor 200-500 m 5--45° peaty or humic iron podzols free 15-40 cm
The soil associations sampled comprised predominantly of soils developed on glacial drifts derived from particular bedrock geologies. Within each 20 km grid square, from the land area which fitted within the above constraints, two sampling sites were selected at random. Soil profiles were exposed by either digging a soil pit or digging back into a cutting. Provided the soil profiles did not exhibit obvious anomalies, for example as a consequence of burrowing animal activity or the presence of boulders, two samples were carefully removed from the entire B horizon. From each, after individual mixing, 200-500 g sub-samples were transfered to clean labelled polythene bags for transport back to the laboratory for analysis. In some areas, grassland replaced Calluna as the dominant vegetation type. If no alternatives for that grid could be found, soils were sampled and the unsatisfactory features were recorded. Of the 20 km grid squares on mainland Scotland that were found to contain suitable sites for sampling, 86% were sampled. In all, 257 samples were collected from 132 grid squares between August and November 1991. In each 20 km grid square, the area of land within the
sampling constraints was invariably much less than 400 kn~. To gain an indication of the variability which might be encountered around each sampling site, a group of 12 appropriate moorland podzol B horizon soils was sampled from a 10 km2 site in the Glendye catchment, 40 km south west of Aberdeen. These were subjected to the same analytical procedures as the other soils in this study, pH values ran~ged from 4.08 to 4.51 (median 4.22). The amount of SO~- adsorbed by the soils on shakin~ with 100 mg 1-1 SOlranged from 0.7 to 2.32/~moles g " (median 1.24). This study gave some indication of the type of potential variability within each 20 km grid square. The soil chemical data was therefore treated as two separate sets as a means of dealing with the variability within the sample population. Thus if similar spatial trends and statistical relationships occurred in both sets of soils, then the relationships observed are less likely to be due to chance alone. Set I and Set 2 represented the 1st and 2nd soil samples taken after random selection from appropriate land within each grid square. Set 1 contained 132 soil samples and Set 2, 125 soil samples. There were fewer samples in Set 2 since duplicates were not obtained from a limited number of sites. Individual soil samples were allocated to their particular texture and parent material classes (Tables 1 and 2). Centroid percentage sand contents for each textural class were derived from the standard particle-size class triangular diagram used by the Soil Survey of England and Wales (HODGSON, 1976). Soils were allocated to seven different parent material classes. The classification was based on the elemental composition of the parent material from which the soils were derived. Values of % SiO 2 composition were obtained from a number of standard geological texts (Table 2). Estimations of SiO 2 for specific lithologies (sandstones, granites, basalts) were straightforward, whereas estimates for major regional lithological units, such as the Lewisian Gneiss, Moine Schist and Dalradian Schist, were more difficult because of the diverse lithological composition within each group. Mean SiO2 values were given when therc was more than one common rock type within each group. Soil samples were sieved and measurements of pH (0,01 M CaCI2), SO ] - adsorption capacity and moisture content were made on the <2 mm fraction. The texture of the soils was assessed by hand. All measurements were carried out on field moist soils. Soil pH measurements were obtained using a glass calomel combined electrode pair after mixing 10 g of soil with 25 ml 0.01 M CaCI 2, and leaving for l h to equilibrate. Sulphate adsorption capacity was determined using the following method. Duplicate 5 g soil samples were shaken for 6 h at room temperature with 25 ml sulphate solution (K2804). One set of duplicate soils was shaken with 100 mg 1-I, another with 10 mg 1-1 SO~4- solutions. The 10 mg 1-1 SO ] - solution relates to known soil solution concentrations whereas 100 mg I 1 SOl- is commonly used to measure SO ] - adsorption capacity (e.g. JOHNSON and HENOERSON, 1979). Previous work has identified the importance of soil and solution pH in adsorption experiments (CHAO et al., 1963; NODVINet al., 1986; HARRISONet al., 1989; GUADALIX Table 1. Per cent sand content and number of samples within each textural class No. of samples Texture Class l 2 3 4 5
Soil texture Sand Loamy sand Sandy loam Sandy silt loam Silt loam
% Sand
Set 1
Set 2
92 82 63 35 10
6 31 45 29 21
4 32 45 27 17
Sulphate and pH of upland podzolic soils in Scotland
129
Table 2. Per cent SiO 2 content and number of samples within each parent material class Parent material class
No. of samples Geology Quartzites and sandstones Granite Lewisian gneiss and Moine schists Greywackes and shales Intermediate igneous rocks Dalradian schists Basic igneous rocks
% SiO2
Set 1
Set 2
74.6 71.3 66.7 63.4 61.4 55.1 49.7
20 15 39 19 7 21 9
18 14 37 19 7 21 7
10 mg / litre equilibration .~
0.1
~
-0.1
,SetlDset2
a -o.2
i
~
i
i
i
i
1
2
4
100 mg / litre equilibration
IISet 1
3
,~ .2
~
3
Textural Class
10 m ~ u i l i b r a t i o n 0.I
PETrlJOHN(1975) Cox et al. (1979) C~IG (1983) PETrlJOHN(1975) Cox et al. (1979) MUELLERand SAXENA(1977) Cox et al. (1979)
100 mg/litre equilibration
Textural Class .~
Reference
"T
2 -O.l
~-0.2 ~
0 1
2
3
4
5
6
Parent Material Class
'
1
'
2
3
4
5
6
7
Parent Material Class
F~c. 1. Mean sulphate adsorption and 95% confidence limits for each textural and parent material class. and PARDO, 1991). All the 100 mg 1-I and 10 mg 1-180~4 solutions used were accordingly adjusted to within _+0.1 units of soil pH using 0.1 M HC1. After shaking, the soils were left to equilibrate for approximately 12 h and then centrifuged, filtered (Whatman No. 1 filter paper) and the filtrate analysed for SO]4- using ion chromatography (Dionex Series 4500i instrument with lonPac AS4A column). Adsorbed SO2- was calculated from the difference in concentration between that in the initial solution and that in the final filtrate. The results are expressed as SOl- adsorbed in pmoles SO] per g oven dry soil.
RESULTS SO~- A d s o r p t i o n T e x t u r e , Mean values of S O ] - adsorption and the upper or lower 95% confidence limits, determined at equilibrium S O ] - concentrations of 10 mg 1- j and 100 mg 1-~ , are shown for each textural class in Fig. 1. Analysis of the two sets of data gave comparable results and similar trends for the soils equilibrated with the 100 mg 1- j SO42- solution. Differences between the two sets of soils were observed when the
soils were shaken with 10 mg 1-1 SO]4-. The results showed that the capacity of soils to retain SO 2increased uniformly from sand (Class 1) to silt loam (Class 5) textured soils. All textural classes showed a net release of SO 2-, in the 10 mg 1-1 equilibration, except the silt loam soils (Class 5) for both Sets 1 and 2. P a r e n t material. Mean values of SO]4- adsorption calculated for each parent material class showed that both sets of data displayed similar trends for the soils equilibrated with 100 mg 1-I SO 2 (Fig. 1). In the soils shaken with 10 mg 1 i SO] the agreement of the results was not as good between Set 1 and Set 2. The 100 mg 1 J results showed a relatively uniform increase in S O ] - adsorption capacity from those soils developed from parent materials with a high silica content (quartzites and sandstones) to those with a low silica content, e.g. basic igneous rocks. Although soils developed on parent material Class 5 (intermediate igneous rocks) do not follow this overall trend, the 95% confidence limits are quite large for this group of soils. Mean S O l - adsorption values
130
D. Barton et al.
calculated for the soils shaken with 10 mg 1-1 SO 2showed that all the parent material classes, excluding Class 7 (basic igneous rocks), tended to release S042-" Regional trends and correlations. Maps showing the spatial variability in SO ] - adsorption capacity in Scotland at both 10 mg 1- l and 100 mg 1- l SO]a- are shown in Fig. 2. Although it is difficult to analyse the data visually, the maps for Set 1 and Set 2 soils equilibrated at 100 mg 1-1 identify areas of low SO]aadsorption capacity (<2/,(moles g-1 SO]4-) concentrated in eastern, northern, central and southern Scotland. Soils with the highest adsorption capacities appear to occur more commonly on the west coast of Scotland and the western central area, where S deposition loads are highest ( U K R G A R , 1990). Maps of S O ] - adsorption capacity derived from soils treated with 10 mg 1-1 SO 2- are divided into soils which adsorb and soils which release SO24- . These appear to show the same broad patterns as for the soils equilibrated with the 100 mg 1-1 solution, with large areas of soils showing a net release of SO]a- concentrated in the northern, eastern and southern parts of Scotland. To provide a more rigorous analysis of the regional data, the relationships between SO]n- adsorption capacity and sulphur deposition were tested using Spearman Rank correlation analysis (Table 3). Sulphur deposition data was supplied by the Warren Spring Laboratory ( U K R G A R , 1990). The results showed significant (P < 0.001) correlations between the amount of SO]- the soils adsorbed from the 100 mg 1-1 SO ] - solution and inputs of wet (marine and non-marine) plus dry deposited S Q~g m -2 a - l ) . A significant (P < 0.001) correlation was also observed between SO~- adsorbed with the 10 mg 1-1 treatment for Set 1 soils but not for Set 2. The relationships were such that the soils with greater capacity to adsorb SO]- were found in those areas that have greater sulphur deposition loads. These results are the opposite trend to what intuitively might be expected, and possibly suggest a coincidental (rather than causal) relationship. The distribution of parent materials (Fig. 3) is such that regions of high S deposition, especially in western central areas, often have a greater abundance of base rich parent materials. Moreover these are likely to result in finer textured soils. For example, 27 out of 36 soils derived from Class 4 (greywackes and shales) were in textural classes 4 and 5. There was a highly significant Spearman Rank correlation (P < 0.001) between parent material and textural class. There was also a significant (P < 0.05) inverse correlation between total S deposition and parent material SiO 2 content. Relationships between soil pH and SO]4- adsorption capacity were also investigated. A significant (P < 0.05) correlation, for Set 1 soils equilibrated in 100 mg 1- ~ SO ]- , was found between the capacity of the soils to adsorb SO42- and soil pH (Table 4). However, this correlation was not apparent for Set 2 soils at the
same SO]- concentration, nor for Set 1 and 2 soils equilibrated in 10 mg l-I SO]-. When the relationship was examined for each individual parent material class, using the combined data sets, significant relationships were only found for the Class 1 (r~ = 0.44, P < 0.01) and Class 2 (r s = 0.41, P < 0.05) soils at the 10 mg 1-1 SO']4- level and Class 1 (rs = 0.39, P < 0.05) soils at the 100 mg 1-1 level. Multiple linear regression analysis was used to examine the relationships among SO~4- adsorption capacity, as a dependent variable, and either texture and S deposition load or parent material and S deposition load, as separate pairs of independent variables. Since the results suggested that parent material and texture were correlated they could not be included in the same regression equation. It was found that for the 10 mg 1 1 results, the combination of both sets of variables accounted for less than 8% (coefficient of multiple determinations, R2 , <8%) of the variability on SO 2- adsorption capacity. Texture (quantified in terms of % sand content) alone gave the greatest coefficient value for the 100 mg 1- l equilibration results (R 2 = 47%) and the inclusion of S deposition load did not improve this value significantly. Parent material (% SiO2 content) accounted for less than 25% (R2 = 25%) of the SOl- adsorption capacity values measured at 100 mg 1-1 and again the inclusion of S deposition did not improve the coefficient value significantly.
Soil pH Texture. Figure 4 shows how mean soil pH varies with textural class for both sets of soils. The results suggest that there was no relationship between soil pH and soil texture. Table 3. Spearman Rank correlations (r0 of SO4 adsorption capacity versus S deposition load Set 1 Equilibrating concentration 10 mg 1-l SO4 100 mg I-I SO4
Set 2
No. of samples
r~
No. of samples
r,
129 132
0.31 *** 0.43***
122 125
0.10 0.33***
• **P < 0.001. Table 4. Spearman Rank correlations (r,) of SO4 adsorption capacity versus soil pH Set 1 Equilibrating concentration I0 mg I -) SO4 I00 mg I -i S04 *P < 0.05.
Set 2
No. of samplcs
r~
No. of samples
r~
129 132
0.08 O.19"
122 125
0.07 0.03
Sulphate and pH of upland podzolic soils in Scotland
Set 1 SO4 adsorption 10 mg/litre equnibration
13
Set 2 S04 adsorption 10 mcj/litre equilibration
S04 (urn(
S04 (umo ABC
BEE,
Set 1 S04 adsorption 100 mg/litre equilbration
S04
!
Set 2 S04 adsorption 100 mg/litre equilibration
g
SO4
m m m
m m.
FIG. 2. Regional pattern of sulphate adsorption for Set I and Set 2 soils equilibrated with 10 and 100 mg I J
so~-.
32
D. Barton et al.
Parent Material Class Distribution
PM Class Basic Igneous Rock,. Dalraclian Schists Intermediate Basic I Greywackes and Sh Lewisian Gneiss anc Granites Quartzites and San
F16.3. Regional distribution of parent material class.
iN
BELC..
ABO ~
Soil pl I
BELOW
4
4
ABO~
Soil pF
3.9
Flo. 5. Regional pattern of soil p H for Set 1 and Set 2 soils.
Set 1 Soil pH
Set 2 Soil pH
e-,
.~-
E"
© C~
C~
e'~
=_..
f,¢3
Sulphate and pH of upland podzolic soils in Scotland
135
~4.
"fll !l 1
2
3 4 Textural Class
5
1
2 3 4 5 6 Parent Material Class
7
Fl6.4. Mean pH and 95% confidence limits for each textural and parent material class. Table 5. Spearman Rank correlation (r~) of rainfall acidity with soil pH Set 1
Rainfall pH H + load
Set 2
No. of samples
rs
No. of samples
rs
132 132
0.48*** -0.07
125 125
0.38*** -0.09
***P < 0.001.
Parent material. Mean pH values were calculated for each parent material class. Results and trends were similar for Sets 1 and 2 (Fig. 4). Excluding parent material Classes 2 and 3, the mean soil pH increased from Class 1 to 7. The relatively high mean pH values found for parent material Classes 2 and 3 may be a consequence of the greater proportion of these soils being situated in relatively unpolluted areas in north and west Scotland. The soils derived from sandstones and quartzites (Class 1) had mean pH values of 3.90 and 3.96 for Set 1 and Set 2, respectively. Soils derived from basic igneous rocks (Class 7) had mean pH values of 4.40 and 4.44 for Sets 1 and 2, respectively. Regional trends and correlations. Regional variations in the pH of podzol B horizons are shown for the two sets of soils in Fig. 5. Unlike the regional SO4 adsorption maps (Fig. 2), there appears to be less variability between Set 1 and Set 2 soils. The most acidic B horizons tend to group together on the southern margin of the central valley of Scotland and also in eastern Scotland. The least acidic soils appear to be concentrated on the west coast. The relationship between volume-weighted mean rainfall pH (Warren Spring data) and soil pH was investigated statistically on a regional basis. Spearman Rank correlation analysis showed significant relationships (Set 1 and Set 2, P < 0.001) between soil pH and rainfall pH (Table 5). No significant correlation was observed between the H + deposition load (keq ha -j a-I), including both wet and dry deposition of H +, and the soil pH, in spite of the fact that such a relationship would be anticipated for sensitive soil types according to the current U.K. critical loads approach.
Multiple linear regression analysis indicated that parent material and rainfall pH accounted for less than 25% (coefficient of determination, R2, = 25%) of the variability in the soil pH results observed. Since both the nature of parent material and rainfall pH appear to influence soil pH, an examination of the relationship between soil and rainfall pH within each parent material class was carried out. Data from the two sets of soil were combined to provide sufficient data within each parent material class for statistical analysis. The graphs (Fig. 6) show that while six of the parent material classes have soils which have been sampled across a wide range of rainfall pHs (4.3-4.9), the group of soils derived from the Lewisian Gneiss and Moine Schists has been sampled from areas which cover a narrower deposition pH range (4.6 to <5). This makes interpretation of the effects of rainfall on soils developed from this parent material class difficult. Spearman Rank correlation showed significant relationships between the pH of the soil B horizons and precipitation pH for the quartzites and sandstones (Class 1) and for the granites (Class 2). The only other parent material class which gave a significant relationship was the basic igneous group (Class 7). The latter correlation was the result of a group of three high pH podzol B horizons in an area of very high base cation deposition in western Scotland. Omitting data for these sites prior to applying the statistical test resulted in no significant trend being observed. Spearman Rank correlations, between soil pH and H + ion deposition load (keq ha- 1 a - l ) , were found to be insignificant for all parent material classes except Class 7 (basic igneous rocks). No relationship was apparent for these soils, when the three high pH soils (pH values greater than 4.9) were excluded. Several of the soils with the highest pH values tended to be found on the west coast of Scotland and the Western Isles. Relatively higher base cation inputs from the atmosphere, as a consequence of the marine climate, may have an effect on soil chemistry at these locations. To investigate this, the relationship between soil pH and base cation inputs in precipitation was examined for the individual parent material classes (Fig. 7). Combined data sets were again analysed using Spearman Rank correlation. To obtain an indication of the relative importance of
136
D. Barton et al. Quartzites & S a n d s t o n e s
Lewisian Gneiss & Moine Schists 5.(
Granites
5.6
5.6
o
o
8
4.8 o o
8
B
o° o
s
4'.4
~'
416
51i
4
rs= 0.62 p < 0.001
~ 3.
o
4.8
3.
4.4
od
5A
414
416
4.8
5
I n t e r m e d i a t e I g n e o u s Rocks
o a
%
c
o
3.2
4.6
4.8
Dalradian Schists
5.6
,¢ gT.°~ ¢ o
o
4.4
is= 0.11
rs= -0.10
4.6
Basic I g n e o u s R o c k s
4.4
m o v
4
4.8
3.~
4-8f
v
8
@
rs= -0.17
4.6
o
~= 0.22
0.61
p < o.oi
4.@
o
om
•
o
3.~
'
~ o
o
~ ~e °
rs=
Greywackes & Shales
4.8
o
oq~
o~
o
4.~
o
o°
4.8
414
5
416
418
Rainfall pH
Rainfall pH
5.6 m
4.8 o
rs= 0_54 p < 0.05 3.
4'.6
414
4.8
'
Rainfall pH Fie,. 6. The relationship between rainfall pH and soil pH with respect to parent material class (& = Spearman Rank Correlation Coefficient).
5.6
Quartzites & S a n d s t o n e s
4.si°~l 5"6i=[
c
4.8 e~
o [3
3.2
8
3
Greywaekes & Shales
5.6
'i!
8
oo
rs= 0.75
F' < o.ool 3
4
5
6
7
I n t e r m e d i a t e I g n e o u s Rocks 5.6
3.
i
o
o oD
8g~ ooB D a
o
2
o
4.8
4
rs= 0.63 p < 0.001 5 6 7
4
~8
~*~
o oG
o
l
Lewisian Gneiss & Moine Schists
Granites
n
rs= 0.30 p < 0.0l
~
~
°
~
°
D a l r a d i a n Schists
5"6I ¢h
4.8
4.8
aG
O
51!
4
3.
4
1
2
3
5
6
Basic I g n e o u s R o c k s
4.8
~
Do
Is= 0.15
o
o
7
3.
l
3
4
5
6
(Na++ C~++ M~+) - H + k e q / h a / yr
o
4
Is= 0.09 2
o
~
o
rs= 0.13 7
3.2~
-i
2
3
4
5
6
(Na++ Ca2++ M~+) - H + keq / h a / y r
o
~
i
~
~
rs= 0.30
~
(N++ Ca2++ Mg+) . H + keq/ha/yr FIG. 7. The relationship between wet deposited base cations minus H + ions and soil pH with respect to Parent Material Class (& := Spearman Rank Corrclation Coefficient).
Sulphate and pH of upland podzolic soils in Scotland base cation inputs, the H ÷ ion precipitation load (keq ha-I a - l ) was subtracted from the sum of the deposited base cations, Na + , Mg2÷ and Ca 2÷ (keq ha- 1 a-l). Although deposition data for K ÷ was not available, this was not thought to be important, since inputs are negligible compared to those of the other base cations ( U K R G A R , 1990). For Class 1 (quartzites and sandstones), Class 2 (granites) and Class 3 (Lewisian Gneiss and Moine Schists), Spearman Rank correlation analysis showed significant correlations, P < 0.001, P < 0.001 and P < 0.01 respectively. Relationships were not significant for the other four soil parent material groups.
DISCUSSION
The results of this study show that parent material and soil particle size are both important factors which influence the capacity of mineral soils to retain SO]a-. Although texture and parent material are not totally independent variables, the importance of the former is clearly illustrated in Fig. 1, which shows that there is more than a 10 fold increase in SO]a- adsorption capacity from a sand to a silt loam for a soil equilibrated with 100 mg 1-I SO 2-. The concentration of secondary Fe and AI hydrous oxides in a soil is known to control SO]- retention (e.g. PARFI~ and SMART, 1978) and the high surface area:volume ratio of fine textured soils will increase the capacity for Fe and A1 hydrous oxide formation in soils. It is also interesting to note that when the soils were equilibrated with 10 mg 1-1 S O l - , a more realistic concentration in terms of the soil solution, only the fine textured soils had any capacity to retain SO ] - . BILLEXret al. (1990) also found that soils equilibrated with realistic concentrations of SO 2- also desorbed rather than adsorbed SO 2-" The importance of parent material in providing an indication of susceptibility has already been incorporated in sensitivity maps of soil acidification (WILSON et al., 1988). Sulphate adsorption, which increased from acid to basic parent materials, will provide a delaying mechanism for soil acidification. Our results suggest that the SO]4- adsorption capacity of most soils may already have been exceeded, if our 10 mg 1- l equilibration conditions are representative of field conditions. However, it is difficult to directly compare these observations with field conditions since certain aspects of the laboratory procedure are likely to influence the results. For example, shaking may increase the desorption of SO 2- as disturbance increases the amount of organic material in competition with SO 2 , for adsorption sites (VANCE and DAVID, 1992). Also, the equilibrium pH established after shaking may be altered from the initial adjusted pH value; a rise in solution pH would tend to promote SO ]- desorption (e.g. NODVtN et al., 1986). Although regional trends in SOl- adsorption capacity are more difficult to analyse, because of the AG 9:2-B
137
inherent variability in any study of this type, SO]adsorption capacity does show a number of interesting trends. These suggest that SO~4- adsorption capacity may be related to the S deposition load. However, as discussed in the results section, this may be a coincidental relationship and may actually be associated with the pattern of geographical distribution of soil parent material. The data showed that there was no relationship between soil pH and texture. Although the lowest soil pH values and lowest mean soil pH values occurred on the soils derived from sandstones and quartzites, soil pH and parent material class did not appear to be significantly related (Fig. 4). On a regional basis, there appeared to be stronger regional differences in soil pH than SO~4- adsorption capacity, with more acidic mineral soil horizons being located in the areas of Scotland with lower rainfall pH. Soil SO 2- saturation facilitates mineral soil acidification, but only the soils derived from quartzites, sandstones and granites show any obvious evidence that they have acidified, following the attainment of a SO2-saturated state. Such soils will have low critical loads and would therefore be the ones to first exhibit a fall in pH in response to excess acid deposition in a given region. The observation that the relationship between soil pH and SO42- adsorption capacity is significant for these soils suggests that they may have become saturated with respect to SO4- and have experienced additional acidification, controlled at least in part by SO 2- leaching. The graphs showing the relationship between rainfall pH and soil pH (Fig. 6) reflect the sensitivity of these soils to acid deposition effects with strong correlations for the most "sensitive" of the parent material classes, namely the sandstones and quartzites (Class 1) and the granites (Class 2). When base cation inputs in precipitation were also included, the strength of these relationships was increased slightly (Fig. 7). In spite of the fact that Ca 2+ has a much greater selectivity coefficient for the exchange sites than Na +, the best correlations for parent material group 1-3 soils were obtained when Na +, Ca 2+ and Mg2~ were all taken into account, probably reflecting the dominance of Na + and Mg 2+ in the maritime deposition of Scotland in general. While this does not establish unequivocally that acid deposition may cause a decline in soil pH, on the other hand a lack of any such relationship for Class 1 and 2 parent materials would call into question the validity of the critical loads approach. The results therefore highlight that base cation inputs from the atmosphere are important in governing which ions occupy cation exchange sites in soils, especially where base cation release by mineral weathering is low or has become small compared with atmospheric inputs. Since the salt effect, where base cations compete with H + ions for cation exchange sites, is more pronounced on acidic parent materials (WIKLANDER, 1975), those soils identified as sensitive
138
D. Barton et al.
to acidification may actually be buffered against falls to very low p H values (e.g. <4) in areas where there is a significant input of marine base cations. It is now well established for peats (SKmA and CRESSER, 1989; SMITH et al., 1993) that competition between atmospheric inputs of H + and of base cations, especially Ca 2÷, regulates peat pH, because of the negligible input of base cations from mineral weathering. It appears that mineral weathering derived inputs of base cations may also play a minor role in sensitive mineral soils. A recent study which examined SO]a- adsorption status and soil p H of forest soils across pollution gradients in Sweden, showed that regional trends in soil acidity closely followed the deposition pattern of acidity and also that soils in areas of greater deposition load were relatively more SO]a- saturated (ERIKSSON et al., 1992). These results on forest soils are in accordance with the findings of this study based on moorland soils. H o w e v e r , these general conclusions, about the relationship between acid deposition loads and soil chemistry in a regional context, must be considered along with other influential factors associated with climate. The influence of a maritime climate and marine derived inputs have been considered in this study, but gradients in rainfall, runoff and temperature that exist between the relatively cool, wet west and the relatively warm, dry east coasts of Scotland (BIRSE and DRY, 1970; DOORNKAMP et al., 1980) will also influence the natural processes of soil acidification. It must be noted that acidification of natural origins might be expected to be greater in wetter regions, although the opposite trend was found in practice. Certain topographical features were considered in the selection of sampling sites, but it was not feasible in this study to be selective for factors such as aspect and position along the slope. A recent study of soils along an altitudinal transect indicated that SO]4adsorption characteristics of individual profiles are dependent upon the position of the profile on a slope, adsorption being more likely on the lower slopes, with the composition of laterally flowing water strongly influencing soil properties (SANGERet al., 1992). Consequently, further detailed examination of the soil properties and site conditions would be required for the totally unequivocal conclusion that acid deposition had a major influence on soil acidity.
CONCLUSIONS The results suggest that soil texture and parent material have a direct effect on SO']4 adsorption and textural measurements should be incorporated in any assessment of soil sensitivity to acid deposition effects in the short term. The critical loads approach suggests that adverse effects on soil p H as a consequence of acid deposition might be expected first on
sensitive soils derived from quartzites, sandstones and granites. Relationships between rain p H and soil p H were found on these parent materials. Base cation inputs from the atmosphere appear to have a moderating influence on the effects of acid deposition, however, and also need to be considered when evaluating soil sensitivity and critical loads for soils. It is possible that, with further development, graphs such as those illustrated in Fig, 7 might prove useful for providing quantitative estimates of the probable response of a soil to changes in acid deposition. For sensitive parent material types, approximate predictions of soil p H might be feasible from a knowledge of H + ion and base cation inputs from the atmosphere. Acknowledgements--The authors would like to thank the landowners, including the Forestry Commission, for granting permission to sample on their land. Thanks also to field assistants Patrick Barton, Luke SangeL Clare Rance and Brian Steenson whose help was much appreciated at various stages of the sampling period. We acknowledge Warren Spring Laboratory for providing data on precipitation chemistry and also the Department of the Environment and the Natural Environment Research Council for providing financial support. Editorial handling: Ron Fuge.
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