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Deciduous leaf litter and cellulose decomposition in soil exposed to heavy atmospheric pollution
Environmental
Pollution(SeriesA) 26 (1981)79-85
D E C I D U O U S LEAF LITTER A N D CELLULOSE DECOMPOSITION IN SOIL EXPOSED TO HEAVY ATMOSPHERIC POLLUTION K. KILLHAM ~k M. WAINWRIGHT
Department of Microbiology, University of Sheffield, Sheffield $10 2TN, Great Britain
ABSTRACT
Atmospheric pollution from a coking plant reduced the decomposition of Acer pseudoplatanus litter left in litter bags in soil at a depth of 2.5 cm by 35 % compared with decomposition at a similar, but unpolluted, site. Cellulose degradation in the top 10 cm of soil was, on the other hand, only marginally inhibited by pollution. Of the total inhibition of litter due to pollution, 57% was calculated to be due to nonparticulate pollutants (acid rain plus gases), whilst the remaining 43 % was due to the effects of atmospheric pollution deposits (APD) which contaminated polluted leaf surfaces. The scarcity of soil arthropods at the polluted site appears to be an important factor reducing litter decomposition.
INTRODUCTION
The accumulation of leaf litter and its subsequent decomposition are important factors determining woodland productivity. Numerous studies have shown that both natural and simulated acid rain can reduce the rate of litter decomposition in woodlands, causing an increase in the amount of organic matter stored in the uppermost soil horizons (Baath et al., 1980). Few studies, however, have been reported on the effects of a total load of atmospheric pollutants on litter degradation. Such effects will, of course, depend on the composition of such effluents, whether, for example, they contain heavy metals (Freedman & Hutchinson, 1980), phenolics (Wainwright, 1979) or particulate materials in addition to acid rain (Wainwright, 1980a). The aim of the work reported here was to determine the effects of atmospheric pollution from a coking plant on deciduous leaf litter and cellulose decomposition in soil and, in particular, to determine whether atmospheric pollution deposits, present on the leaf surface, affect these processes. 79 Environ. Pollut. Ser. A. 0143-1471/81/0026-0079/$02.50 © Applied Science Publishers Ltd, England, 1981 Printed in Great Britain
80
K. K 1 L L H A M , M. W A I N W R I G H T
MATERIALS A N D M E T H O D S
Sites and soil types The polluted and unpolluted sites had essentially the same soil type (brown earths showing signs of podzolisation) and vegetation (deciduous woodland with Acer pseudoplatanus predominating) and both their altitude (80 and 60 m above mean sea level for polluted and unpolluted sites, respectively) and climate (Table 1) were similar. The only marked difference between the two sites was that the polluted site was 500 m downwind of a coking plant (Chapeltown, South Yorkshire, SK 365954; operating since 1929), while the other site was relatively unpolluted (Fitzwilliam, West Yorkshire, SE 166417). However, the pH, total C and N contents of the soils TABLE 1 CHEMICAL PROPERTIES OF SOILS AND CLIMATIC DATA (1979) FOR THE TWO SITES
Soil pH
Total soil C (?/o) Total soil N (?/o) Mean daily maximum temperature (°C) Mean daily minimum temperature (°C) Mean annual rainfall (mm)
Mean number of days on which rain fell Number of days with snow lying on the ground
Polluted site
Unpolluted site
3.3 14.6 0.93 12.7 6.2 830 181 23
4.9 9.8 0.82 12.9 6.3 805 173 27
differed considerably. When soil samples were analysed along a transect away from the pollution source the soil properties eventually approximated to those found at the unpolluted site, showing that differences in the pH and chemical properties of the polluted soils resulted directly from exposure to the pollution load. Sycamore leaves at the polluted site were covered in a thin layer of atmospheric pollution deposits (APD) whilst those from the unpolluted site were largely uncontaminated.
Litter decomposition The rate of litter decomposition over a one-year period at the sites was determined using litter bags (Edwards & Heath, 1963). Freshly fallen sycamore leaves were cut into 2.5 cm diameter discs and fifty discs were placed into heat-sealed nylon bags (10 × 7cm) of varying mesh size. A 5/z mesh was used to exclude all but microorganisms, 0.5 mm mesh to allow for access to mites, springtails, enchytraeids and most other soil arthropods as well as micro-organisms and a 1 cm mesh to enable the entry of most soil organisms, including earthworms. Bags (72 for each mesh size) were buried below the sycamore canopy to a depth of 2.5 cm and the original litter layer replaced. Every month, six bags of each mesh size were removed and the area of leaf-discs decomposed measured. Discs cut from
CELLULOSE DECOMPOSITION IN POLLUTED SOIL
81
APD-contaminated leaves obtained from the polluted site, as well as discs cut from uncontaminated leaves (two bags per mesh size per leaf type), were used. Discs where APD had been brushed from the leaves (two bags per mesh size per month) were also included. All experiments were performed at both sites.
Determination of the rate of cellulose decomposition The rate of cotton cellulose powder (Whatman C F l l ) decomposition over 7 months was determined by packing cellulose-amended soils (2 ~ w/w) from both sites into open-ended plastic tubes (10 cm length x 6.5 cm diameter, twenty-one tubes per soil type). The tubes also served as soil corers and were buried flush with the soil surface beneath sycamore canopies. Control tubes containing unamended soil were also included. The indigenous litter layer was replaced and three tubes (plus a control) for each soil type removed every month to determine the amount of added cellulose remaining (total cellulose minus native cellulose). Cellulose content of the soil was determined using the method of Updegraff (1969).
Enumeration of soil arthropods Freshly fallen sycamore leaves (2.5g) were placed in l cm mesh nylon nets (15 cm x 15 cm, twent~,-eight nets per site) and distributed in the uppermost litter layer beneath the canopy from which they originated. The nets were staked into position and removed from the field after six months. The arthropod population of the litter was then determined using a modified Berlese apparatus (Kiihnelt, 1961). RESULTS AND DISCUSSION
The following conclusions regarding the effects of atmospheric pollution from a coking plant on litter decomposition can be drawn from the data shown in Figs I and 2: (1) For any one litter type at either site, decomposition was greatest in the 1 cm mesh litter bags, followed by that found in the 0.5 mm mesh bags, whilst negligible decomposition was found in the 5/~m mesh bags. (2) Both APD contaminated and uncontaminated litter decomposed faster at the unpolluted than at the polluted site. (3) Lowest rates of decomposition were found when APD contaminated litter was exposed at the polluted site, whilst, conversely, highest rates were found when uncontaminated litter was left at the unpolluted site. (4) Rates of decomposition of uncontaminated litter left at the polluted site were similar to those found when contaminated litter was left at the unpolluted site. Two approaches to litter bag experiments appear in the literature. In the first, measurements of the loss in weight of air dried leaf discs over a time period are determined, whilst, in the second approach, loss of leaf area during decomposition is
82
K. KILLHAM, M. WAINWRIGHT
80 ' ~ , Q 7o
g b
•S
0
Iq
D
j
Ie
M
A
M
j
j
Months
Fig. 1. Litter decomposition over a one-year period in 1 cm mesh bags. © - - © uncontaminated leaves left at unpolluted site; A - - / k , contaminated leaves left at unpolluted site; 0 - - 0 , uncontaminated leaves left at polluted site; [I--I-q, contaminated leaves left at polluted site. Significantly different from control ( O - C ) ) * * p = 0.05; * p = 0.1.
,oo o
\ 5O.
5O
A
S
6
N
b
J
i= I~
A
I~
j
j
Months Fig. 2. Litter decomposition over a one-year period in 0-5 mm mesh bags. O - - O , uncontaminated leaves left at unpolluted site; A - - A , contaminated leaves left at unpolluted site; Q - - Q , uncontaminated leaves left at polluted site; I-q--[:], contaminated leaves left at polluted site. Significantly different from control ( O - O) ** P = 0.05; * p = 0. I.
CELLULOSE DECOMPOSITION IN POLLUTED SOIL
83
the criterion used. The last approach has been criticised (Satchell, 1974) because decomposition, particularly when caused by micro-organisms, can occur without a perceptible loss in leaf area. We verified this criticism to some extent in the present work since, whilst loss of leaf area in the 5/~m mesh bags was never greater than 2 ~o, a 6 ~ loss in leaf weight occurred over a one-year period. The presence of APD on polluted leaves, and its possible loss in situ or during air drying, precluded the use of decomposition rates based on loss of dry weight in the experiments described here. It should be borne in mind, however, that the method used to determine litter decomposition rates at the two sites will tend to emphasise the role of the soil fauna at the expense of that played by micro-organisms in the decomposition process. When contaminated litter was exposed (1 cm mesh bags for one year at the polluted site), a 35 ~ reduction in litter decomposition was seen compared with that when uncontaminated litter was left at the unpolluted site. From the data shown in Fig. 1 it can be calculated that non-particulate pollution (acid rain plus gases (NP)) accounts for 57 ~ of this reduction in litter decomposition whilst particulates (APD) account for the remaining 43 ~o. NP = Duu - Dup: 98 - 78: 2 0 ~ (i.e. 5 7 ~ of total reduction) APD = Dup - Dcp = 78 - 63 = 15 ~ (i.e. 43 ~ of total reduction) NP + APD = 20 + 15 = 35 ~ total reduction where Duu = ~ decomposition of uncontaminated litter at the unpolluted site. Dup = ~ decomposition of uncontaminated litter at the polluted site. Dcp = ~o decomposition of contaminated litter at the polluted site. The influence of the pollution deposits was further illustrated by the fact that removal (by brushing) of APD from the leaf surface resulted in decomposition rates similar to those found for uncontaminated litter at both sites (Fig. 3). Although the degradation of cotton cellulose was inhibited in the polluted soil, the effect was not marked (Fig. 4), showing that cellulose degrading micro-organisms are only marginally affected by air pollution from the coking plant. This agrees with a previous finding (Wainwright, 1980b) that microbial activity is not markedly impaired when deciduous woodland soil is subjected to air pollution from a coking plant over a period of one year. A preliminary study of the arthropods present in polluted and unpolluted litter showed a reduction in numbers and diversity in the former (Table 2). While some of these are not herbivorous (Kiihnelt, 1961), the reduction in arthropod numbers by atmospheric pollution is likely to account in large part for the observed reduction in litter decomposition found at this site. The atmospheric pollution deposits were found to be rich in phenols (Wainwright, 1979) and naphtha compounds, and have a high sulphur content (Killham & Wainwright, 1981) and as a result may prove directly toxic to soil animals. On the other hand, they may retard litter
84
K. KILLHAM, M. WAINWRIGHT
100 90
05ram mesh
0.5~n mesh
-~ 80
/// /-/I
//j
///
i
Icm mesh
/ / / / z /
~__ - ~ -
1cm mesh
I [i
f / /
///
2O
/// f// f// ///
"-
/
Unpolluted site
/
/
-
-
Polluted site
Fig. 3. Effect of removal of atmospheric pollution deposits from the leaf surface by brushing, on litter decomposition over a one-year period, cross-hatched columns: uncontaminated leaves; open columns, brushed contaminated leaves; hatched columns, contaminated leaves.
12,
0
A
S
6
N
O
J
~
M
Months
Fig. 4. Effect of atmospheric pollution on cellulose degradation over 7 months in soil. O - - O , uncontaminated soil left at unpolluted site; A - - A , contaminated soil left at unpolluted site; 0 - - 0 , uncontaminated soil left at polluted site; l--I--I--l, contaminated soil left at polluted site. Significantly different from control ( O - O ) ** p = 0.05; * p = 0-1.
CELLULOSE DECOMPOSITION IN POLLUTED SOIL
85
TABLE 2 MEAN NUMBER OF SOIL ARTHROPODS OBTAINED FROM SYCAMORE LITTER (10 g) AT POLLUTED AND UNPOLLUTED SI'FES
Chiiopoda Acari Collembola Araneae
Polluted site
Unpolluted site
1 _+0.3 3 _+0.5 3 +_0.4 4 -t- 1.5
12 _ 2-5 9 +_2-0 16 _+2.0 5 _ i.3
Means of seven replicates (_+ SD), each replicate bulked from litter (2.5 g) exposed in four litter bags (1 cm) on the soil surface at the two sites. d e c o m p o s i t i o n by reducing the p a l a t a b i l i t y of leaf litter to soil f a u n a , either directly or by killing p h y l l o p l a n e m i c r o - o r g a n i s m s which m a y act as a s u b s t a n t i a l n u t r i e n t source for litter animals. This last possibility however, appears remote as s c a n n i n g electron microscope studies of A P D c o n t a m i n a t e d leaves showed the presence of a diverse a r r a y o f p h y l l o p l a n e m i c r o - o r g a n i s m s . T o date most studies o n the effects o f atmospheric p o l l u t i o n o n litter d e c o m p o s i t i o n have ignored the c o n t r i b u t i o n m a d e by particulates to i m p a i r m e n t of the process. T h e results of this study show that A P D has a n effect o n litter d e g r a d a t i o n which is similar to that p r o d u c e d by n o n - p a r t i c u l a t e p o l l u t i o n (acid rain plus gases). The effect of A P D o n n u t r i e n t cycling in w o o d l a n d ecosystems is, however, likely to be more limited t h a n t h a t of acid rain, b o t h in terms of the n u m b e r of effluents c o n t a i n i n g these p o l l u t a n t s a n d in the extent to which they are distributed f r o m the p o l l u t a n t source. REFERENCES
BAATH,E., BERG,B., LOHN,V., LUNGREN,B., LUNDKVIST,H., ROSSWALL,T., SODERSTROM,B. & WIRI~N, A. (1980). Soil organisms and litter decomposition in a Scots pine forest--Effects of experimental acidification. In Effects of acid precipitation on terrestrial ecosystems, ed. by T. C. Hutchinson and M. Havas. NA TO Conference Series 1, Vol. 4, 375-80. EDWARDS,C. A. & HEATH,G. W. (I 963). The role of soil animals in the breakdown of leaf material. In Soil organisms, ed. by J. Doeksen and J. Van der Drift, 76-84. North Holland, Amsterdam. FREEDMAN, B. & HUTCHINSON, T. C. (1980). Smelter pollution near Sudbury, Ontario, Canada, and effects on forest litter decomposition. In Effects ofacidprecipitation on terrest rial ecosystems, ed. by T.C. Hutchinson and M. Havas. NATO Conference Series 1, Vol. 4, 395-434. KILLHAM,K. & WAINWRIGHT,M. (198I). Closed combustion method for the rapid determination of total S in atmospheric polluted soils and vegetation. Environ. Pollut. (Series B), 2(2), 81-5. Ki2HNELT,W. (1961). Soil biology with special reference to the animal kingdom. London, Faber and Faber. SATCHELL, J. E. (1974). Litter-interface of animate-inanimate matter. In Biology of plant litter decomposition, ed by C. H. Dickinson and G. J. F. laugh, xxxii-xxxiii. London, Academic Press. UFDEGR^rF, D. M. (1969). Semi-micro determination of cellulose in biological materials. Analyt. Biochem., 32, 420-4. WAINWRIGHT,M. (1979). Assay of phenol o-hydroxylaseactivity in soil. Soil Biol. Biochem., 11,549-5 I. WAINWRIGHT,M. (1980a). Man-made emissions of sulphur and the soil. Int. J. Environ. Studies, 14, 279-88. WAINWRIGHT,M. (1980b). Effect of exposure to atmospheric pollution on microbial activity in soil. PI. Soil, 55, 199-204.
Pollution(SeriesA) 26 (1981)79-85
D E C I D U O U S LEAF LITTER A N D CELLULOSE DECOMPOSITION IN SOIL EXPOSED TO HEAVY ATMOSPHERIC POLLUTION K. KILLHAM ~k M. WAINWRIGHT
Department of Microbiology, University of Sheffield, Sheffield $10 2TN, Great Britain
ABSTRACT
Atmospheric pollution from a coking plant reduced the decomposition of Acer pseudoplatanus litter left in litter bags in soil at a depth of 2.5 cm by 35 % compared with decomposition at a similar, but unpolluted, site. Cellulose degradation in the top 10 cm of soil was, on the other hand, only marginally inhibited by pollution. Of the total inhibition of litter due to pollution, 57% was calculated to be due to nonparticulate pollutants (acid rain plus gases), whilst the remaining 43 % was due to the effects of atmospheric pollution deposits (APD) which contaminated polluted leaf surfaces. The scarcity of soil arthropods at the polluted site appears to be an important factor reducing litter decomposition.
INTRODUCTION
The accumulation of leaf litter and its subsequent decomposition are important factors determining woodland productivity. Numerous studies have shown that both natural and simulated acid rain can reduce the rate of litter decomposition in woodlands, causing an increase in the amount of organic matter stored in the uppermost soil horizons (Baath et al., 1980). Few studies, however, have been reported on the effects of a total load of atmospheric pollutants on litter degradation. Such effects will, of course, depend on the composition of such effluents, whether, for example, they contain heavy metals (Freedman & Hutchinson, 1980), phenolics (Wainwright, 1979) or particulate materials in addition to acid rain (Wainwright, 1980a). The aim of the work reported here was to determine the effects of atmospheric pollution from a coking plant on deciduous leaf litter and cellulose decomposition in soil and, in particular, to determine whether atmospheric pollution deposits, present on the leaf surface, affect these processes. 79 Environ. Pollut. Ser. A. 0143-1471/81/0026-0079/$02.50 © Applied Science Publishers Ltd, England, 1981 Printed in Great Britain
80
K. K 1 L L H A M , M. W A I N W R I G H T
MATERIALS A N D M E T H O D S
Sites and soil types The polluted and unpolluted sites had essentially the same soil type (brown earths showing signs of podzolisation) and vegetation (deciduous woodland with Acer pseudoplatanus predominating) and both their altitude (80 and 60 m above mean sea level for polluted and unpolluted sites, respectively) and climate (Table 1) were similar. The only marked difference between the two sites was that the polluted site was 500 m downwind of a coking plant (Chapeltown, South Yorkshire, SK 365954; operating since 1929), while the other site was relatively unpolluted (Fitzwilliam, West Yorkshire, SE 166417). However, the pH, total C and N contents of the soils TABLE 1 CHEMICAL PROPERTIES OF SOILS AND CLIMATIC DATA (1979) FOR THE TWO SITES
Soil pH
Total soil C (?/o) Total soil N (?/o) Mean daily maximum temperature (°C) Mean daily minimum temperature (°C) Mean annual rainfall (mm)
Mean number of days on which rain fell Number of days with snow lying on the ground
Polluted site
Unpolluted site
3.3 14.6 0.93 12.7 6.2 830 181 23
4.9 9.8 0.82 12.9 6.3 805 173 27
differed considerably. When soil samples were analysed along a transect away from the pollution source the soil properties eventually approximated to those found at the unpolluted site, showing that differences in the pH and chemical properties of the polluted soils resulted directly from exposure to the pollution load. Sycamore leaves at the polluted site were covered in a thin layer of atmospheric pollution deposits (APD) whilst those from the unpolluted site were largely uncontaminated.
Litter decomposition The rate of litter decomposition over a one-year period at the sites was determined using litter bags (Edwards & Heath, 1963). Freshly fallen sycamore leaves were cut into 2.5 cm diameter discs and fifty discs were placed into heat-sealed nylon bags (10 × 7cm) of varying mesh size. A 5/z mesh was used to exclude all but microorganisms, 0.5 mm mesh to allow for access to mites, springtails, enchytraeids and most other soil arthropods as well as micro-organisms and a 1 cm mesh to enable the entry of most soil organisms, including earthworms. Bags (72 for each mesh size) were buried below the sycamore canopy to a depth of 2.5 cm and the original litter layer replaced. Every month, six bags of each mesh size were removed and the area of leaf-discs decomposed measured. Discs cut from
CELLULOSE DECOMPOSITION IN POLLUTED SOIL
81
APD-contaminated leaves obtained from the polluted site, as well as discs cut from uncontaminated leaves (two bags per mesh size per leaf type), were used. Discs where APD had been brushed from the leaves (two bags per mesh size per month) were also included. All experiments were performed at both sites.
Determination of the rate of cellulose decomposition The rate of cotton cellulose powder (Whatman C F l l ) decomposition over 7 months was determined by packing cellulose-amended soils (2 ~ w/w) from both sites into open-ended plastic tubes (10 cm length x 6.5 cm diameter, twenty-one tubes per soil type). The tubes also served as soil corers and were buried flush with the soil surface beneath sycamore canopies. Control tubes containing unamended soil were also included. The indigenous litter layer was replaced and three tubes (plus a control) for each soil type removed every month to determine the amount of added cellulose remaining (total cellulose minus native cellulose). Cellulose content of the soil was determined using the method of Updegraff (1969).
Enumeration of soil arthropods Freshly fallen sycamore leaves (2.5g) were placed in l cm mesh nylon nets (15 cm x 15 cm, twent~,-eight nets per site) and distributed in the uppermost litter layer beneath the canopy from which they originated. The nets were staked into position and removed from the field after six months. The arthropod population of the litter was then determined using a modified Berlese apparatus (Kiihnelt, 1961). RESULTS AND DISCUSSION
The following conclusions regarding the effects of atmospheric pollution from a coking plant on litter decomposition can be drawn from the data shown in Figs I and 2: (1) For any one litter type at either site, decomposition was greatest in the 1 cm mesh litter bags, followed by that found in the 0.5 mm mesh bags, whilst negligible decomposition was found in the 5/~m mesh bags. (2) Both APD contaminated and uncontaminated litter decomposed faster at the unpolluted than at the polluted site. (3) Lowest rates of decomposition were found when APD contaminated litter was exposed at the polluted site, whilst, conversely, highest rates were found when uncontaminated litter was left at the unpolluted site. (4) Rates of decomposition of uncontaminated litter left at the polluted site were similar to those found when contaminated litter was left at the unpolluted site. Two approaches to litter bag experiments appear in the literature. In the first, measurements of the loss in weight of air dried leaf discs over a time period are determined, whilst, in the second approach, loss of leaf area during decomposition is
82
K. KILLHAM, M. WAINWRIGHT
80 ' ~ , Q 7o
g b
•S
0
Iq
D
j
Ie
M
A
M
j
j
Months
Fig. 1. Litter decomposition over a one-year period in 1 cm mesh bags. © - - © uncontaminated leaves left at unpolluted site; A - - / k , contaminated leaves left at unpolluted site; 0 - - 0 , uncontaminated leaves left at polluted site; [I--I-q, contaminated leaves left at polluted site. Significantly different from control ( O - C ) ) * * p = 0.05; * p = 0.1.
,oo o
\ 5O.
5O
A
S
6
N
b
J
i= I~
A
I~
j
j
Months Fig. 2. Litter decomposition over a one-year period in 0-5 mm mesh bags. O - - O , uncontaminated leaves left at unpolluted site; A - - A , contaminated leaves left at unpolluted site; Q - - Q , uncontaminated leaves left at polluted site; I-q--[:], contaminated leaves left at polluted site. Significantly different from control ( O - O) ** P = 0.05; * p = 0. I.
CELLULOSE DECOMPOSITION IN POLLUTED SOIL
83
the criterion used. The last approach has been criticised (Satchell, 1974) because decomposition, particularly when caused by micro-organisms, can occur without a perceptible loss in leaf area. We verified this criticism to some extent in the present work since, whilst loss of leaf area in the 5/~m mesh bags was never greater than 2 ~o, a 6 ~ loss in leaf weight occurred over a one-year period. The presence of APD on polluted leaves, and its possible loss in situ or during air drying, precluded the use of decomposition rates based on loss of dry weight in the experiments described here. It should be borne in mind, however, that the method used to determine litter decomposition rates at the two sites will tend to emphasise the role of the soil fauna at the expense of that played by micro-organisms in the decomposition process. When contaminated litter was exposed (1 cm mesh bags for one year at the polluted site), a 35 ~ reduction in litter decomposition was seen compared with that when uncontaminated litter was left at the unpolluted site. From the data shown in Fig. 1 it can be calculated that non-particulate pollution (acid rain plus gases (NP)) accounts for 57 ~ of this reduction in litter decomposition whilst particulates (APD) account for the remaining 43 ~o. NP = Duu - Dup: 98 - 78: 2 0 ~ (i.e. 5 7 ~ of total reduction) APD = Dup - Dcp = 78 - 63 = 15 ~ (i.e. 43 ~ of total reduction) NP + APD = 20 + 15 = 35 ~ total reduction where Duu = ~ decomposition of uncontaminated litter at the unpolluted site. Dup = ~ decomposition of uncontaminated litter at the polluted site. Dcp = ~o decomposition of contaminated litter at the polluted site. The influence of the pollution deposits was further illustrated by the fact that removal (by brushing) of APD from the leaf surface resulted in decomposition rates similar to those found for uncontaminated litter at both sites (Fig. 3). Although the degradation of cotton cellulose was inhibited in the polluted soil, the effect was not marked (Fig. 4), showing that cellulose degrading micro-organisms are only marginally affected by air pollution from the coking plant. This agrees with a previous finding (Wainwright, 1980b) that microbial activity is not markedly impaired when deciduous woodland soil is subjected to air pollution from a coking plant over a period of one year. A preliminary study of the arthropods present in polluted and unpolluted litter showed a reduction in numbers and diversity in the former (Table 2). While some of these are not herbivorous (Kiihnelt, 1961), the reduction in arthropod numbers by atmospheric pollution is likely to account in large part for the observed reduction in litter decomposition found at this site. The atmospheric pollution deposits were found to be rich in phenols (Wainwright, 1979) and naphtha compounds, and have a high sulphur content (Killham & Wainwright, 1981) and as a result may prove directly toxic to soil animals. On the other hand, they may retard litter
84
K. KILLHAM, M. WAINWRIGHT
100 90
05ram mesh
0.5~n mesh
-~ 80
/// /-/I
//j
///
i
Icm mesh
/ / / / z /
~__ - ~ -
1cm mesh
I [i
f / /
///
2O
/// f// f// ///
"-
/
Unpolluted site
/
/
-
-
Polluted site
Fig. 3. Effect of removal of atmospheric pollution deposits from the leaf surface by brushing, on litter decomposition over a one-year period, cross-hatched columns: uncontaminated leaves; open columns, brushed contaminated leaves; hatched columns, contaminated leaves.
12,
0
A
S
6
N
O
J
~
M
Months
Fig. 4. Effect of atmospheric pollution on cellulose degradation over 7 months in soil. O - - O , uncontaminated soil left at unpolluted site; A - - A , contaminated soil left at unpolluted site; 0 - - 0 , uncontaminated soil left at polluted site; l--I--I--l, contaminated soil left at polluted site. Significantly different from control ( O - O ) ** p = 0.05; * p = 0-1.
CELLULOSE DECOMPOSITION IN POLLUTED SOIL
85
TABLE 2 MEAN NUMBER OF SOIL ARTHROPODS OBTAINED FROM SYCAMORE LITTER (10 g) AT POLLUTED AND UNPOLLUTED SI'FES
Chiiopoda Acari Collembola Araneae
Polluted site
Unpolluted site
1 _+0.3 3 _+0.5 3 +_0.4 4 -t- 1.5
12 _ 2-5 9 +_2-0 16 _+2.0 5 _ i.3
Means of seven replicates (_+ SD), each replicate bulked from litter (2.5 g) exposed in four litter bags (1 cm) on the soil surface at the two sites. d e c o m p o s i t i o n by reducing the p a l a t a b i l i t y of leaf litter to soil f a u n a , either directly or by killing p h y l l o p l a n e m i c r o - o r g a n i s m s which m a y act as a s u b s t a n t i a l n u t r i e n t source for litter animals. This last possibility however, appears remote as s c a n n i n g electron microscope studies of A P D c o n t a m i n a t e d leaves showed the presence of a diverse a r r a y o f p h y l l o p l a n e m i c r o - o r g a n i s m s . T o date most studies o n the effects o f atmospheric p o l l u t i o n o n litter d e c o m p o s i t i o n have ignored the c o n t r i b u t i o n m a d e by particulates to i m p a i r m e n t of the process. T h e results of this study show that A P D has a n effect o n litter d e g r a d a t i o n which is similar to that p r o d u c e d by n o n - p a r t i c u l a t e p o l l u t i o n (acid rain plus gases). The effect of A P D o n n u t r i e n t cycling in w o o d l a n d ecosystems is, however, likely to be more limited t h a n t h a t of acid rain, b o t h in terms of the n u m b e r of effluents c o n t a i n i n g these p o l l u t a n t s a n d in the extent to which they are distributed f r o m the p o l l u t a n t source. REFERENCES
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