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
Earthworms in Soil Restoration
638 Restoration Ecology DECEMBER 2008
been used in the case studie