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Seasonal changes in the sulphate content of deciduous woodland soils exposed to atmospheric pollution
Environmental Pollution 47 (1987) 195-204
Seasonal Changes in the Sulphate Content of Deciduous Woodland Soils Exposed to Atmospheric Pollution Wendy Nevell & M. Wainwright Department of Microbiology, University of Sheffield, Sheffield, SI0 2TN, Great Britain (Received 26 February 1986; revised version received 16 March 1987; accepted 3 April 1987)
A BSTRA CT Seasonal changes in soil pH, sulphate concentration and totaLS were measured in two brown earth soils, sampled from deciduous woodlands. One site studied was exposed to severe atmospheric pollution from a coking works, while the other site was relatively unpolluted but located in an area receiving wet and dry deposited acidity of greater than 1"0 and 2.4 kg H + ha- xyear - 1, respectively The pH of soil at the heavily polluted site was lower than the relatively unpolluted soil at each monthly sample point, except during November. Annual average sulphate concentrations (LiCl-extractable) were highest in the soil exposed to coking pollution, where they peaked during summer and autumn. A marked difference in totaLS wasJbund in soils from the two sites, the heavily polluted soil showing the highest concentration with peaks again occurring during late summer and autumn. Only 4"0% (w/w) of the totaLS of the heavily polluted soil occurred as LiCLextractable sulphate, compared to 21"4% (w/w)for the relatively unpolluted soil, showing that organic sulphur is increased in brown earths following exposure to severe atmospheric pollution from the coking works.
INTRODUCTION The cycling o f sulphur in soils receiving atmospheric sulphur deposition is potentially important because it may influence the mobility of sulphate and 195 Environ. Pollut. 0269-7491/87/$03'50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
196
Wendy Nevell, M. Wainwright
therefore the leaching of cations (Cole & Johnson, 1977; Strick et al. 1982; Wainwright & Nevell, 1984). Cole & Johnson (1977) showed that seasonal variations occurred in the sulphate of a Douglas fir ecosystem, with concentrations of this ion peaking in autumn, just after a 2-month summer drought, and then decreasing during a following period of heavy rains. Similarly, Christophersen & Wright (1981) found a marked seasonal pattern of sulphate contents in coniferous soils in which sulphate accumulated during summer and winter, while being washed out of soils in spring and autumn. Soils which are exposed to localised, heavy atmospheric pollution generally contain larger concentrations of total sulphur and sulphate than relatively unpolluted soils (Wainwright, 1984). They also tend to oxidise reduced forms of sulphur at a faster rate (Lettl et al., 1981). The aim of this study was to determine seasonal changes in total sulphur and soil sulphate concentrations in relation to variations in soil moisture and pH in deciduous woodland soils exposed to atmospheric pollution.
METHODS Samples of an acidic, deciduous woodland soil were collected from beneath Acer pseudoplatanus L. growing at a site exposed to heavy atmospheric pollution from a coking works (Chapeltown, South Yorkshire SK 367954) and at a relatively unpolluted site (Fitzwilliam, West Yorkshire, SE 418166). Unfortunately, we were unable to replicate the sites because of the absence of other, suitable polluted sites in the immediate area. Samples were collected in the fourth week of each month from March 1985 to February 1986. Five sub-samples from the top 10cm of soil were taken and bulked into one large sample. Soils from both sites were typical brown earths located on carboniferous sandstones and shales from the Upper Coal Measures. Characteristics of the soils are shown in Table 1. Total organic C and total N were determined by the Walkely and Black, and macro-Kjeldahl methods, respectively (Hesse, 1971), while total S was determined using a LECO CS-244 Carbon Sulphur Determinator with a HF 100 Induction Furnace. Cation exchange capacity was determined by the method of Brown (1943). Climate and vegetation (mixed deciduous) at the sites are similar, but the polluted soils have been exposed to coking pollution for over 50 years. The present SO2 concentrations annually average 150#gm -3, but occasionally exceed 200pgm -3. The vegetation at the polluted site is generally covered with a thin layer of soot deposits, while this covering is absent from the relatively unpolluted site. Further details of the two sites are given elsewhere (Killham & Wainwright, 1984). The soils were sampled in
Sulphate content of air polluted soils
197
TABLE 1 Physical and Chemical Characteristics of the Heavily Polluted and Relatively Unpolluted Soils
Characteristic'
Soils
Moisture content (% w/w) (mean annual) Organic matter (% w/w) pH (in water) Cation exchangecapacity (meq. 100g -1) Exchangeable bases Base saturation (%) Total organic-C (% w/w) TotaI-N (% w/w) Total-S (% w/w) SO42-S (/lgg- 1 dry soil)
Polluted soils
Relatively unpolluted
5"1 37.4 3.9
3"3 16.2 5.2
14"1 3.7 26.2 13.6 1.3 0.56 204
9"2 4.2 45.6 9.4 1.0 0'06 116
October, when the top 3 cm of soil was collected after the leaf litter had been removed. Soil moisture content (% w/w) was measured within 24 h of sampling by oven drying samples (10 g) at 105°C to constant weight. Soil water potential was determined using a pressure plate apparatus. Soil pH was determined using a glass electrode on a soil slurry (1:10 soil:distilled water shaken for 15 min). Sulphur ions were extracted with a 1:10 soil: LiCI (0.1 M) slurry shaken for 15 min then filtered through Whatman No. 1 filter paper. Sulphate-S was determined turbidimetrically (Hesse, 1971), while total sulphur was determined on air dry soil samples (sieved to < 2 mm) using a LECO CS-244 Carbon Sulphur Determinator with a HF 100 Induction Furnace. Samples (25 mg) were weighed into clay crucibles and approximately 1 g of iron accelerator and 1 g of combustion accelerator were added. The samples were then combusted at approximately 2000°C and the SO 2 evolved was measured by infra-red analysis. Total sulphur was calculated automatically using a coke sample (0.032% S) as standard. Blank measurements were obtained by combusting the iron and combustion accelerators together.
RESULTS AND DISCUSSION The seasonal changes in soil moisture content, pH and sulphur content of the brown earth soils are shown in Figs 1-3 and summarised in Table 1. The soil moisture content (% w/w) of the heavily polluted soil was consistently
198
Wendy Nevell, M. Wainwright
.+ 10
0
b)
5.3
J
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.
4.7"
•
4.5"
4.3"
•
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•
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Fig. 1. Monthly changes in (a) soil moisture content and (b) pH of the heavily polluted ( 0 - - 0 ) and relatively unpolluted (B--II) brown earth soils over a 1-year period. (Means of triplicates, standard deviation values never exceeded 10%.) higher than that of the relatively unpolluted soil, presumably because of its higher organic matter content and therefore high water-holding capacity (Brady, 1974) (Fig. 1). Soil moisture contents for both soils were highest in samples collected during the winter m o n t h s and lowest in those collected in September and October; these seasonal changes in soil moisture reflect the monthly rainfall values recorded for the Sheffield area during the same sampling period. Exceptionally high moisture contents were recorded in the soil samples collected in February despite the relatively low rainfall recorded during that m o n t h (Fig. i). This was probably due to snow covering the
Sulphate content o/ air polluted soils
199
a:l 15o
¢q,~.
50
0 ............ b)
500] • 450 I
1 30
250]
20.1151005~ ~
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0 J " F'M'A'M' J ' J'A'S 'O'N'D' Fig. 2. Monthly changes in amounts of LiCl-extractable sulphate in heavily polluted
(O=O) and relatively unpolluted ( I - - m ) brown earth soils over a 1-yearperiod. (Means of triplicates.) Results (_+ standard deviation) expressed as (a) /~gS g-1 field moist and (b) /~gS g- 1dry soil (* significantlydifferent from value for relatively unpolluted soil, p = 0"05). ground for most of the m o n t h leading to the soil being frozen and badly drained. The pH of soil from the heavily polluted site was lower than that of the soil obtained from the relatively unpolluted site of each monthly sampling except for November (Fig. lb). The low soil pH at the heavily polluted site was attributed by Killham & Wainwright, (1984) to the input of pollutants from the coking works, because the pH of the relatively unpolluted soil decreased when samples were transferred to the heavily polluted site. The pH of the heavily polluted soil averaged 3-9 throughout the year and showed
Wendy Nevell, M. Wainwright
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0.3' if)
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Fig. 3. M o n t h l y changes in total sulphur content o f heavily polluted ( O - - O ) a n d relatively unpolluted ( m - m ) b r o w n earth soils over a l-year period. (Means o f triplicates. Results (_+ s t a n d a r d deviation expressed as % w/w dry soil.))
little variation except for a peak (pH4"6) in the February sample. This exceptionally high pH value coincided with a high soil moisture content (Fig. la, Table 2) and corresponds to a similar increase in pH observed when this soil was saturated in the laboratory (Nevell & Wainwright, 1986). The pH of the relatively unpolluted soil fluctuated markedly throughout the year and peaked irregularly in February, May, June and October (Fig. lb). Only the peak in the February sample could be attributed to a high soil moisture content and the result of soil saturation. The seasonal changes in LiC1extractable sulphate and total sulphur content of soils from the two sites are TABLE 2
W a t e r Potential Values for the Heavily Polluted and Relatively U n p o l l u t e d Brown E a r t h Soils at Different Soil Moisture C o n t e n t s
Soil moisture content (% w/w )
10 20 30 40 50 60
Water potential ( MPa) Heavily polluted Relative unpolluted soil soil -95 - 80 - 5"0 -0-1 - 0-08 - 0"07
- 100 - 90 - 9.0 -0"08 - 0"075 - 0.065
Sulphate content of a i r polluted soils
201
shown in Figs 2 and 3. Lithium chloride was used as extractant because it removes the soluble sulphate fraction from soils, and also slows or stops microbial activity during extraction and storage of the extracts. The annual average sulphate concentration of the heavily polluted soil was larger than that of the relatively unpolluted soil (Table 1) but was significantly greater (p--0.05) only during certain months. When the results are expressed as /~g Sg-1 field moist soil, sulphate concentrations were only significantly higher for the heavily polluted soil during July, August, September and October (Fig. 2a). Moreover, they were significantly lower than for the relatively unpolluted soil during March. When the results are expressed as /~g Sg- 1 dry soil (Fig. 2b) the sulphate concentrations at the coking works site were significantly higher than at the relatively unpolluted site for eight months of the year and showed no significant difference from the latter during the remaining four months. The sulphate concentration in the heavily polluted soil tended to peak during the late summer and autumn months (July to October) which coincided with the drier months of the year (Fig. 3). Cole & Johnson (1977) and Christopherson & Wright (1981) also found a marked seasonal pattern in sulphate concentration in forest soils with peaks occurring during the dry season and then decreasing during the heavy rains which followed. At the coking works site the larger sulphate concentrations may have resulted from the high ambient SO2 levels here. Soils have a large capacity to sorb sulphur gases and to oxidise them to sulphate (Bremner & Banwart, 1976). However, such sorption is greater in moist than in dry soils (Bremner & Banwart, 1976; Ghiorse & Alexander, 1976). Therefore, the high soil sulphate concentrations at Chapeltown during the months of low soil moisture content may have resulted from internal production of sulphate ions (i.e. oxidation and/or mineralisation reactions) or from increased interception by the sycamore canopies of both gaseous and particulate sulphur pollutants from the atmosphere. These atmospheric inputs may be washed from the canopy into the soil during rain events. This appears likely since, in SO2 polluted areas, the sulphate concentration in rain is increased during passage through tree canopies. Interception of particulates by the canopy was observed to be particularly marked during summer, due to a covering of aphid honeydew on the leaves. This sticky, sugary substance is secreted by aphids, populations of which have been shown to increase in atmospherically polluted environments (Dohmen et al., 1984). The concentration of soil sulphate at both sites was high during January. This may reflect the high rainfall during this month (Table 3) and therefore increased wet deposition of sulphur pollutants. This seems possible because both sites lie within an area subjected to high levels of atmospheric SO2. Alternatively, continued wetting of the soils by the high rainfall may have
202
Wen@ Nevell, M. Wainwright TABLE 3 Meteorological Data for Sheffield Area (March 1985-February 1986)a
Month
Mar. '85 April May June July Aug. Sept. Oct. Nov. Dec. Jan. '86 Feb. a
Daily temperature (°C) Mean Mean Mean o f max. min. max. + rain.
8"3 11'6 14-4 16"8 20.4 18.4 18'8 14.4 6.5 8'2 5'4 1-0
1.7 4.8 6.8 8"6 6-6 12.7 11-4 11.0 8-7 1.4 0.6 -3.1
5.0 8"2 10.2 12"7 16.6 14.9 14.9 11 '0 4.0 6.1 3.0 - 1.0
Rainfall (ram) (total per month) 48-8 69'7 63"4 76"5 46.4 87'9 14.9 33'6 78.2 78-0 174.1 37.2
Number o f days on which o f snow snow fell on ground
7 1 0 0 0 0 0 0 2 1 11 17
2 0 0 0 0 0 0 0 3 1 9 25
From Weston Park Museum meteorological station, Sheffield.
increased the desorption of adsorbed sulphate, resulting in a 'flush' of LiCIextractable sulphate (Nevell & Wainwright, 1986). A marked difference in the amount of total sulphur was found at the two sites, annually averaging 0-61% (w/w dry soil) for the heavily polluted site, compared with only 0-07% for the relatively unpolluted site (Fig. 3). These results reflect the higher concentrations of atmospheric sulphur incident on the heavily polluted soil (both as gaseous and particulate sulphur), as well as the larger sulphate adsorbing capacity of the soil at this site. Little seasonal variation in the total sulphur occurred in the soil at the relatively unpolluted site, but in the heavily polluted soil the total suphur concentration peaked during the late summer to autumn period, when sulphate concentrations were large. This presumably reflects the ability of sycamore canopies to intercept sulphur pollutants from the atmosphere. Tree canopies have frequently been observed to increase the ion content (including sulphur) of throughfall and stemflow (Wainwright, 1978; Skeffington, 1983). The lower total-S content during the wetter winter and spring months may, on the other hand, have resulted from leaching of sulphate into the lower soil horizons. The results show that only 4-0% of the total sulphur in the heavily polluted soil occurred as LiCl-extractable sulphate, whereas 21.4% of the total sulphur occurred in this form in the relatively unpolluted soil. Therefore, exposure to heavy atmospheric pollution from the coking plant
Sulphate content of air polluted soils
203
increased the a m o u n t of organic sulphur in nearby soils. Organic sulphur c o m p o u n d s are likely to enter the soil from leaf litter, roots, animal faeces, plant exudates, or sulphate may be incorporated into organic matter by microorganisms (Freney, 1967). Recent studies have shown that 35Ssulphate may be rapidly converted into organic matter in soils, provided that sufficient energy is available (Fitzgerald et al., 1982). On the other hand, Johnson et al., (1982) found that 90% of sulphate accumulated by a forest ecosystem occurred as sulphate adsorption in sesquioxide rich sub-surface soils. Barrow & Shaw (1977) suggested that sulphate adsorption can be regarded as a two-stage process in which the second stage involves a decrease in sulphate availability. Sulphate may therefore be weakly or tightly bound in soils, with only the former or reversibly adsorbed sulphate being water soluble (Meiwes et al., 1980; Johnson & Reuss, 1984). Because sulphate adsorption is pH dependent, being enhanced at low pH (Singh, 1984), the process is likely to be an important factor in sulphur retention in polluted soil.
ACKNOWLEDGEMENT Financial assistance from N E R C for a post-graduate studentship for W.N. is gratefully acknowledged.
REFERENCES Barrow, N. J. & Shaw, T. C. (1977). The slow reactions between soil and anions: 7. Effect of time and temperature of contact between an adsorbing soil and sulfate. Soil Sci., 124, 347-54. Brady, N. C. (1974). The nature and properties of soils. Macmillan, London. Bremner, J. M. & Banwart, W. L. (1976). Sorption of sulfur gases by soils. Soil Biol. Biochern., 8, 79-83. Brown, I. C. (1943). A rapid method for determining exchangeable hydrogen and total exchangeable bases of soils., Soil Science, 56, 353-7. Christopherson, N. & Wright, R. F. (1981). Sulfate budget and a model for sulphate concentrations in stream water at Birkenes, a small forested catchment in southernmost Norway. Water Resour. Res., 17, 377-89. Cole, D. W. & Johnson, D. W. (1977). Atmospheric sulfate additions and cation leaching in a Douglas fir ecosystem. Water Resour. Res., 13, 313-17. Dohmen, G. P., McNeiil, S. & Bell, J. N. B. (1984). Air pollution increases Aphis fabae pest potential. Nature, 307, 52-3. Fitzgerald, J. W., Strickland, T. C. & Swank, W. T. (1982). Metabolic rate or inorganic sulphate in soil samples from undisturbed and managed forest ecosystems. Soil Biol. Biochem., 14, 529 36.
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Freney, J. R. (1967). Sulphur-containing inorganics. In Soil biochemistry, Vol. 1. ed. by A. D. McLaren and G. H. Peterson, Arnold, London. Ghiorse, W. C. & Alexander, M. (1976). Effect of micro-organisms on the sorption and fate of sulfur dioxide and nitrogen dioxide in soil, J. Environ, QuaL, 5, 227-30. Hesse, P. R. (1971). A textbook of soil chemical analysis, J. Murray, London, 322-3. Johnson, D. W. & Reuss, J. O. (1984). Soil mediated effects of atmospherically deposited sulphur and nitrogen. PhiL Trans. R. Soc. Lond. B, 305, 383-92. Johnson, D. W., Henderson, G. S., Huff, D. D., Lindberg, S. E., Richter, D. D., Shriner, D. S., Todd, D. E. & Turner, J. (1982). Cycling of organic and inorganic sulphur in a chestnut oak forest. Oecologia, 54, 141-8. Killham, K. & Wainwright, M. (1984). Chemical and microbiological changes in soil following exposure to heavy atmospheric pollution. Environ. Pollut., Series A, 33, 121-31. Lettl, A., Langkramer, O., Lochman, V. & Jacks, M. (1981). Effects of industrial emissions with high sulphur dioxide content on thiobaciili and oxidative ability of spruce forest soils towards inorganic sulphur compounds. Folia MicrobioL, 26, 151-7. Meiwes, K. J., Khanna, P. K. & Ulrich, B. (1980). Retention of sulphate by an acid brown earth and its relationship with the atmospheric input of sulphur to forest vegetation. Z. Pflanzenernaehr. Bodenkd., 143, 402-11. Nevell, W. & Wainwright, M. (1986). Increases in extractable sulphate following soil submergence with water, dilute sulphuric acid or acid rain. Environ. Pollut., Series B, 12, 301-11. Singh, B. R. (1984). Sulfate sorption by acid forest soils: 3. Desorption of sulphate from adsorbed surfaces as a function of time, desorbing ion, pH and amount of adsorption. Soil Sci., 138, 346 53. Skeffington, R. A. (1983). Soil properties under three species of tree in southern England in relation to acid deposition and throughfall. In Effect of accumulations of air pollutants in forest ecosystems, ed. by B. Ulrich and J. Pankrath, 219-31, Reidel, Dordrecht. Strick, J. E. Schindler, S. C., David, M. B., Mitchell, M. J. & Nakas, J. P. (1982). Importance of organic sulfur constituents and microbial activity to sulfur transformations in an Adirondack forest soil. Northeast Environ. Sci., 1, 161-9. Wainwright, M. (1978). Distribution of sulphur oxidation products in soils and on Acer pseudoplatanus L. growing close to sources of atmospheric pollution. Environ. Pollut., 17, 153-60. Wainwright, M. (1984). Sulfur oxidation in soils. Advances in Agronomy, 37, 349-96. Wainwright, M. & Nevell, W. (1984). Microbial transformations of sulphur in atmospheric-polluted soils. Rev. Environ. Health, 4, 339-56.
Seasonal Changes in the Sulphate Content of Deciduous Woodland Soils Exposed to Atmospheric Pollution Wendy Nevell & M. Wainwright Department of Microbiology, University of Sheffield, Sheffield, SI0 2TN, Great Britain (Received 26 February 1986; revised version received 16 March 1987; accepted 3 April 1987)
A BSTRA CT Seasonal changes in soil pH, sulphate concentration and totaLS were measured in two brown earth soils, sampled from deciduous woodlands. One site studied was exposed to severe atmospheric pollution from a coking works, while the other site was relatively unpolluted but located in an area receiving wet and dry deposited acidity of greater than 1"0 and 2.4 kg H + ha- xyear - 1, respectively The pH of soil at the heavily polluted site was lower than the relatively unpolluted soil at each monthly sample point, except during November. Annual average sulphate concentrations (LiCl-extractable) were highest in the soil exposed to coking pollution, where they peaked during summer and autumn. A marked difference in totaLS wasJbund in soils from the two sites, the heavily polluted soil showing the highest concentration with peaks again occurring during late summer and autumn. Only 4"0% (w/w) of the totaLS of the heavily polluted soil occurred as LiCLextractable sulphate, compared to 21"4% (w/w)for the relatively unpolluted soil, showing that organic sulphur is increased in brown earths following exposure to severe atmospheric pollution from the coking works.
INTRODUCTION The cycling o f sulphur in soils receiving atmospheric sulphur deposition is potentially important because it may influence the mobility of sulphate and 195 Environ. Pollut. 0269-7491/87/$03'50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
196
Wendy Nevell, M. Wainwright
therefore the leaching of cations (Cole & Johnson, 1977; Strick et al. 1982; Wainwright & Nevell, 1984). Cole & Johnson (1977) showed that seasonal variations occurred in the sulphate of a Douglas fir ecosystem, with concentrations of this ion peaking in autumn, just after a 2-month summer drought, and then decreasing during a following period of heavy rains. Similarly, Christophersen & Wright (1981) found a marked seasonal pattern of sulphate contents in coniferous soils in which sulphate accumulated during summer and winter, while being washed out of soils in spring and autumn. Soils which are exposed to localised, heavy atmospheric pollution generally contain larger concentrations of total sulphur and sulphate than relatively unpolluted soils (Wainwright, 1984). They also tend to oxidise reduced forms of sulphur at a faster rate (Lettl et al., 1981). The aim of this study was to determine seasonal changes in total sulphur and soil sulphate concentrations in relation to variations in soil moisture and pH in deciduous woodland soils exposed to atmospheric pollution.
METHODS Samples of an acidic, deciduous woodland soil were collected from beneath Acer pseudoplatanus L. growing at a site exposed to heavy atmospheric pollution from a coking works (Chapeltown, South Yorkshire SK 367954) and at a relatively unpolluted site (Fitzwilliam, West Yorkshire, SE 418166). Unfortunately, we were unable to replicate the sites because of the absence of other, suitable polluted sites in the immediate area. Samples were collected in the fourth week of each month from March 1985 to February 1986. Five sub-samples from the top 10cm of soil were taken and bulked into one large sample. Soils from both sites were typical brown earths located on carboniferous sandstones and shales from the Upper Coal Measures. Characteristics of the soils are shown in Table 1. Total organic C and total N were determined by the Walkely and Black, and macro-Kjeldahl methods, respectively (Hesse, 1971), while total S was determined using a LECO CS-244 Carbon Sulphur Determinator with a HF 100 Induction Furnace. Cation exchange capacity was determined by the method of Brown (1943). Climate and vegetation (mixed deciduous) at the sites are similar, but the polluted soils have been exposed to coking pollution for over 50 years. The present SO2 concentrations annually average 150#gm -3, but occasionally exceed 200pgm -3. The vegetation at the polluted site is generally covered with a thin layer of soot deposits, while this covering is absent from the relatively unpolluted site. Further details of the two sites are given elsewhere (Killham & Wainwright, 1984). The soils were sampled in
Sulphate content of air polluted soils
197
TABLE 1 Physical and Chemical Characteristics of the Heavily Polluted and Relatively Unpolluted Soils
Characteristic'
Soils
Moisture content (% w/w) (mean annual) Organic matter (% w/w) pH (in water) Cation exchangecapacity (meq. 100g -1) Exchangeable bases Base saturation (%) Total organic-C (% w/w) TotaI-N (% w/w) Total-S (% w/w) SO42-S (/lgg- 1 dry soil)
Polluted soils
Relatively unpolluted
5"1 37.4 3.9
3"3 16.2 5.2
14"1 3.7 26.2 13.6 1.3 0.56 204
9"2 4.2 45.6 9.4 1.0 0'06 116
October, when the top 3 cm of soil was collected after the leaf litter had been removed. Soil moisture content (% w/w) was measured within 24 h of sampling by oven drying samples (10 g) at 105°C to constant weight. Soil water potential was determined using a pressure plate apparatus. Soil pH was determined using a glass electrode on a soil slurry (1:10 soil:distilled water shaken for 15 min). Sulphur ions were extracted with a 1:10 soil: LiCI (0.1 M) slurry shaken for 15 min then filtered through Whatman No. 1 filter paper. Sulphate-S was determined turbidimetrically (Hesse, 1971), while total sulphur was determined on air dry soil samples (sieved to < 2 mm) using a LECO CS-244 Carbon Sulphur Determinator with a HF 100 Induction Furnace. Samples (25 mg) were weighed into clay crucibles and approximately 1 g of iron accelerator and 1 g of combustion accelerator were added. The samples were then combusted at approximately 2000°C and the SO 2 evolved was measured by infra-red analysis. Total sulphur was calculated automatically using a coke sample (0.032% S) as standard. Blank measurements were obtained by combusting the iron and combustion accelerators together.
RESULTS AND DISCUSSION The seasonal changes in soil moisture content, pH and sulphur content of the brown earth soils are shown in Figs 1-3 and summarised in Table 1. The soil moisture content (% w/w) of the heavily polluted soil was consistently
198
Wendy Nevell, M. Wainwright
.+ 10
0
b)
5.3
J
5.1 4.9 pH
.
4.7"
•
4.5"
4.3"
•
i._/•
~
•
4,1"
a.9.. 3.7
J
•--°\,o_.Jo~.' 'F
'M
" A'M
' J
' J
' A ' S " O"
x N'D
'
Fig. 1. Monthly changes in (a) soil moisture content and (b) pH of the heavily polluted ( 0 - - 0 ) and relatively unpolluted (B--II) brown earth soils over a 1-year period. (Means of triplicates, standard deviation values never exceeded 10%.) higher than that of the relatively unpolluted soil, presumably because of its higher organic matter content and therefore high water-holding capacity (Brady, 1974) (Fig. 1). Soil moisture contents for both soils were highest in samples collected during the winter m o n t h s and lowest in those collected in September and October; these seasonal changes in soil moisture reflect the monthly rainfall values recorded for the Sheffield area during the same sampling period. Exceptionally high moisture contents were recorded in the soil samples collected in February despite the relatively low rainfall recorded during that m o n t h (Fig. i). This was probably due to snow covering the
Sulphate content o/ air polluted soils
199
a:l 15o
¢q,~.
50
0 ............ b)
500] • 450 I
1 30
250]
20.1151005~ ~
•
~~~l~f
0 J " F'M'A'M' J ' J'A'S 'O'N'D' Fig. 2. Monthly changes in amounts of LiCl-extractable sulphate in heavily polluted
(O=O) and relatively unpolluted ( I - - m ) brown earth soils over a 1-yearperiod. (Means of triplicates.) Results (_+ standard deviation) expressed as (a) /~gS g-1 field moist and (b) /~gS g- 1dry soil (* significantlydifferent from value for relatively unpolluted soil, p = 0"05). ground for most of the m o n t h leading to the soil being frozen and badly drained. The pH of soil from the heavily polluted site was lower than that of the soil obtained from the relatively unpolluted site of each monthly sampling except for November (Fig. lb). The low soil pH at the heavily polluted site was attributed by Killham & Wainwright, (1984) to the input of pollutants from the coking works, because the pH of the relatively unpolluted soil decreased when samples were transferred to the heavily polluted site. The pH of the heavily polluted soil averaged 3-9 throughout the year and showed
Wendy Nevell, M. Wainwright
200 0.7
0.6 ¸
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0.3' if)
.L
0.2'
o
0.1'
J
F
M
A
M
J
J
A
S
0
N
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Fig. 3. M o n t h l y changes in total sulphur content o f heavily polluted ( O - - O ) a n d relatively unpolluted ( m - m ) b r o w n earth soils over a l-year period. (Means o f triplicates. Results (_+ s t a n d a r d deviation expressed as % w/w dry soil.))
little variation except for a peak (pH4"6) in the February sample. This exceptionally high pH value coincided with a high soil moisture content (Fig. la, Table 2) and corresponds to a similar increase in pH observed when this soil was saturated in the laboratory (Nevell & Wainwright, 1986). The pH of the relatively unpolluted soil fluctuated markedly throughout the year and peaked irregularly in February, May, June and October (Fig. lb). Only the peak in the February sample could be attributed to a high soil moisture content and the result of soil saturation. The seasonal changes in LiC1extractable sulphate and total sulphur content of soils from the two sites are TABLE 2
W a t e r Potential Values for the Heavily Polluted and Relatively U n p o l l u t e d Brown E a r t h Soils at Different Soil Moisture C o n t e n t s
Soil moisture content (% w/w )
10 20 30 40 50 60
Water potential ( MPa) Heavily polluted Relative unpolluted soil soil -95 - 80 - 5"0 -0-1 - 0-08 - 0"07
- 100 - 90 - 9.0 -0"08 - 0"075 - 0.065
Sulphate content of a i r polluted soils
201
shown in Figs 2 and 3. Lithium chloride was used as extractant because it removes the soluble sulphate fraction from soils, and also slows or stops microbial activity during extraction and storage of the extracts. The annual average sulphate concentration of the heavily polluted soil was larger than that of the relatively unpolluted soil (Table 1) but was significantly greater (p--0.05) only during certain months. When the results are expressed as /~g Sg-1 field moist soil, sulphate concentrations were only significantly higher for the heavily polluted soil during July, August, September and October (Fig. 2a). Moreover, they were significantly lower than for the relatively unpolluted soil during March. When the results are expressed as /~g Sg- 1 dry soil (Fig. 2b) the sulphate concentrations at the coking works site were significantly higher than at the relatively unpolluted site for eight months of the year and showed no significant difference from the latter during the remaining four months. The sulphate concentration in the heavily polluted soil tended to peak during the late summer and autumn months (July to October) which coincided with the drier months of the year (Fig. 3). Cole & Johnson (1977) and Christopherson & Wright (1981) also found a marked seasonal pattern in sulphate concentration in forest soils with peaks occurring during the dry season and then decreasing during the heavy rains which followed. At the coking works site the larger sulphate concentrations may have resulted from the high ambient SO2 levels here. Soils have a large capacity to sorb sulphur gases and to oxidise them to sulphate (Bremner & Banwart, 1976). However, such sorption is greater in moist than in dry soils (Bremner & Banwart, 1976; Ghiorse & Alexander, 1976). Therefore, the high soil sulphate concentrations at Chapeltown during the months of low soil moisture content may have resulted from internal production of sulphate ions (i.e. oxidation and/or mineralisation reactions) or from increased interception by the sycamore canopies of both gaseous and particulate sulphur pollutants from the atmosphere. These atmospheric inputs may be washed from the canopy into the soil during rain events. This appears likely since, in SO2 polluted areas, the sulphate concentration in rain is increased during passage through tree canopies. Interception of particulates by the canopy was observed to be particularly marked during summer, due to a covering of aphid honeydew on the leaves. This sticky, sugary substance is secreted by aphids, populations of which have been shown to increase in atmospherically polluted environments (Dohmen et al., 1984). The concentration of soil sulphate at both sites was high during January. This may reflect the high rainfall during this month (Table 3) and therefore increased wet deposition of sulphur pollutants. This seems possible because both sites lie within an area subjected to high levels of atmospheric SO2. Alternatively, continued wetting of the soils by the high rainfall may have
202
Wen@ Nevell, M. Wainwright TABLE 3 Meteorological Data for Sheffield Area (March 1985-February 1986)a
Month
Mar. '85 April May June July Aug. Sept. Oct. Nov. Dec. Jan. '86 Feb. a
Daily temperature (°C) Mean Mean Mean o f max. min. max. + rain.
8"3 11'6 14-4 16"8 20.4 18.4 18'8 14.4 6.5 8'2 5'4 1-0
1.7 4.8 6.8 8"6 6-6 12.7 11-4 11.0 8-7 1.4 0.6 -3.1
5.0 8"2 10.2 12"7 16.6 14.9 14.9 11 '0 4.0 6.1 3.0 - 1.0
Rainfall (ram) (total per month) 48-8 69'7 63"4 76"5 46.4 87'9 14.9 33'6 78.2 78-0 174.1 37.2
Number o f days on which o f snow snow fell on ground
7 1 0 0 0 0 0 0 2 1 11 17
2 0 0 0 0 0 0 0 3 1 9 25
From Weston Park Museum meteorological station, Sheffield.
increased the desorption of adsorbed sulphate, resulting in a 'flush' of LiCIextractable sulphate (Nevell & Wainwright, 1986). A marked difference in the amount of total sulphur was found at the two sites, annually averaging 0-61% (w/w dry soil) for the heavily polluted site, compared with only 0-07% for the relatively unpolluted site (Fig. 3). These results reflect the higher concentrations of atmospheric sulphur incident on the heavily polluted soil (both as gaseous and particulate sulphur), as well as the larger sulphate adsorbing capacity of the soil at this site. Little seasonal variation in the total sulphur occurred in the soil at the relatively unpolluted site, but in the heavily polluted soil the total suphur concentration peaked during the late summer to autumn period, when sulphate concentrations were large. This presumably reflects the ability of sycamore canopies to intercept sulphur pollutants from the atmosphere. Tree canopies have frequently been observed to increase the ion content (including sulphur) of throughfall and stemflow (Wainwright, 1978; Skeffington, 1983). The lower total-S content during the wetter winter and spring months may, on the other hand, have resulted from leaching of sulphate into the lower soil horizons. The results show that only 4-0% of the total sulphur in the heavily polluted soil occurred as LiCl-extractable sulphate, whereas 21.4% of the total sulphur occurred in this form in the relatively unpolluted soil. Therefore, exposure to heavy atmospheric pollution from the coking plant
Sulphate content of air polluted soils
203
increased the a m o u n t of organic sulphur in nearby soils. Organic sulphur c o m p o u n d s are likely to enter the soil from leaf litter, roots, animal faeces, plant exudates, or sulphate may be incorporated into organic matter by microorganisms (Freney, 1967). Recent studies have shown that 35Ssulphate may be rapidly converted into organic matter in soils, provided that sufficient energy is available (Fitzgerald et al., 1982). On the other hand, Johnson et al., (1982) found that 90% of sulphate accumulated by a forest ecosystem occurred as sulphate adsorption in sesquioxide rich sub-surface soils. Barrow & Shaw (1977) suggested that sulphate adsorption can be regarded as a two-stage process in which the second stage involves a decrease in sulphate availability. Sulphate may therefore be weakly or tightly bound in soils, with only the former or reversibly adsorbed sulphate being water soluble (Meiwes et al., 1980; Johnson & Reuss, 1984). Because sulphate adsorption is pH dependent, being enhanced at low pH (Singh, 1984), the process is likely to be an important factor in sulphur retention in polluted soil.
ACKNOWLEDGEMENT Financial assistance from N E R C for a post-graduate studentship for W.N. is gratefully acknowledged.
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