Anecic Earthworm Species Research Articles

Association between heavy metal uptake and growth and reproduction in the anecic earthworm, Alma nilotica (Grube 1855).

Elevated heavy metal concentrations in soils are a cause for concern as they are hazardous to soil organisms including earthworms which are considered as ecosystem engineers. Current ecotoxicity tests predominantly use temperate earthworm species, and thus there is the need to include a broader genera of native species to improve ecological risk assessment. Alma nilotica, is a tropical anecic earthworm species that survives well under laboratory conditions and has potential for use in ecotoxicology testing but lacks published toxicity data for important pollutants. Growth and reproduction bioassays were carried out with A. nilotica to determine the relationship between the concentrations of Cu, Zn, Pb and Cr in spiked soils and their bioaccumulation and toxic effects. Positive linear relationships were found between soil-metal and internal earthworm-metal concentrations. Cu did not inhibit growth up to 35 days of exposure but became toxic with longer exposure duration. Zn was not regulated by A. nilotica although it is an essential metal that is well regulated by Eisenia sp. commonly used in standard ecotoxicity tests, showing differences in metal regulation by earthworms of different ecological categories. Based on bioaccumulation factors (BAFs), growth inhibition and reproduction effects the metals were ranked in decreasing toxicity as Pb > Cr > Zn > Cu. The mean 20% Internal Effects Concentrations (IEC20s) for reproduction were 1.04, 2.9, 8.3 and 224.2 mg metal kg-1 earthworm for Pb, Cr, Zn and Cu respectively. These data can contribute to the improvement of metal risk assessment particularly in tropical contexts.

Earthworms reduce nutrient loss from loess soil slopes under simulated rain

Burrowing, feeding, and casting activities of earthworms affect nutrient cycling and hydrological processes that take place in sloping lands. However, the effects exerted by earthworms on nutrient losses associated with runoff and sediment during rain remains unclear, particularly in the Loess Plateau, where soil erosion and nutrient loss have led to severe environmental issues. In this study, our purpose was to quantify the effects exerted by the anecic earthworm species, Metaphire guillelmi, on nutrient loss via runoff and erosion in loess soil slopes. Six artificial grass slopes (mesocosms, 15°), comprising three with earthworms (100 individuals m−2) for 28 days, and three without earthworms, were subjected to simulated rain (90 mm h−1) for 60 min. Our results indicated that earthworms had (i) increased the content of NO3–-N from 8.1 to 151.8 mg kg−1 and OP from 51.0 to 71.8 mg kg−1, but decreased the content of OC from 6.6 to 5.2 g kg−1, TN from 0.76 to 0.62 g kg−1, TP from 0.73 to 0.68 g kg−1 and NH4+-N from 3.7 to 2.4 mg kg−1 at surface 2 cm soil; (ii) increased water-stable macroaggregates from 45.1 to 48.3% and soil saturated water conductivity from 0.07 to 0.28 mm min−1, and (iii) greatly reduced the cumulative runoff from 65.7 to 15.3 mm and sediment occurance from 0.44 to 0.16 kg m−2 on slopes. Consequently, earthworm activities decreased runoff-associated nutrients (decreased NH4+-N from 19.5 to 2.8 mg m−2, NO3–-N from 108.2 to 26.2 mg m−2, and SP from 2.2 to 0.1 mg m−2) loss by 75.8–96.2%, and sediment-bound nutrient (decreased OC from 2.4 to 0.8 g m−2, TN from 0.3 to 0.1 g m−2, TP from 0.3 to 0.1 g m−2) loss by 63.8–68.5%. Hierarchical partitioning and variation partitioning analyses showed that runoff and sediment yields and their interaction with the soil physical characteristics were the dominant pathways via which earthworms regulated nutrient loss during rainfall events. This study highlights the importance of soil fauna in regulating erosion and nutrient loss in the Loess Plateau, and indicates that these factors should be considered in future water and soil conservation management.

Two distinct ecological behaviours within anecic earthworm species in temperate climates

Earthworm species in temperate climates have usually been classified into three main ecological categories according to their morpho-anatomical, physiological and ecological traits: epigeic, endogeic and anecic. However, since these ecological categories were first defined, many studies on the ecological traits of widespread anecic species: Lumbricus centralis (Bouché, 1972), Lumbricus terrestris (Linnaeus, 1758), Aporrectodea longa longa (Ude, 1885) and Aporrectodea giardi (Ribaucourt, 1901) have revealed two distinct feeding behaviours, as well as differences in their growth rates and burrowing behaviour. In this review we highlight that within anecic earthworms, Lumbricus anecic species (here after “LAS”) mainly consume fresh plant-derived materials on the soil surface modifying the quantity and spatial organisation of said materials. In contrast, Aporrectodea anecic species (here after “AAS”) consume mainly aged plant-derived materials already incorporated into the soil and only a small proportion of surface-available plant-derived materials. Furthermore, the AAS have a denser and more complex burrow network than LAS. This suggests that AAS burrow into the soil to search for soil organic matter incorporated in the soil whereas the LAS essentially focus on burying the surface litter into their burrow. Consequently, LAS seem to benefit from easily assimilated substrates, grow faster and reach maturity in a shorter time span than AAS species. This distinction between anecic Lumbricus and Aporrectodea earthworms is expected to have different consequences for soil trophic network and soil functioning such as carbon and nutrient cyclings, water regulation and soil structure maintenance.

Growth and Pb Uptake of Brassica campestris Enhanced by Two Ecological Earthworm Species in Relation to Soil Physicochemical Properties

A comprehensive understanding of the influence of earthworms on the growth and Pb accumulation of leafy vegetables is significant for soil management and human health. This study was aimed to evaluate the different influences of two ecological earthworm species on the growth and Pb accumulation of Brassica campestris in a Pb-contaminated soil and their relationship with soil physico-chemical properties. In a 30-day microcosm experiment, the anecic and native earthworm species Amynthas aspergillum and the epigeic species Eisenia fetida were inoculated in soil artificially contaminated with Pb at different levels (i.e., 0, 100, 500, and 1,000 mg kg−1), and B. campestris was grown. With a survival rate of 81%–100%, A. aspergillum was more tolerant to Pb contamination than E. fetida with 46%–84%. At the same time, earthworm inoculation significantly increased soil Pb bioavailability (p < 0.05). At the 500 and 1,000 mg kg−1 Pb levels, the treatments with earthworm inoculation showed higher plant biomass, leaf area, and chlorophyll concentration than the treatments without earthworm. The principal component analysis (PCA) showed that earthworm inoculation exerted a stronger effect on soil properties than Pb contamination, but the latter had a stronger effect on plant growth and Pb accumulation. Compared with A. aspergillum, E. fetida had a greater effect on soil cation exchange capacity, available Pb, and plant growth and Pb accumulation. In contrast, A. aspergillum had a greater effect on soil C and N contents than E. fetida. The co-inertia analysis revealed that plant Pb accumulation was positively correlated with soil available Pb and CEC. The leaf chlorophyll concentration was closely related to soil Eh, pH, and Dissolved organic carbon. The findings of this study showed that in the Pb-contaminated soils, earthworm inoculation exerted a strong effect on soil physicochemical properties and the growth and Pb accumulation of the leafy vegetable B. campestris. Both the epigeic earthworm species E. fetida and the anecic species A. aspergillum were associated with higher Pb accumulation or concentration in B. campestris, which may bring a possible risk to food security.

Long-term cattle grazing shifts the ecological state of forest soils.

Cattle grazing profoundly affects abiotic and biotic characteristics of ecosystems. While most research has been performed on grasslands, the effect of large managed ungulates on forest ecosystems has largely been neglected. Compared to a baseline seminatural state, we investigated how long‐term cattle grazing of birch forest patches affected the abiotic state and the ecological community (microbes and invertebrates) of the soil subsystem. Grazing strongly modified the soil abiotic environment by increasing phosphorus content, pH, and bulk density, while reducing the C:N ratio. The reduced C:N ratio was strongly associated with a lower microbial biomass, mainly caused by a reduction of fungal biomass. This was linked to a decrease in fungivorous nematode abundance and the nematode channel index, indicating a relative uplift in the importance of the bacterial energy‐channel in the nematode assemblages. Cattle grazing highly modified invertebrate community composition producing distinct assemblages from the seminatural situation. Richness and abundance of microarthropods was consistently reduced by grazing (excepting collembolan richness) and grazing‐associated changes in soil pH, Olsen P, and reduced soil pore volume (bulk density) limiting niche space and refuge from physical disturbance. Anecic earthworm species predominated in grazed patches, but were absent from ungrazed forest, and may benefit from manure inputs, while their deep vertical burrowing behavior protects them from physical disturbance. Perturbation of birch forest habitat by long‐term ungulate grazing profoundly modified soil biodiversity, either directly through increased physical disturbance and manure input or indirectly by modifying soil abiotic conditions. Comparative analyses revealed the ecosystem engineering potential of large ungulate grazers in forest systems through major shifts in the composition and structure of microbial and invertebrate assemblages, including the potential for reduced energy flow through the fungal decomposition pathway. The precise consequences for species trophic interactions and biodiversity–ecosystem function relationships remain to be established, however.

Response of temperate anecic earthworm individual biomass to species interactions

Earthworms contribute to a wide range of ecosystem services provided by the soil. Nevertheless, synecology of these organisms is still not properly elucidated especially in terms of species interactions. The aim of the present study was to determine the effects of anecic earthworm species interactions on their individual biomass. These effects were measured using three epi-anecic species, Lumbricus rubellus rubellus (Hoffmeister, 1843), Lumbricus centralis (Bouché, 1972), Lumbricus terrestris (Linné 1758), and three strict-anecic species, Aporrectodea caliginosa meridionalis (Bouché, 1972), Aporrectodea nocturna (Evans, 1946), Aporrectodea giardi (Savigny, 1826). Twenty-one pairs of individuals were established following five assemblages: monospecific pairwise assemblages of epi- and strict-anecic earthworms (2 × 3 treatments), bispecific pairwise assemblages within epi- and within strict-anecic earthworms (2 × 3 treatments) and bispecific pairwise assemblages with one epi- and one strict-anecic earthworm (3 × 3 treatments). Treatments were maintained in mesocosms for 30 days under controlled conditions (food provided at the soil surface at the beginning of the experiment) and changes in the earthworm individual biomass were measured. Strict-anecic earthworms in monospecific or bispecific assemblages maintained their initial biomass. In contrast, epi-anecic earthworms exhibited an increase of 12.4% and 23.7% of their biomass in monospecific and bispecific assemblages, respectively. In bispecific assemblages combining one epi- and one strict-anecic earthworm, epi-anecic earthworms solely gained biomass leading to a total increase of a 6.9%. Surprisingly, the biomass' changes were not homogenous within the two sub-categories as the six earthworm species exhibited species-specific responses. The greatest increases in individual biomass were recorded for epi-anecic earthworms in the bispecific assemblages. This study provides further evidence for the distinction between the two anecic sub-categories, as it indicates that species interactions is positive only for epi-anecic earthworm biomass.

The role of hedgerows in soil functioning within agricultural landscapes

Intensification of agriculture has led to major losses of hedgerows and field margins worldwide. Soil sample extraction, in situ time series of soil moisture, temperature and soil water quality analyses, annual earthworm sampling and arbuscular mycorrhizal (AM) fungi sampling enabled comparison of soil functions between typical hedgerows, grass field margins, pasture and arable (mainly winter wheat) fields in a temperate, lowland setting. Mean bulk density (upper 50 cm), surface compaction and soil moisture content were significantly lower while organic matter content and porewater dissolved organic carbon concentrations were significantly greater in hedgerow soils, than margins or fields. Mean nitrate and phosphate concentrations were three and ten times larger, respectively, in soil solutions under hedgerows than arable fields while ammonium concentrations were least in arable fields. Saturated hydraulic conductivity was significantly greater under hedgerows (median = 102 mm hr−1) where it took an average of one hour longer for soils to reach maximum moisture content following rainfall, than adjacent arable (median = 3 mm hr−1) or pasture fields and margins (median = 27 mm hr−1). Hedgerow soils had a greater proportion of flow through micropores and less macropore flow than other soils. The pasture and margin soils had the largest proportion of macropore flow (>85%) and more (and larger) anecic earthworm species, such as Lumbricus terrestris which produce vertical burrows. Earthworm density, biomass and diversity were greater in pasture and margin soils, followed by hedgerow soils, and tended to be lowest in arable soils. For both total and AM fungi, hedgerow soils hosted a distinct and heterogeneous soil community, margin and pasture communities were diverse but clustered together, and arable communities formed a distinct cluster, with low inter-sample variation and significantly lowest AM fungal richness. The findings demonstrate that soils under hedgerows, which should be conserved, can provide important functions on farmland including storing organic carbon, promoting infiltration and storing runoff, increasing earthworm diversity and hosting distinct AM communities.

Microplastic transport in soil by earthworms

Despite great general benefits derived from plastic use, accumulation of plastic material in ecosystems, and especially microplastic, is becoming an increasing environmental concern. Microplastic has been extensively studied in aquatic environments, with very few studies focusing on soils. We here tested the idea that microplastic particles (polyethylene beads) could be transported from the soil surface down the soil profile via earthworms. We used Lumbricus terrestris L., an anecic earthworm species, in a factorial greenhouse experiment with four different microplastic sizes. Presence of earthworms greatly increased the presence of microplastic particles at depth (we examined 3 soil layers, each 3.5 cm deep), with smaller PE microbeads having been transported downward to a greater extent. Our study clearly shows that earthworms can be significant transport agents of microplastics in soils, incorporating this material into soil, likely via casts, burrows (affecting soil hydraulics), egestion and adherence to the earthworm exterior. This movement has potential consequences for exposure of other soil biota to microplastics, for the residence times of microplastic at greater depth, and for the possible eventual arrival of microplastics in the groundwater.

Exotic earthworms dispersion through protected forest areas and their potential impacts on nitrous oxide production

It is generally accepted that human activities are responsible for the dispersal of exotic earthworms in northeastern North America. We know little, however, about the relative effects of concurrent human activities on the structure of these earthworm communities in protected forest areas, nor on their impacts on soil biological activities. Our first objective was to infer the relative importance of recreational fishing and road traffic on the structure of Lumbricidae communities in Mont-Tremblant National Park, the oldest conservation area in the province of Quebec, Canada. Our second objective was to test the relationship between earthworm species abundances and soil properties related to microbial and nitrogen dynamics. We sampled earthworm communities around 61 lakes, which included 23 heavily-fished lakes and 20 non-fished lakes located near roads, as well as 18 non-fished lakes located in remote areas of the park. Our results revealed that fishing and proximity to roads both have a positive effect on the abundance of earthworms, as does the soil pH. Fishing activities had a greater effect than road proximity on the abundance and diversity of earthworm communities, notably on the abundance of the anecic species Lumbricus terrestris. To assess at a finer scale the effects of earthworm community structure on soil microbial and nitrogen dynamics, we collected and analyzed soils from 47 sampling points around two lakes with high earthworm densities. Exploratory redundancy analysis found a negative correlation between epigeic and anecic earthworm species, with the former correlating positively to microbial biomass and the latter correlating positively to nitrification and denitrification. Confirmatory path analysis established a positive indirect effect of Lumbricus terrestris, the preferred fishing bait, on potential soil nitrous oxide emissions. We conclude that the human-mediated dispersion of earthworms in the most pristine ecosystems of Quebec affects ecosystem functioning and thus requires a review of current policies regarding the use of live-bait by fishermen.

Organic matter composition and the protist and nematode communities around anecic earthworm burrows

By living in permanent burrows and incorporating organic detritus from the soil surface, anecic earthworms contribute to soil heterogeneity, but their impact is still under-studied in natural field conditions. We investigated the effects of the anecic earthworm Lumbricus centralis on fresh carbon (C) incorporation, soil organic matter composition, protists, and nematodes of a Cambisol under grassland. We used plant material labelled with stable isotope tracers to detect fresh C input around earthworm-occupied burrows or around burrows from which the earthworm had been removed. After 50 days, we sampled soil (0–10 cm depth) in concentric layers around the burrows, distinguishing between drilosphere (0–8 mm) and bulk soil (50–75 mm). L. centralis effectively incorporated fresh C into the drilosphere, and this shifted soil organic matter amount and chemistry: total soil sugar content was increased compared to unoccupied drilosphere and bulk soil, and the contribution of plant-derived sugars to soil organic matter was enhanced. Earthworms also shifted the spatial distribution of soil C towards the drilosphere. The total abundance of protists and nematodes was only slightly higher in earthworm-occupied drilosphere, but strong positive effects were found for some protist clades (e.g. Stenamoeba spp.). Additional data for the co-occurring anecic earthworm species Aporrectodea longa showed that it incorporated fresh C less than L. centralis, suggesting that the two species may have different effects on soil C distribution and organic matter quality.

Different impacts of native and exotic earthworms on rhizodeposit carbon sequestration in a subtropical soil

Earthworms are known to regulate the sequestration of soil and leaf litter carbon (C). However, their impacts on the more accessible rhizospheric C, which represents a major energy source for soil food webs and an essential factor for C sequestration, are still unclear. Previous studies indicate that earthworms regulate the dynamics of SOC and leaf litter-C by increasing C accessibility to microbiota. However, in the case of labile rhizodeposit-C, microbiota might not require any pre-conditioning by earthworms and may rapidly metabolize most of this root-derived C. Consequently, potential pathways by which earthworms may affect the fate of rhizodeposit-C would be to regulate the biomass and/or activity of rhizosphere microbiota and, further, to mineralize/stabilize microbial products. A 13CO2 labelling experiment was performed to determine the impacts of four different earthworm species on the fate of tree rhizodeposit-C in a subtropical soil. We hypothesized that endogeic earthworm species, representing primarily geophagous species, would closely interact with soil microbiota and sequester the microbially metabolized rhizodeposit-C more efficiently than epigeic and anecic earthworm species. We found that irrespective of ecological group affiliation, the three native earthworms did not affect rhizodeposit-C sequestration. In contrast, the exotic endogeic species stimulated the immobilization of rhizodeposit-C in the biomass of root-associated bacteria and/or arbuscular mycorrhizal fungi and, further, accessed the microbiota-metabolized rhizodeposit-C more efficiently. As a consequence, the exotic endogeic earthworm species transiently tripled rhizodeposit-C retention in soil. We propose that the weak linkages between native earthworms and rhizodeposits-related microbiota limit earthworm impacts on rhizodeposit-C sequestration. However, the exotic endogeic species Pontoscolex corethrurus may potentially alter rhizodeposit-C dynamics in invaded areas by shifting rhizosphere microbial community composition. This work highlights a distinct mechanism by which earthworms can regulate C dynamics and indicates a significant contribution of invasive earthworm species to belowground processes.

Micro-scale dry bulk density variation around earthworm (Lumbricus terrestris L.) burrows based on X-ray computed tomography

The burrowing activity of earthworms leads to soil structural changes by compaction of surrounded soil matrix, which represents the so-called drilosphere. This is a distinct volume around the resulting macropore, which controls various soil processes. In the aim to detect the bulk density variation around earthworm burrows, two repacked soil columns from a silt loam with defined water content and bulk density were created, separately inoculated with the anecic earthworm species Lumbricus terrestris, stored under constant conditions for 7weeks and finally analyzed by means of X-ray computed tomography. A basic voxel approach was developed to conduct a gapless analysis of the drilosphere and surrounding soil matrix. The measured Hounsfield Units of basic voxels (pixel size of 0.25mm and slice thickness of 0.625mm) were converted into the corresponding dry bulk density (BD). The initial mean BD of an entire horizontal soil slice was 1.34Mgm−3 (core 1) and 1.38Mgm−3 (core 2). Due to burrowing L. terrestris compacted the inner boundary of the drilosphere (burrow wall) up to a BD of at least 1.75Mgm−3 which equates to more than 30% compared to the soil matrix. The BD decreased from the inner boundary of the drilosphere to its outer boundary, which is a transition zone to the surrounding soil matrix. However, the BD decrease was not concentrically uniform but revealed heterogeneous patterns of zones with different BD-classes. This locally heterogeneous BD distribution is an evidence of radial earthworm forces acting anisotropically. We conclude that BD heterogeneity in the drilosphere of L. terrestris might have notable implications for the understanding of lateral transfer of water and solutes in the soil profile.

A simple and effective method to keep earthworms confined to open-top mesocosms

Earthworms can have a profound effect on a myriad of soil physical, chemical and microbial parameters. To better understand their role in the soil, they are often studied under controlled conditions. However, a persistent problem in such controlled experiments is the ability of earthworms to escape from experimental units with open tops (e.g. for plant growth). Here, we tested whether adhesive hook tape applied to the inside of mesocosms is effective in confining them to their experimental units. A mesocosm study was set up with hook tape treatments (control, one layer, two layers), mesocosm material (polyvinylchloride – PVC, polypropylene – PP) and earthworm species (Lumbricus rubellus (Hoffmeister), Aporrectodea caliginosa (Savigny), Lumbricus terrestris (L.)+Aporrectodea longa (Ude)) as different factors to study the escape of earthworms during 24h. In the treatments without hook tape, individuals of L. rubellus and A. caliginosa escaped, with highest escape rates (80%) for L. rubellus from the PP mesocosms, and lowest escape rates (20%) for A. caliginosa from the PVC mesocosms. When hook tape was applied, in either one or two layers, no individuals of those species escaped. The two anecic earthworm species, L. terrestris and A. longa did not escape from any mesocosms, irrespective of the presence of hook tape. As not a single earthworm escaped from the hook tape treatments, we conclude that applying hook tape is a simple, inexpensive and effective method to keep earthworms confined to experimental units.

Herbivore behavior in the anecic earthworm species Lumbricus terrestris L.?

Earthworms dominate invertebrate biomass in many non-acidic terrestrial ecosystems and represent key detritivores. Anecic earthworms live in the soil but primarily feed on soil surface litter. Recent studies indicate that predation of seeds and seedlings may also contribute to earthworm nutrition. However, the biological significance of this behavior is unknown. Here, we present to our knowledge the first photographic and video evidence that the anecic earthworm species Lumbricus terrestris L. attacks living plants and damages leaves still attached to plants. This behavior could be observed repeatedly and in different locations, indicating that it may be a common process. Though based on uncontrolled observations and in situations of low litter availability, this herbivore behavior suggests that the role of earthworms within ecosystems should not be restricted to litter burial and decomposition, but should include aboveground herbivory. Our observations do not allow us to determine with certainty if the earthworms directly fed on fresh plant material or – more likely – transported it to its permanent burrows to facilitate microbial decay. We propose that this behavior and its implications for the composition and functioning of plant communities merits further scientific attention. For instance, given the detrimental effects of some common molluscicides on earthworm performance, assumed mollusk effects on plant growth and seedling survival reported in previous studies could be – at least in part – due to the action of anecic earthworms. Future studies should investigate the biological significance and the context-dependency of this behavior.

Experimental assessment of ecological restoration options for compacted forest soils

It is ecologically undesirable to solve forest soil compaction due to mechanized harvesting at large spatial scales using agricultural mechanical soil loosening techniques. We therefore examined whether a stimulation of biological activity through litter manipulation, liming and/or inoculation of the anecic earthworm species Lumbricus terrestris could significantly contribute to the ecological restoration of compacted forest soils by comparing the impact of these treatments on the soil within and beside compacted wheel tracks. The replacement of native litter by litter with a better quality resulted in a faster litter decomposition. However, maximal decay rates were obtained only when litter manipulation, earthworm inoculation and liming were combined. Anecic earthworms were initially absent as soils were probably too acid. Liming as well as litter manipulation had a small positive influence on the numbers of retraced L. terrestris, inducing positive feedback mechanisms on soil pH and litter decomposition rates. None of the treatments, however, had a significant effect on the compaction degrees within or beside tracks within the small study period. L. terrestris realized only a small decrease of bulk density beside the tracks. Within the tracks a similar number of L. terrestris was retrieved, but effects on the compaction degree were negligible. Liming decreased penetration resistance, but only in the absence of anecic earthworms. Endogeic earthworms were overall more abundant than the anecics, especially within tracks where soil water contents and pH values were higher. However, endogeics only had a marginal effect on litter decomposition and although they positively influence soil structure, they could not realize a reduction of the compaction degree, quantified by bulk density and penetration resistance, due to different burrowing habits. Our results indicated that a positive impact of anecic earthworms on the structure of compacted forest soils can be obtained in the long-term, at least in case soil conditions (acidity, nutrient availability and moisture content) are favourable. This can be achieved by conversion of forests towards tree species with high quality litter.

The action of an animal ecosystem engineer: Identification of the main mechanisms of earthworm impacts on soil microarthropods

Non-trophic interactions are shaping soil food web structure and functions. Particularly, the action of ecosystem engineers, such as earthworms, are likely to fundamentally impact the abiotic and biotic properties of their environment. The present study aimed to identify the main mechanisms through which earthworms belonging to varying ecological groups – epigeic, endogeic and anecic species – affect soil microarthropods by reviewing the literature on this topic and by performing meta-analyses. Earthworm ecological groups differed considerably in their impacts on microarthropods, whereas effects did not vary significantly between microarthropod taxa at the habitat scale. Inconsistent impacts of epigeic species on soil microarthropods are most likely due to differences in earthworm densities. Effects can thus be positive in the case of moderate densities or negative in the case of high densities and associated distinct changes in the physical structure of the upper soil organic layers. By contrast, impacts of endogeic earthworms appeared to be mainly negative and were primarily due to competition with microarthropods for food resources. Consequently, negative impacts on soil microarthropods intensified with increasing earthworm density and biomass. This interaction between endogeic earthworms and microarthropods is better referred to as amensalism due to the competitive predominance of earthworms. Impacts of anecic earthworm species differed significantly from that of endogeic ones; they were neutral at the habitat scale and positive on the microhabitat scale. Moreover, impacts were independent of earthworm densities due to the quasi-territorial behaviour of anecic earthworms. Positive effects were mainly attributed to the formation of stable microhabitats by anecic species; namely burrows/middens, rich in nutrients and microorganisms. The present study points to the relevance of the non-trophic biotic interactions that drive the composition of belowground food webs by identifying the most essential mechanisms underlying the impacts of animal ecosystem engineers on soil microarthropods. Moreover, as earthworms emerge as important biological invaders, the results of the present study may help to fully appreciate, estimate and model the consequences of this momentous global change phenomenon. Particularly, the spread of exotic epigeic and endogeic earthworm species likely threatens soil microarthropod density, diversity and functions.

Quantitative image analysis of earthworm-mediated soil displacement

The way soil is disrupted and deformed by earthworm movement is hard to quantify non-destructively. Two anecic earthworm species, Aporrectodea longa (Ude) and Lumbricus terrestris L., were compared in their effect on the soil around them as they burrow. Image analysis (particle image velocimetry, PIV) was used successfully to quantify the distance and direction of soil particle displacement by earthworm locomotion giving a unique insight into their effect on the soil structure development. The data collected from both species using PIV show a decline in soil displacement at increasing distance from the earthworm’s body. The quantity of soil being displaced was more to the sides of the earthworms (radially) than in front of the prostomium (axially). Also, L. terrestris displaced more soil than A. longa both axially and radially. The findings from this study suggest that PIV image analysis is a viable tool for examining soil displacement by earthworms and the method used has the potential to be applied to other soil organisms.

Earthworm effects on plant growth do not necessarily decrease with soil fertility

Earthworms are known to generally increase plant growth. However, because plant-earthworm interactions are potentially mediated by soil characteristics the response of plants to earthworms should depend on the soil type. In a greenhouse microcosm experiment, the responsiveness of plants (Veronica persica, Trifolium dubium and Poa annua) to two earthworm species (in combination or not) belonging to different functional groups (Aporrectodea. caliginosa an endogeic species, Lumbricus terrestris an anecic species) was measured in term of biomass accumulation. This responsiveness was compared in two soils (nutrient rich and nutrient poor) and two mineral fertilization treatments (with and without). The main significant effects on plant growth were due to the anecic earthworm species. L. terrestris increased the shoot biomass and the total biomass of T. dubium only in the rich soil. It increased also the total biomass of P. annua without mineral fertilization but had the opposite effect with fertilization. Mineral fertilization, in the presence of L. terrestris, also reduced the total biomass of V. persica. L. terrestris did not only affect plant growth. In P. annua and V. persica A. caliginosa and L. terrestris also affected the shoot/root ratio and this effect depended on soil type. Finally, few significant interactions were found between the anecic and the endogeic earthworms and these interactions did not depend on the soil type. A general idea would be that earthworms mostly increase plant growth through the enhancement of mineralization and that earthworm effects should decrease in nutrient-rich soils or with mineral fertilization. However, our results show that this view does not hold and that other mechanisms are influential.

Effects of an endogeic and an anecic earthworm on the competition between four annual plants and their relative fecundity

Competition between plants for essential resources determines the distribution of biomasses between species as well as the composition of plant communities through effects on species reproductive potentials. Soil organisms influence plant competitive ability and access to resources; thus they should modify plant community composition. The effects of an endogeic ( Aporrectodea caliginosa) and an anecic ( Lumbricus terrestris) earthworm species on the competition between grass ( Poa annua), two forbs ( Veronica persica and Cerastium glomeratum) and legume ( Trifolium dubium) were investigated in a greenhouse experiment. We established two types of plant communities: monocultures and polycultures of the four species. L. terrestris increased the biomass of P. annua and V. persica (in monocultures as well as in polycultures). However, the presence of L. terrestris allowed the grass to produce the highest biomass in polycultures suggesting that this earthworm species promoted the growth of P. annua against the other plant species. In monocultures as well as in polycultures, the presence of L. terrestris to increased the number of seeds of T. dubium and the total seed mass of V. persica. These results suggest that L. terrestris enhanced the short term competitive ability of P. annua by promoting its growth. The increased number of seeds of T. dubium in the presence of L. terrestris suggests that this earthworm species could enhance the long-term competitive ability of this legume and may increase its number of individuals after several generations.

Impact of five different tillage systems on soil organic carbon content and the density, biomass, and community composition of earthworms after a ten year period

To assess the impact of different types of soil tillage on the density, biomass, and community composition of earthworms, a long-term field study was performed in which soils were tilled in different ways for ten years. This study included five different types of tillage: (i) plough, (ii) grubber, (iii) disc harrow, (iv) mulch sowing, and (v) direct sowing. At the end of the experiment the earthworm density, biomass, and community composition, and the SOC (soil organic carbon) content were determined. The results show that density, biomass, and community composition of earthworm populations varied in relation to the type of soil tillage used. The density of anecic earthworm species decreased when soils were managed by conventional ploughing, relative to reduced tillage practices, whereas conversely the density of endogeic species increased. Additionally, the varying types of soil tillage influenced the abundance and biomass of different earthworm species in different ways. The density of Aporrectodea caliginosa was positively influenced by ploughing, whereas Aporrectodea longa, Lumbricus castaneus, and Satchellius mammalis showed a positive relationship to the grubber and Allolobophora chlorotica to direct sowing. We attribute these changes to modifications in the vertical distribution of SOC and varying potentials for mechanical damage of earthworms by tillage. A decrease in tillage intensity modified the vertical SOC distribution in the topsoil and consequently revealed positive effects on earthworm biodiversity, thus sustaining soil functioning.