Earthworms in Soil Restoration: Lessons Learned from United Kingdom Case Studies of Land Reclamation
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Earthworms in Soil Restoration: Lessons Learnedfrom United Kingdom Case Studies of LandReclamation
Kevin R. Butt1,2
Restoration ecology requires theoretical consideration ofa habitats former structure and function before the practiceof ecological restoration is applied. However, experiencehas shown that this does not always occur and aspects suchas soil ecology have often been an afterthought. Here, casestudy material relates the use of earthworms at selectedsites in the United Kingdom. Due to their soil-forming ca-pabilities, these organisms may be essential to reconstruc-tion of soils when drastic activities have despoiled an area.
While describing in brief the type of work undertaken,these case studies seek to illustrate some of the misunder-standings/problems/deliberately negative acts that have toooften accompanied use of earthworms in soil restoration.From such experiences, implications for practice are sug-gested that should lead to a greater understanding andappropriate utilization of earthworms in future projects.
Key words: case studies, earthworms, landfill site, recla-mation, soil restoration.
For the past 20 years, the science of restoration ecologyhas grown from virtually nothing to an expanding area ofresearch and associated practice. Seminal publicationsthat brought together thinking of numerous experts in thefield (e.g., Jordan et al. 1987; Perrow & Davy 2002) arenow regarded as key texts in educational spheres with thenumber of research publications in this area continuing togrow rapidly (e.g., Ormerod 2003). Many research proj-ects sensibly focus on well-defined habitats, giving specificemphasis to the plants that are present and how theirestablishment and survival will lead to the desired restora-tion trajectory. However, during this process, much lessemphasis may be given to the soils in which these plantsare expected to grow. Habitat degradation may rapidlylead to localized (easily observed) faunal extinctionsfollowed by the gradual or perhaps rapid destruction ofthe flora. Often, though, soil-related problems are notaddressed, and in extreme cases, the soil may be deliber-ately removed. Restoration or even rehabilitation of a hab-itat thereafter becomes increasingly difficult.In many soils, earthworms are essential components of
the fauna. As detritivores, they are partially responsible forthe breakdown and recycling of dead organic matter. Thismay involve direct incorporation of vegetation such as leavesor by production of feces (casts) deposited at the soil surfaceand thereby assisting burial. The intimate mixing of soil by
horizontal burrowing (endogeic) species, such as Aporrecto-dea caliginosa and Allolobophora chlorotica, causes mineralcomponents and organic fragments to become closely associ-ated. The crumb structure of earthworm casts is unique andan ideal substrate for promoting plant growth due to a richassemblage of microorganisms and nutrients compared withthe contents of the surrounding soil. Passage of soil throughthe gut of an earthworm therefore adds to soil status andimproves soil quality (e.g., Edwards & Bohlen 1996).Other activities of some (deep burrowing) earthworms,
such as Lumbricus terrestris and Ap. longa, include the for-mation of vertical burrows. These provide channels thatallow circulation of air and also permit rainwater infiltra-tion, leading to reduced erosion through surface run-off.Earthworm and soilwater relationships are now thought tobe of some importance, particularly within agricultural andrestored sites (e.g., Shipitalo et al. 2004). The very presenceof earthworms may be a contributory factor in pedogenesis,and because they actively change and ameliorate their soilenvironment, they are now regarded as ecosystem engi-neers (Lavelle et al. 1997). With increased attention placedon the restoration of derelict and degraded land, there isa need to ensure that soil rehabilitation is achieved usingthe best practicable option but at acceptable cost. Thiscompromise may lead to use of subsoil, lacking organicmatter and a resident fauna. In extreme situations, such aslandfill caps, such material may also be deliberately com-pacted creating a particularly hostile environment fordevelopment of sustainable earthworm populations.Earthworms, rightly or not, have therefore often been tar-
geted as organisms to introduce (inoculate) into soils in theprocess of rehabilitation. The following seeks to explore suchpractices through examination of case study material drawn
1 School of Built and Natural Environment, University of Central Lancashire,Preston PR1 2HE, U.K.2Address correspondence to K. R. Butt; email email@example.com
2008 Society for Ecological Restoration Internationaldoi: 10.1111/j.1526-100X.2008.00483.x
DECEMBER 2008 Restoration Ecology Vol. 16, No. 4, pp. 637641 637
from four locations in the United Kingdom. These sites, atHillingdon (Marfleet 1985; Butt et al. 1993), Calvert (Buttet al. 1997, 2004), Stockley Park (Hallows 1993; Butt 1999),and Hallside (Craven 1995; Bain et al. 1999), have been thefocus of seven earthworm inoculation trials. Specific aspectsdrawn from these case studies (Table 1) where a variety ofinoculation techniques were used (Table 2) cannot fail todemonstrate some of their achievements and successes,which are already documented in the literature and reviewedby Butt (1999). However, a specific aim here is to criticallyassess each of the operations by reference to problems, mis-takes, or deliberate acts that failed to assist the specificallystated or inferred objectives. Thereafter, implications forpractice in reclamation schemes are examined.The case studies chosen all relate to sites in Britain,
simply because the author had a direct input into theirestablishment or was involved indirectly (usually throughmonitoring) at a later stage. Nevertheless, results have awider bearing, and the positive aspects demonstrated canbe transferred and the potential problems avoided in simi-lar reinstated soils across temperate systems worldwide.
Locations and Initial Soil Conditions
Case study sites are linked through poor initial site condi-tions but no more than might be anticipated on reclaimedindustrial sites. No major toxins were present within thesites, but all were deficient in good quality soils. Hilling-don had a subsoil topped with sewage sludge to provideadequate physical conditions for earthworms, whereasHallside has sewage sludge plowed into the raw, shale-richcolliery spoil substrate. The sewage sludge content mayinitially have been conducive to epigeic earthworms butcontained little mineral soil for geophagous, endogeic spe-cies such as Allolobophora chlorotica or deep-burrowinganecic animals such as Lumbricus terrestris.Due to the level of compaction (1.62 g/cm3) above an
active landfill, high clay content, and no addition of organicmatter, the subsoil medium at Calvert was not ideal forearthworm inoculation. However, of great value was theinitial support from site managers to assist experiments ofthis nature plus a reclaimed soil (in extremis) to trial a noveltechnique (earthworm inoculation unit [EIU]). By compari-son, the regraded, passive landfill at Stockley Park wasmuch more conducive to earthworms because a grass cover-ing was established for amenity sport (golf) over a thin layerof stony soil. All the sites had certain deficiencies/problemsbut with appropriate management could have supportedselected earthworms. Nevertheless, the species selected foruse were not always ideal for the given soil conditions.
Choice of Earthworm Species, Number, Technique,and Timing
The relative merits of major earthworm inoculation tech-niques are provided in Table 1. Examples of each have T
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been used in the case studies presented (Table 2) plusa variant at Stockley Park, where broadcast inoculationwas supplemented by earthworms being dug in to slotscreated in the turf of the receptor site. This cannot beregarded as a standard practice or one that will assistearthworm survival. Here, as on a golf course, the slotswere closed (trodden down) after earthworm insertionpotentially leading to immediate mortality. Timing of thisparticular operation (over winter months) may havenecessitated this practice to avoid deposited animals freez-ing on the soil surface.Use of broadcast inoculation at Hillingdon in 1984 was
then seen as best practice, but the method of collectionwas a potential problem from the outset. This was becausemass spraying of soil with formalin will have harmed the4,000 collected animals if not washed immediately. Likelyspecies used, determined by resampling the donor field,were Aporrectodea caliginosa, Ap. longa, and Lumbricusterrestris. All may not have been ideal candidates forthe site due to the high organic matter content. Litter-dwelling and other shallow-working species such as Redworm (L. rubellus) and Allolobophora chlorotica, respec-tively, would have been more suitable (but less easily col-lected). By contrast, the species used at Stockley Parkwere obtained commercially. They did not therefore sufferfrom any chemical exposure prior to use, but once again,the majority (commercially bred epigeic Eisenia veneta) ofthe 1.5 million inoculated (Hallows 1993) were not a suit-able choice. This was confirmed during monitoring in lateryears, when none of this species was located (Butt 1999).Three trials saw development of the EIU technique
(e.g., Butt et al. 1997). This mass rearing in plastic bags(Lee 1995) meant that species selection was possible, soappropriate species could be used as starter cultures.However, this in itself was part of the development pro-cess and species selection shifted in favor of Ap. longaand Al. chlorotica or a combination of both species overL. terrestris. The latter was a poor candidate species for
this site because it was less able to burrow into compactedclays than, e.g., Ap. longa (Kretzschmar 1991). However,site managers were initially keen to use L. terrestris con-trary to the judgment of the scientists involved.During development and trialing of this technique at
Calvert, the size of the unit was reduced (2 rather than 4L) for ease of handling and inoculation into site. To pre-vent hatchling escape, sealed units were used during the3-month cultivation phase. Netting was also pegged imme-diately above each EIU site following inoculation becausesite workers after the first trial reported that birds (corvidsand gulls) from the nearby active landfill area had shownsignificant interest in the EIUs and likely reduced earth-worm numbers through predation. In addition to monitor-ing the earthworms, their interaction with trees was alsoinvestigated in collaboration with the Forestry Commis-sion. Over a period of a decade, Alder (Alnus glutinosa)had a significant positive effect on overall earthworm den-sity, likely due to nitrogen additions. However, the earth-worm treatments had no significant effect on tree growth,most likely as a result of extremely hostile soil conditionswith low organic matter content and high compaction(Butt et al. 2004). The third trial at Calvert (March 2003)used Ap. caliginosa, Ap. longa, and Octolasion cyaneumfrom stock sources (e.g., Lowe & Butt 2005). This com-pared inoculation of earthworms into two adjacent 400-m2
plots, one of which had 40 tonnes of composted greenwaste (CGW) added as a surface dressing. At most recentsampling (3 years), the inoculated earthworm species wereall recorded from the CGW plot, but only very low num-bers of Ap. longa were found in the control plot.Site managers at Hallside (Scottish Greenbelt Company
[SGC]) considered that provision of earthworms throughuse of the EIU technique might be of value to assist wil-low (Salix sp.) and poplar (Populus sp.) short rotationcoppice production (Craven 1995). SGC therefore com-missioned 2,000 (3 L) EIUs following the design of Buttet al. (1997) but with a substrate similar to that spread on
Table 2. Relative merits of earthworm inoculation techniques (adapted from Butt et al. 1997).
Technique Advantages Disadvantages
Broadcast after chemical/physical extraction
High densities possible; species selectionpossible
Protective microenvironment absent; nococoon transfer; mainly deep-burrowing(anecic) worms; worms may be injuredduring extraction; laborious andexpensive; damage to collection site
Turf transfer Protective microenvironment; cocoonstransferred
Densities usually low; little control overspecies/numbers; mainly surface-dwelling (epigeic and endogeic)worms; cutting machines/laborrequired; damage to collection site
EIU Protective microenvironment; cocoons andall life stages present; high densitiespossible; control over numbers; speciesselection/combinations possible; nocollection site to be damaged
Laborious and expensive
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site (colliery spoil and sewage sludge). The earthwormstarter culture comprised 8,000 L. terrestris (obtained froma commercial supplier), and EIUs were inoculated intosite by a commercial labor force. However, as at Calvert,this species was not suitable to the given soil conditions,and none were recorded during monitoring for the follow-ing 2 years (Bain et al. 1999).
At Hillingdon, the site itself was not stable, and signs in1990 revealed methane seepage that had reduced earth-worm numbers to zero in places, such that no organic mat-ter incorporation had occurred. However, monitoring ofthis site was not continuous, so it was difficult to draw firmc...