Using Collembola to assess the risks of applying metal-rich sewage
sludge to agricultural land in western Scotland
Lorna J. Cole
∗, David I. McCracken, Garth N. Foster, Mark N. Aitken
Conservation and Ecology Department, Environment Division, SAC Auchincruive, Ayr, KA6 5HW, Scotland UKReceived 6 December 1999; received in revised form 5 April 2000; accepted 19 April 2000
Abstract
The recent United Kingdom ban on the disposal of sewage sludge at sea has led to the prediction that land application of sludge will become more widespread. The positive aspect of recycling nutrients may, however, be offset by the risk of contamination by heavy metals that are frequently present in sludge. The environmental impact of applying metal-rich sewage sludge to agricultural land was, therefore, assessed using Collembola. A combination of pitfall trapping and suction sampling was used to monitor epigeal/hemiedaphic Collembola on a small plot field trial in the west of Scotland. Four sludge treatments were investigated: cadmium-rich sludge, zinc-rich sludge, uncontaminated sludge and a no-sludge control. It was found that the abundance of Lepidocyrtus cyaneus Tullberg and Isotoma viridis Bourlet was significantly lower in plots receiving cadmium-rich sludge than those receiving uncontaminated sludge. Isotoma anglicana Lubbock was not influenced by the presence of metals in sludge and Isotomurus palustris (Müller) was actually favoured by the application of metal-rich sludge. Other aspects of collembolan ecology, and the efficiency of the two sampling methods, are also discussed. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Ecotoxicology; Sewage sludge; Heavy metals; Collembola; Agriculture
1. Introduction
Recent estimates suggest that, in the United King-dom, 500,000 t of sewage sludge are applied annually to agricultural land (Smith, 1996). However, the Com-mission of the European Communities (CEC) recent ban on the disposal of sewage sludge at sea (Commis-sion of the European Communities, 1991) has led to the prediction that the recycling of sludge to agricul-tural land will become more widespread throughout Europe (Rund, 1995). Probably, the principal envi-ronmental risk posed by fertilising agricultural land
∗Corresponding author. Tel.:+44-1292-525-291;
fax:+44-1292-525-333.
E-mail address: l.cole@au.sac.ac.uk (L.J. Cole).
with sludge lies in the fact that it often contains sig-nificant quantities of heavy metals such as cadmium, zinc, lead and copper (Petruzzelli et al., 1994). These elements, collectively known as potentially toxic el-ements (PTEs), are persistent and will accumulate in the upper layers of the soil where they can reach levels that are potentially toxic to plants and animals (van Straalen et al., 1987).
The CEC’s 1986 Directive on The protection of
the environment, and in particular the soil, when sewage sludge is used in agricultural land addresses
the problem of PTEs in sewage sludge (Commission of the European Communities, 1986). This Directive controls the amount of heavy metals entering the en-vironment through sludge application to agricultural land by setting maximum metal levels for sludge
and soil. However, despite the ample information on how PTEs in sludge affect micro-organisms (Brookes and McGrath, 1984; Smith, 1996), there is com-paratively little information on how they influence invertebrates. Risk assessment studies tend to focus on economically important pests (e.g. Hemiptera: Aphidae), polyphagous predator groups (e.g. Aranae and Coleoptera) and macro-decomposers (e.g. earth-worms). For example, earthworms and spiders col-lected from plots treated with contaminated sludge were shown to have elevated metal concentrations indicating that the metals were in bio-available forms (Benninger-Truax and Taylor, 1993). Green peach aphids (Appelia schwartzi Börner) feeding on plants grown on contaminated sludge were found to have decreased fecundity and longevity than those feeding on plants grown on uncontaminated sludge (Pimentel and Warneke, 1989). The metals in sludge are, there-fore, biologically available and have the potential to reduce fitness.
When a pollutant is added to agricultural land, changes in the structure of the microbial community tend to precede changes in the composition of the plant or invertebrate community. Groups that feed di-rectly on soil fungi and bacteria (e.g. Collembola and Acarina) may, therefore, be more susceptible to metal pollution than predatory or phytophagous groups which are further removed from the effects of the pollutant. Previous field studies have indicated that although the addition of metal contaminated sludge did not adversely affect the abundance of Collem-bola, it influenced the community structure (Lübben, 1989; Bruce et al., 1999). Lübben (1989) found that
Sminthurinus aureus (Lubbock), Willemia intermedia
Mills and Isotoma notabilis Schäffer were sensitive to sewage sludge artificially contaminated with metal salts (zinc, cadmium, copper, nickel, chromium and lead), while Folsomia candida (Willem) and
Mesapho-rura spp. were not. Collembolan communities would
therefore appear to be sensitive to metal-contaminated sludge with species-specific differences in metal sen-sitivity occurring.
Collembola are of agronomic importance not only as a consequence of their regulatory role in decompo-sition (Seastedt, 1984), but also because they are an important source of prey for polyphagous predators (Hopkin, 1997). As Collembola live in close associa-tion with the soil micro-flora and fauna, it is likely that
they will give an earlier indication of ecosystem dis-turbance than predatory groups. Furthermore, Collem-bola are better suited than larger more mobile inverte-brates (e.g. Coleoptera) in small plot agricultural field trials.
In this study, the risks of applying metal-rich sewage sludge to agricultural land were assessed in the field using epigeal Collembola as indicators. A sludge rich in zinc, which is an essential micro-nutrient, and a sludge rich in cadmium, which is an ecological impu-rity, were investigated on a small-plot field trial. The metal-rich sludges were derived from sewage treat-ment works with naturally high inputs of each specific metal. It was considered essential to use naturally con-taminated sludge since this provided a more appro-priate measure of exposure (and, hence, risk) under field conditions. The use of sludges artificially con-taminated with metal salts was discounted as metals are more available in such sludges and they therefore do not represent a natural situation (Smith, 1996). Al-though the use of naturally contaminated sludge meant that the sludges differed in other respects (see Table 1), these were taken into account in the interpretation of the results.
2. Materials and methods
2.1. Study site
The trial site was situated on a well-drained sandy clay loam topsoil overlying a sandy loam subsoil (Eu-ric Cambisol) at SAC Auchincruive, west of Scotland. The topsoil contained 210 g kg−1 clay and 26 g kg−1 organic carbon. The research was conducted as part of a larger scale experiment instigated in 1994. The larger experiment consisted of 23 sludge treatments established across three blocks in a randomised block design (i.e. a total of 69 plots each 6 m×8 m). Epigeal Collembola were monitored on four of these sludge treatments (i.e. a total of 12 plots): no sludge control, uncontaminated sludge, cadmium-rich sludge and zinc-rich sludge (Table 2). The metal-rich sludges were derived from sewage treatment works with nat-urally high inputs of the specific metal.
Table 2
The four sludge treatments studied at SAC Auchincruive, Scotland, and the maximum metal concentrations set by current CEC legislation (Commission of the European Communities, 1986)a
Treatment Mean soil concentration Mean sludge concentration
Control 85.5 mg Zn kg−1soil –
<0.33 mg Cd kg−1soil
Uncontaminated sludge 94.8 mg Zn kg−1soil 725 mg Zn kg−1 sludge
0.33 mg Cd kg−1 soil 1.94 mg Cd kg−1 sludge
Zinc-rich sludge 270 mg Zn kg−1soil 6619 mg Zn kg−1 sludge
– 17.2 mg Cd kg−1 sludge
Cadmium-rich sludge 2.2 mg Cd kg−1 soil 48.9 mg Cd kg−1 sludge
– 1244 mg Zn kg−1 sludge
CEC maximum concentrations 300 mg Zn kg−1soil 4000 mg Zn kg−1 sludge 3 mg Cd kg−1 soil 40 mg Cd kg−1 sludge
aMean cadmium and zinc concentrations in sludge and soil at SAC Auchincruive in 1996 and maximum soil and sludge concentrations set by CEC Directive 86/278/EEC (Commission of the European Communities, 1986).
sludge was incorporated into the soil using small plot equipment and then the plots were sown with Ital-ian ryegrass (Lolium multiflorum Lam.) which was cut twice each year. A 1.2-m permanent grass strip surrounded each plot to prevent soil transfer between plots during cultivation. The input of organic matter to the soil was standardised across sludge treatments, and sludge application did not alter soil pH. During 1996, the grass was cut on 24 May and 26 July (i.e. prior to the May and August Collembola sampling dates: see below).
2.2. Sampling techniques
Sampling was conducted between April and August 1996. A combination of pitfall trapping and suction sampling was utilised in order to provide an effec-tive sampling regime for epigeal Collembola (Berbiers et al., 1989; Frampton, 1994).
Dietrick Vacuum insect nets (D-Vac) have previ-ously been used to collect Collembola by suction (Purvis and Curry, 1978; Frampton, 1988), but can become heavy and uncomfortable with prolonged use. Modified Ryobi RSV3100E sweeper-vac, have been found to be as, or more, efficient at sampling Carabidae, Araneae, Staphlinidae, Isopoda and Aphi-didae (Harwood, 1994; MacLeod et al., 1994). This technique was, therefore, adopted in the current study as a light-weight alternative to sample Collembola. Following the process developed by MacLeod et al. (1994), fine weave (<0.5 mm) nylon/cotton mix voile
bags were placed in the sampler nozzle and renewed
after each sample. Suction samples were taken on 30 April, 31 May, 22 June and 4 August. For each suction sample, an area of 706 cm2 was partitioned off and extracted for 60 s. On all sampling dates, five such samples were taken randomly from each of the 12 plots under investigation. To standardise across dates, suction samples were taken on sunny days between 10:00–17:00 h when the grass was dry.
Pitfall traps consisted of plastic beakers (150 ml volume, diameter 6 cm) containing 20 ml of the preser-vative ethylene glycol. Five pitfalls were inserted in each plot on 24 April, 27 May, 20 June and 1 Au-gust, and these were retained in situ for seven days. To avoid the ‘digging-in’ effect (Joosse and Kapteijn, 1968), the pitfall traps in May, June and August were inserted into the same holes as the April pitfalls.
Both the suction samples and pitfall traps were pro-cessed by first separating the organic and inorganic matter by repeated flotation in a saturated salt solution. The organic matter (including Collembola) was then passed through two graded sieves (15 mm and 45mm)
to separate the Collembola from the larger arthropods and plant debris.
‘splitter’, separating Collembola into species on the basis of very small differences in morphology (Hop-kin, 1997), the information derived from this study was maximised.
2.3. Statistical analysis
The five samples collected from each plot on a sam-pling date were pooled to obtain an indication of over-all collembolan community structure for each plot on each specific sampling date. The pooled data were used in statistical analyses and the pitfall and suction data sets were investigated separately.
Prior to analyses, the variance to mean ratios were calculated to determine if species abundances were normally distributed. For the majority of species it was found that the variance and means were about equal indicating that the distribution was random (Sokal and Rohlf, 1995). Square-root transformation was, there-fore, more appropriate and effective at normalising the data than the more common log transformation. It was deemed inappropriate to use different transforma-tions for different species, and consequently the abun-dance of all species was square-root transformed. It was, however, recognised that even after transforma-tion the variances for some species were still hetero-geneous and, consequently, the significance value was
Table 3
Results of ANOVAs and Tukey tests performed on Collembola species obtained by suction sampling at SAC Auchincruivea
Species Date Treatment
F-value Probability Location of difference F-value Probability Location of difference
Total 20.04 <0.001 May and Jun>Apr, May>Aug 5.17 <0.01 U>C
Isotoma viridis 20.91 <0.001 Aug>All 16.75 <0.001 All>Cd
Isotoma notabilis 7.29 <0.001 May>Jun 2.12 N.S. –
Isotoma anglicana 129.75 <0.001 May>Apr and Aug>Jun 24.14 <0.001 All>C Isotomurus palustris 86.70 <0.001 May>Jun>Aug and Apr 12.51 <0.001 Zn and Cd>C Isotomurus maculatus 24.80 <0.001 May>Apr and Jun>Aug 4.76 <0.01 U>C Isotoma/Isotomurus juvenile 56.36 <0.001 May>Jun and Apr>Aug 2.66 N.S. – Lepidocyrtus cyaneus 5.01 <0.01 Aug>Apr 15.77 <0.001 All>Cd Heteromurus nitidis 8.12 <0.001 Aug>All 8.12 <0.001 U>C Tomocerus longicornis 13.34 <0.001 Aug>Apr and May, Jun>Apr 1.34 N.S. –
Sminthurus viridis 4.60 <0.01 Jun>Apr 23.15 <0.001 C>All Cd>U and Zn Sminthurinus aureus 27.22 <0.001 May and Jun>Apr>Aug 2.48 N.S. –
Sminthurides malmgreni 13.91 <0.001 Apr>All 1.89 N.S. –
Ceratophysella denticulata 31.76 <0.001 Apr and May>Jun and Aug 6.08 <0.01 Zn>C
aAs no block effect was found for any of the species tested, this information is omitted from the table. The following codes are used for treatments: C, control; U, uncontaminated sludge; Zn, zinc-rich sludge; and Cd, cadmium-rich sludge.
set at 0.01 to avoid wrongly rejecting the null hypoth-esis (Day and Quinn, 1989).
Effects of date, treatment and block were tested for using three-way analyses of variance (without replication). Examination of the data prior to analysis indicated that interaction did not occur and it was, therefore, possible to perform the tests without repli-cation hence enabling block effects to be investigated. For each dataset, separate ANOVAs were performed on the abundance of total Collembola and on fre-quently occurring species (i.e. those occurring in over 50% of samples). Tukey multiple comparison tests were then applied to locate significant differences identified by ANOVA.
3. Results
Table 4
Results of ANOVAs and Tukey tests performed on Collembola species obtained from pitfall trapping at SAC Auchincruivea
Species Date Treatment
F-value Probability Location of difference F-value Probability Location of difference
Total 7.58 <0.001 All>Apr 6.14 <0.01 U>Cd
Isotoma viridis 12.14 <0.001 Aug>All 8.97 <0.001 U>Cd
Isotoma notabilis 7.85 <0.001 Apr>All 4.51 <0.01 U>C
Isotoma anglicana 62.93 <0.001 May>Jun and Aug>Apr 3.47 N.S. – Isotomurus palustris 25.28 <0.001 All>Apr 6.32 <0.01 All>C Isotomurus maculatus 17.87 <0.001 May and Apr>Aug, May>Jun 8.76 <0.001 U>C and Cd Isotoma/Isotomurus juvenile 31.12 <0.001 All>Aug, May>Apr 12.61 <0.001 All>C Lepidocyrtus cyaneus 5.83 <0.01 Aug>Apr and May 10.60 <0.001 C and U>Cd Heteromurus nitidis 7.65 <0.001 Aug>Apr and May 4.21 0.01 U>C
Sminthurus viridis. 0.39 N.S. – 3.99 N.S. –
Sminthurinus aureus 23.72 <0.001 May and Jun>Apr, Jun>Aug 1.39 N.S. –
Sminthurinus elegans 0.97 N.S. – 1.40 N.S. –
Sminthurides pumilis 17.95 <0.001 August>All 0.78 N.S. –
Ceratophysella denticulata 1.21 N.S. – 4.56 <0.01 Zn>Cd
aAs no block effect was found for any of the species tested, this information is omitted from the table. The following codes are used for treatments: C, control; U, uncontaminated sludge; Zn, zinc-rich sludge; and Cd, cadmium-rich sludge.
to merit separate statistical analysis: F. candida, F.
fimetarioides (Axelson), I. tigrina (Nicoleti), Entomo-brya nivalis (Linné), Lepidocyrtus lignorum
(Fabrici-cus), Pseudosinella decipiens Denis, Tomocerus minor (Lubbock), Dicyrtoma saundersi (Lubbock),
Dicyr-toma ornata (Nicolet) and Anurida pygmea (Börner).
Results of the ANOVAs for the suction data are presented in Table 3 and for the pitfall data in Table 4. While no block effect was found for any of the species investigated, effects of treatment and date were apparent.
3.1. Seasonal variation indicated by suction sampling
The influence of season on collembolan abun-dance was species-specific. The abunabun-dance of total Collembola, Isotomurus/Isotoma juveniles, Isoto-murus palustris (Müller) and IsotoIsoto-murus maculatus Schaeffer increased from April to May, reaching
Table 5
Mean grass height (in cm) in experimental plots for each treatment and sampling date showing standard deviations in brackets
Control Uncontaminated Zinc-rich Cadmium-rich
April 13.0 (±2.02) 31.5 (±3.76) 30.7 (±3.32) 27.1 (±3.54)
May 10.9 (±2.74) 10.1 (±2.91) 10.1 (±2.05) 9.2 (±2.19)
June 16.7 (±3.18) 43.0 (±8.06) 43.7 (±4.10) 39.2 (±5.14)
August 9.1 (±1.61) 7.4 (±1.77) 7.7 (±2.67) 8.5 (±2.87)
their peak in May suction samples, before decreasing again in August (Table 3). Tomocerus longicornis,
Lepidocyrtus cyaneus (Tullberg), Heteromurus ni-tidis (Templeton) and I. viridis (Bourlet) increased
as the summer progressed, occurring in their high-est abundance in August suction samples (Table 3).
Ceratophysella denticulata (Bagnall) was more
abun-dant in April and May than in June or August and
Sminthurides malmgreni (Tullberg) occurred almost
exclusively in April samples (Table 3). I. anglicana occurred in its lowest abundance in June when the grass was longest, while Sminthurus viridis (Linné) occurred in its highest (Tables 3 and 5).
3.2. Seasonal variation indicated by pitfall trapping
Isotoma viridis, Sminthurides pumilis (Krausbauer), H. nitidis and L. cyaneus occurred in their highest
Fig. 1. Effect of sludge treatment on the abundance of total Collembola, Heteromurus nitidis and Isotomurus maculatus caught by suction sampling (showing mean number square-root transformed). Error bars show standard deviations.
exception of S. pumilis, which did not occur in suction samples in sufficient numbers for statistical analysis to be applied, these species were also more abundant in August suction samples. In agreement with the suction sample results, Isotoma/Isotomurus juveniles peaked in May pitfalls (Table 4). Isotoma notabilis was significantly more abundant in April pitfalls than on any other sampling date (Table 4). As the higher abundance in April pitfalls was not replicated in suc-tion samples, it is possible that it was a consequence of an increase in activity due to the digging in effect (Joosse and Kapteijn, 1968).
3.3. Treatment effects indicated by suction sampling
The addition of uncontaminated sludge significantly increased the abundance of H. nitidis, I. maculatus and total Collembola above that of the control plots (Table 3 and Fig. 1). This increase was not significant in plots receiving contaminated sludge (i.e. zinc and cadmium-rich sludge), hence, indicating adverse ef-fects of the metal-rich sludges. Cadmium-rich sludge was clearly shown to adversely affect L. cyaneus and
I. viridis and both species occurred in their lowest
abundance in plots receiving this sludge (Table 3 and Fig. 2). Isotomurus palustris was significantly more abundant in plots receiving contaminated sludge (but
not uncontaminated sludge) than control plots (Table 3 and Fig. 3), while C. denticulata was significantly more abundant in plots receiving zinc-rich sludge than control plots (Table 3 and Fig. 4). Isotoma anglicana was favoured in all plots receiving sludge irrespective of metal contamination (Table 3 and Fig. 3).
Sminthurus viridis was the only species to occur in
its highest abundance in control plots, and this species was also significantly more abundant in plots receiv-ing cadmium-rich sludge than those receivreceiv-ing uncon-taminated or zinc-rich sludge (Table 3 and Fig. 4).
3.4. Effects of treatment indicated by pitfall trapping
Pitfall trapping also indicated that cadmium-rich sludge adversely affected I. viridis and L. cyaneus and a significantly lower number of both species occurred in pitfalls set in plots receiving cadmium-rich sludge than those in plots receiving uncontaminated sludge (Table 4 and Fig. 5). Furthermore, the abundance of L.
cyaneus in plots receiving cadmium-rich sludge was
significantly lower than that of control plots.
Pitfall trapping indicated that numbers of I.
nota-bilis, I. maculatus and H. nitidis were significantly
Fig. 2. Effect of sludge treatment on the abundance of Isotoma viridis and Lepidocyrtus cyaneus caught by suction sampling (showing mean number square-root transformed). Error bars show standard deviations.
the total collembolan abundance and the abundance of I. maculatus was significantly lower in pitfalls es-tablished in plots receiving cadmium-rich sludge than in those receiving uncontaminated sludge (Table 4, Figs. 5 and 6). This difference was not significant in the suction sampling results (Table 3).
Isotoma/Isotomurus juveniles and I. palustris had
a significantly higher abundance in pitfalls placed in plots receiving sludge than those placed in control plots (Table 4 and Fig. 7). In agreement with the suction results, C. denticulata occurred in its
high-Fig. 3. Effect of sludge treatment on the abundance of Isotoma anglicana and Isotomurus palustris caught by suction sampling (showing mean number square-root transformed). Error bars show standard deviations.
est abundance in pitfalls established in plots receiving zinc-rich sludge (Table 4 and Fig. 7).
4. Discussion
4.1. Seasonal variation in epigeal Collembola
Fig. 4. Effect of sludge treatment on the abundance of Sminthurus viridis and Ceratophysella denticulata caught by suction sampling (showing mean number square-root transformed). Error bars show standard deviations.
their maximum population density in April (e.g. S.
malmgreni), others in May (e.g. I. anglicana and Iso-toma/Isotomurus juveniles) and others in August (e.g. H. nitidis, I. viridis and L. cyaneus). Joosse (1969)
suggested that seasonal differences in species abun-dance could be related to humidity. This hypothesis was not supported for all species, and while some hy-drophilic species (e.g. S. malmgreni) peaked in April as predicted, others (i.e. I. viridis and T.
longicor-Fig. 5. Effect of sludge treatment on the abundance of total Collembola, Isotoma viridis and Lepidocyrtus cyaneus caught by pitfall trapping (showing mean number square-root transformed). Error bars show standard deviations.
nis) peaked in August (Joosse, 1970; Fjellberg, 1980).
It would therefore appear that species-specific differ-ences in seasonal abundance could not simply be ex-plained by their humidity preference.
4.2. Effect of treatment on epigeal Collembola
Fig. 6. Effect of sludge treatment on the abundance of Heteromurus nitidis, Isotoma notabilis and Isotomurus maculatus caught by pitfall trapping (showing mean number square-root transformed). Error bars show standard deviations.
to be primarily the result of an increase in food sup-ply (Lübben, 1989; Pimentel and Warneke, 1989). In this study, a higher abundance of total Collembola, H.
nitidis and I. notabilis was found in plots receiving
uncontaminated sludge (but not those receiving con-taminated sludge) when compared to control plots. It
Fig. 7. Effect of sludge treatment on the abundance of Isotomurus palustris, Isotoma/Isotomurus juveniles and Ceratophysella denticulata caught by pitfall trapping (showing mean number square-root transformed). Error bars show standard deviations.
plots receiving uncontaminated sludge, thus indicating that these species were particularly sensitive to the cadmium-rich sludge.
It is important to note that the efficiency of several sampling methods has been found to decrease in long grass (Greenslade, 1964; Harwood, 1994). In studies where treatment influences grass height, the possibility that treatment effects may be attributed to changes in sampling efficiency must be considered. In this study, the lower abundance of L. cyaneus and I. viridis in plots receiving cadmium-rich sludge could not simply be attributed to a reduced sampling efficiency in longer grass, as grass height was actually lower in plots re-ceiving cadmium-rich sludge (Table 5). It is therefore likely that the observed decrease in these species was a true effect of treatment. Furthermore, the observed changes in abundance found by suction sampling were largely mirrored in the pitfall data, further suggesting that grass height did not unduly influence sampling ef-ficiency. As additional differences between the sludges existed, we cannot however conclude that it was the high levels of cadmium in the cadmium-rich sludge that adversely influenced these species (see Table 1).
Toxicity studies conducted in the laboratory provide a useful indication as to what levels of metals could potentially effect animals in the field. Smit and van Gestel (1996) found an EC50 value of 185 mg kg−1
soil for the effect of zinc on the population size of
F. candida, while Crommenruijn et al. (1993) found
an EC50-reproduction value in F. candida of 227 mg
Cd kg−1soil. From Table 2, it can be seen that while the soil concentration of zinc in the study site was higher than that shown to effect F. candida in the lab-oratory, the concentration of cadmium was not. Lab-oratory studies, however, only investigate the direct effects of a contaminant (i.e. effects on growth rate, reproduction and survival) and it is possible that the indirect effects of cadmium were more pronounced than those of zinc. Cadmium may indirectly affect Collembola through altering their habitat structure and food (Posthuma et al., 1993). It is possible that the lower grass height in the cadmium-rich plots re-sulted in a more variable, and hence less favourable, microclimate (Pimentel and Warneke, 1989). Further-more, species that are able to digest fungal cell walls, where heavy metals are frequently accumulated, are exposed to metal concentrations far in excess of those found in the soil (Siepel, 1994).
The fact that total Collembola and I. maculatus appeared to be adversely affected by the application of cadmium-rich sludge when sampled by pitfall trapping, but not when sampled by suction sampling, may indicate sublethal effects of cadmium on mobil-ity. The detoxification of heavy metals is thought to be energetically costly, and a decrease in glycogen reserves after metal exposure has been found for sev-eral invertebrate species (Richards and Ireland, 1978; Bodar et al., 1990; Reddy and Bhagyalakshmi, 1994). The presence of metals may, therefore, be expected to reduce activity by decreasing the energy available and isopods exposed to heavy metals have been shown to have reduced activity levels (Hopkin, 1989; Sørensen et al., 1997). Computer-automated video tracking could provide a quick and effective method of mon-itoring metal pollution in the laboratory (Sørensen et al., 1997) and the present results indicate that I.
maculata might be a suitable test species.
Sminthurus viridis was the only species to occur in
its highest abundance in control plots. The abundance of S. viridis was also higher in the plots receiving cadmium rich sludge than in the plots receiving un-contaminated or zinc-rich sludge. It is possible that this species was adversely affected by the higher grass yield in the zinc-rich and digested sludge plots, or was sampled less efficiently in long grass. How-ever, as S. viridis occurred in its highest abundance in June when the grass was longest, neither explanation is likely. It is therefore suggested that S. viridis, be-ing phytophagous, did not benefit from the increase in micro-organisms which accompanies sludge ap-plication. The application of sludge can also favour predatory carabid beetles and this may account for the lower abundance of S. viridis observed in uncon-taminated sludge plots (Larsen et al., 1996). As this species consumes living plant tissue, which generally has lower concentrations of heavy metals than fungi (Curry, 1994), it may actually have had a competitive advantage over mycophagous Collembola in plots receiving cadmium-rich sludge.
4.3. Sampling protocol
give poorer quantitative data (Greenslade, 1964). Pit-fall traps also tend to under-represent plant climbing species (e.g. S. viridis), which are well represented in suction samples, but appear to be more effective at catching strictly surface-active species (Bruce, 1997). In this study, suction sampling and pitfall trapping both yielded nine significant treatment effects. This indicates neither method was superior in assessing the ecological impact of sewage sludge. Effects for some species were only detected by suction sampling (e.g.
S. viridis and I. anglicana) while others were only
detected by pitfall trapping (e.g. Isotoma/Isotomurus juveniles and I. notabilis). In agreement with other au-thors, the most effective sampling regime for epigeal Collembola would, therefore, appear to be a combina-tion of both pitfall traps and succombina-tion samples (Berbiers et al., 1989; Frampton, 1994). Limiting resources may make such a choice unfeasible and when selecting between methods, the efficiency of each in collect-ing specific groups of species should be considered. For example, if a treatment predominantly effects plant-climbing species (e.g. some pesticides), suction sampling would be the preferred method as it is more efficient at sampling such species (Bruce, 1997).
5. Conclusion
The addition of uncontaminated sewage sludge increased the abundance of total Collembola, H.
ni-tidis and I. notabilis in comparison to control plots.
However, a similar increase was not observed in plots receiving cadmium and zinc-rich sludges, thus indicating adverse effects of contaminated sludge on Collembola. Cadmium-rich sludge also reduced the abundance of L. cyaneus and I. viridis and both species were found in their lowest abundance in plots receiving this sludge. The soil concentration of cad-mium in the experimental plots was below that found to adversely affect Collembola in laboratory studies, hence suggesting that cadmium was indirectly effect-ing these species. Alternatively, other toxic substances in the cadmium-rich sludge may have influenced these species. It is possible that these adverse ef-fects at species level (and the associated alteration of community structure) could alter the decomposition rate, soil fertility, and the abundance of polyphagous predators. Further research is therefore required to
as-certain whether any such wider environmental effects occur and ultimately to determine if current legisla-tion adequately protects the full range of invertebrates associated with agricultural land.
Acknowledgements
SAC receives financial support from the Scottish Executive Rural Affairs Department. This experiment is part of a larger project carried out in collaboration with ADAS, IACR, MLURI and WRc and we thank all collaborators for their assistance, particularly Dr Brian Chambers of ADAS. L.J. Cole was in receipt of a MacLagen Ph.D. scholarship while undertaking this work. We are indebted to Dr Arne Fjellberg for his in-valuable help in Collembola identification. Thanks are also due to Brian Laird and Mark Cole for their tech-nical assistance and to Dr Geoff Frampton for advice.
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