Impact of earthworms on the diversity of microarthropods in a vertisol (Martinique)

In a study of a 15-year-old pasture in Martinique (French West Indies), abundance and organization of microarthropod communities were correlated with the spatial distribution of the earthworm Polypheretima elongata (Megascolecidae). In patches of high earthworm density (133 individuals m^–2), microarthropod density was significantly higher (80000 individuals m^–2) than in patches with few earthworms (31 worms m^–2 and 49000 microarthropods m^{– 2}). The diversity of microarthropod communities followed a similar pattern, the Shannon index for Collembola communities being, respectively, 3.12 and 1.82 in and outside earthworm patches. These results suggest that mesofauna abundance and diversity might be at least partly determined by the activity of larger invertebrates, as a result of the dramatic effects that the latter group exerts upon soil structure, pore distribution and food resources. Received: 7 February 1997     

Soil properties and metal accumulation by earthworms in the Siena urban area (Italy)

This paper reports the results of a study focused on the metal (Cd, Co, Cr, Cu, Ni, Pb, Sb, U and Zn) distribution in soils and uptake and accumulation by earthworms Nicodrilus caliginosus (Savigny) from urban, peri-urban, green-urban and non-urban zones of Siena municipality (central Italy). The main goal was to define the influence of soil properties and metal soil contents on the uptake of these contaminants by earthworms. Data indicated that Cd, Cu, Pb, Sb and Zn soil contents increased in the following order: non-urban < green-urban < peri-urban < urban soils, suggesting that vehicular traffic affects the distribution of these metals. Pb and Sb were the main soil contaminants and their highest enrichments were found in urban sites where stop-and-go traffic occurs. Concentrations of these traffic-related metals in earthworms showed a distribution pattern similar to that in soil, suggesting that soil contamination influenced the uptake of Cd, Cu, Pb, Sb and Zn by N. caliginosus. There were significant positive correlations between Cd, Pb and Sb earthworm concentrations and their soil contents. The lack of correlation for Cu and Zn could be due to the physiological regulation of these elements by earthworms. Statistical analysis pointed out that the uptake and accumulation of Cd, Cu, Pb, Sb and Zn by earthworms were affected by some soil physicochemical properties such as the organic carbon and carbonate contents that are able to rule the bioavailability of metals in soils.     

Removal of an invasive shrub (Chinese privet: Ligustrum sinense Lour) reduces exotic earthworm abundance and promotes recovery of native North American earthworms

This study investigated the possibility of a facilitative relationship between Chinese privet (Ligustrum sinense) and exotic earthworms, in the southeastern region of the USA. Earthworms and selected soil properties were sampled five years after experimental removal of privet from flood plain forests of the Georgia Piedmont region. The earthworm communities and soil properties were compared between sites with privet, privet removal sites, and reference sites where privet had never established. Results showed that introduced European earthworms (Aporrectodea caliginosa, Lumbricus rubellus, and Octolasion tyrtaeum) were more prevalent under privet cover, and privet removal reduced their relative abundance (from >90% to ∼70%) in the community. Conversely, the relative abundance of native species (Diplocardia michaelsenii) increased fourfold with privet removal and was highest in reference sites. Soils under privet were characterized by significantly higher pH relative to reference plots and privet removal facilitated a significant reduction in pH. These results suggest that privet-mediated effects on soil pH may confer a competitive advantage to European lumbricid earthworms. Furthermore, removal of the invasive shrub appears to reverse the changes in soil pH, and may allow for recovery of native earthworm fauna.     

Modelling interacting effects of invasive earthworms and wildfire on forest floor carbon storage in the boreal forest

In forest ecosystems, earthworms and wildfire are two ecological agents that cause carbon (C) stored in the forest floor to be transferred to the atmosphere as greenhouse gases, either through heterotrophic respiration (earthworms) or through periodical combustion (wildfire). For centuries, wildfire has been an important ecological driver in the boreal forests of Canada where most fire emissions to the atmosphere originate from the forest floor. In contrast, earthworms are recent invaders, having been introduced to the Canadian boreal during the 20th century. Their spread is mainly associated with anthropogenic activities. We examined stand-level effects of earthworms and wildfire on forest floor C by adapting an earthworm-C simulation model for the boreal and using it in combination with a forest C accounting model. Because the overall impact of an invasive species depends on its areal extent, we used a spatial model of earthworm spread to calculate the total predicted change in C storage at the landscape-level following earthworm invasions in northeastern Alberta. Depending on the ecological groups of earthworms modelled in stand-level simulations, the forest floor C stock was reduced by 49.7–94.3% after 125 years, although the majority of this reduction occurred 35–40 years after initiation of the invasion. Because earthworm activities reduced the amount of forest floor C available for burning, emissions from wildfire were lower in the presence of earthworms. Spatial modelling of earthworm effects within the 5,905,400 ha Alberta–Pacific Forestry Management Area projected that forest floor C stocks in the invaded stands decreased 50,875 Mg C by 2006, and 2,706,354 Mg C by 2056, compared with the same area if earthworms were not present. Loss of forest floor C averaged over the 50 year simulation was 10 g m² yr⁻¹; similar in magnitude to estimates for C loss in the Canadian boreal due to wildfire or harvesting. These results indicate effects of non-native earthworms on the forest floor should be included in predictions of forest ecosystem C budgets to ensure accurate attribution of emissions to heterotrophic respiration versus combustion.     

Interactions between earthworms, litter and trees in an old-growth beech forest

 Forty plots were selected in an old-growth beech forest (Biological Reserve of La Tillaie, Fontainebleau State Forest, France), to embrace the whole range of site conditions and phases of vegetation dynamics. Soils are sandy, thus the nutrient status of the topsoil is very poor except when trees have access to an underlying limestone layer. The study was focused on the role of calcium in the sustainability of the beech ecosystem. Calcium is mostly redistributed through leaf litter accretion and the activity of litter-consuming organisms, but other sources are fallen wood and uprooted mounds. In each of the 40 plots, earthworm species were sampled, and measurements were taken in order to describe humus profiles, growth of adult trees, litter quantity and quality, and access to lime. Densities of soil-dwelling earthworms, calcium content of beech leaf litter, height of tallest trees and depth of the limestone layer were correlated, indicating a gradient of soil fertility which mainly results from long-term interactions between soil organisms and trees in varying geological conditions. Possible causal relationships and implications of calcium turnover for nature conservation were discussed in the light of existing knowledge. Received: 6 July 1998     

Orientation behaviour of earthworms (Lumbricidae) towards different crops

Summary In an outdoor experiment the distribution of earthworms (Lumbricus spp., Allolobophora spp.) under different plant species was studied. Earthworms avoid uncovered areas, but show different reactions under vegetation. This effect reappears in the same manner every year. A particular preference was shown for clover whereas onion was least preferred. Possible reasons for this behaviour are the extent of covering, the moisture regime and exudates of the plants. According to the results obtained the deciding factor seems to be the extent of cover whereas odorous substances are of secondary importance.Dedicated to the late Prof. Dr. M.S. Ghilarov     

Oral sealing using glue: a new method to distinguish between intestinal and dermal uptake of metals in earthworms

Earthworms may take up chemicals from soil and pore water, both through their skin (dermal) and by ingestion (oral). It remains unclear, however, what the relative importance of these pathways is. To assess bioavailability of pollutants in soil to earthworms, it is necessary that the contribution of each pathway is known. Lumbricus rubellus were sealed by means of medical histoacryl glue, to block ingestion of soil particles and pore water. For 6 d, these earthworms showed good survival and vitality and no soil ingestion was found. Equal metal uptake was found by sealed and unsealed earthworms exposed to an inert sand matrix continuously flushed with contaminated water. Therefore, pore water uptake via ingestion contributes little to metal accumulation. Uptake rates of Cd, Cu and Pb in sealed and unsealed earthworms exposed to two contaminated field soils were similar. Uptake and elimination kinetics of Zn were significantly lower in sealed earthworms exposed to one of the two field soils. Body concentrations of Cu and Pb could be completely attributed to the dermal route. For internal Cd and Zn concentrations, however, 0-17 and 21-30%, respectively, were derived from ingestion. It is concluded that for metals the dermal route is the uptake route of importance. The sealing method described here may be useful in a variety of earthworm nutrition and contamination-effect studies.     

Role of earthworms in nitrogen cycling during the cropping phase of shifting agriculture (Jhum) in north-east India

We investigated the role of earthworms in the N cycle in a shifting agriculture system under a 5- and a 15-year Jhum system fallow period intervening between two croppings on the same site. Earthworms participated in the N cycle through worm cast egestion, mucus production, and dead tissue decomposition. Soil N was initially depleted by volatilization during slash and burn operations, and subsequently during cultivation processes. These losses were more pronounced under the 15-year Jhum system. We also studied the addition of N to the system in crop residues, through weed recycling, or in compost applied as organic manure under both the 5- and the 15-year Jhum systems. The total soil N made available for uptake by the plant through the activity of earthworms in this agro-ecosystem was higher than the total input of N to the soil through the addition of slashed vegetation, inorganic and organic manure, and recycled crop residue and weeds. Therefore, in highly leached soils of the humid tropics, worm activity is particularly, important because of rapid incorporation of litter into the mineral soils and because of local concentrations of nutrients in the surface soil layers.     

Changes in biochemical properties of cow manure during processing by earthworms (Eisenia andrei, Bouché) and the effects on seedling growth

The biochemical changes in fresh cow manure caused by the earthworm Eisenia andrei (Bouché) were measured over a period of four months, under controlled laboratory conditions. Earthworms were introduced into each of four plastic containers (0.4 × 0.27 × 0.15 m) containing fresh cow manure (2500 g), and four containers containing manure but without earthworms served as controls. Earthworms reduced the pH and decreased the moisture content in the manure. The C:N ratio of the manure with or without earthworms decreased progressively from 36 to 21. The ash and total nitrogen contents increased greatly for a few weeks after the introduction of earthworms, reflecting a rapid breakdown of carbon compounds and mineralization of nitrogen by the earthworms. CO₂ evolution decreased rapidly (44%) one week after the introduction of earthworms, and continued at a lower rate throughout the 17 weeks (51% reduction as compared to 22% without earthworms), indicating increasing stability of the organic matter. Earthworms reduced microbial biomass early in the process, but enhanced nitrogen mineralization and increased the rates of conversion of ammonium-nitrogen into nitrate. The major general effect of earthworms on the organic wastes was to accelerate the maturation of the organic wastes as demonstrated by enhanced growth of lettuce and tomato seedlings.     

Interactions between earthworms, trees, soil nutrition and metal mobility in amended Pb/Zn mine tailings from Guangdong, China

This paper reports research that attempts to rehabilitate toxic Pb/Zn mine tailings, in Guangdong, China, to achieve a healthy functional soil that supports sustainable vegetation. We studied the effects of the earthworm Pheretima guillelmi on the growth of a woody legume Leucaena leucocephala on Pb/Zn mine tailings diluted with varying amounts of mineral soil in pot experiments. L. leucocephala grew successfully on tailings with a 25% (w/w) soil amendment, but P. guillelmi only survived and actively burrowed with a 50% soil amendment. The presence of earthworms improved the yield of plants by 10-30%. Whilst earthworms marginally increased available N and P in soil, they increased uptake of phosphorus (by about 10%) to above-ground plant tissues. Six-month-old plants were more sensitive than 10-month plants to metal stress. P. guillelmi increased bioavailable metal concentrations in the amended spoils, accompanied by a direct increase of metal uptake by the plants. Increased metal uptake by plants was largely due to the higher dry matter production stimulated by earthworm activity, but this increased the rate of metal uptake into plants from spoil by at least 16% and as much as 53%. These results demonstrate that we should broaden the ecological context of phytoremediation by considering the plant-soil-animal interactions that influence metal mobility.     

Critical body residues (CBRs) for ecotoxicological soil quality assessment: copper in earthworms

The term ‘critical body residue’ (CBR) was defined as the lowest observed total body concentration of a contaminant in an organism, which is associated with the occurrence of adverse toxic effects in either individuals or populations of a defined age or stage of development. In this study, internal toxicity thresholds were determined for copper in the clitellated adult stage of earthworms (Lumbricus rubellus and Aporrectodea caliginosa). The objective was to assess the applicability of CBRs as a practical tool in soil quality assessment of contaminated sites and as a means of a sustainable protection of earthworm fauna. Laboratory studies showed that body concentrations of Cu were generally in agreement with the chemically available CaCl₂-extractable fraction in soil, but that there was also some evidence of internal pH-related homeostatic regulation. Toxicological correlates of body Cu concentrations with adverse effects on cocoon production (fecundity) suggested an approximate sublethal internal threshold of about 40 mg kg⁻¹, with mortality occurring at about 60 mg kg⁻¹. Adult L. rubellus sampled from areas with a wide range of metal pollution showed body Cu concentrations with a minimum of 8 mg kg⁻¹ and a maximum of 60 mg kg⁻¹. Beyond this apparent physiological tolerance range, environmental management directed at optimal earthworm population survival may not be sustainable in contaminated fields. Studies of L. rubellus colonizing a metal-contaminated experimental sludge-treated field showed that a reduced rate of colonization can already be associated with an average body Cu concentration of 25 mg kg⁻¹. However, in this particular field situation mixture effects of other metals that were also present in the soil and the occurrence of avoidance behaviour during colonization may have contributed to this low internal toxicity threshold. It is concluded that the CBR approach seems to be a feasible option for use as a tool in a bioavailability-based soil quality assessment, even for essential trace metals like copper, but that further insight may be needed to establish the uncertainty and reliability of the application in environmental quality assessment and decision making.     

Effect of tillage on earthworms over short- and medium-term in conventional and organic farming

Earthworms play an important role in many soil functions and are affected by soil tillage in agricultural soils. However, effects of tillage on earthworms are often studied without considering species and their interactions with soil properties. Furthermore, many field studies are based on one-time samplings that do not allow for characterisation of temporal variation. The current study monitored the short (up to 53 days) and medium term (up to 4 years) effects of soil tillage on earthworms in conventional and organic farming. Earthworm abundances decreased one and three weeks after mouldboard ploughing in both conventional and organic farming, suggesting direct and indirect mechanisms. However, the medium-term study revealed that earthworm populations in mouldboard ploughing systems recovered by spring. The endogeic species Aporrectodea caliginosa strongly dominated the earthworm community (76%), whereas anecic species remained <1% of all earthworms in all tillage and farming systems over the entire study. In conventional farming, mean total earthworm abundance was not significantly different in reduced tillage (153 m⁻²) than mouldboard ploughing (MP; 130 m⁻²). However, reduced tillage in conventional farming significantly increased the epigeic species Lumbricus rubellus from 0.1 m⁻² in mouldboard ploughing to 9 m⁻² averaged over 4 years. Contrastingly, in organic farming mean total earthworm abundance was 45% lower in reduced tillage (297 m⁻²) than MP (430 m⁻²), across all sampling dates over the medium-term study (significant at 3 of 6 sampling dates). Reduced tillage in organic farming decreased A. caliginosa from 304 m⁻² in mouldboard ploughing to 169 m⁻² averaged over 4 years (significant at all sampling dates). Multivariate analysis revealed clear separation between farming and tillage systems. Earthworm species abundances, soil moisture, and soil organic matter were positively correlated, whereas earthworm abundances and penetration resistance where negatively correlated. Variability demonstrated between sampling dates highlights the importance of multiple samplings in time to ascertain management effects on earthworms. Findings indicate that a reduction in tillage intensity in conventional farming affects earthworms differently than in organic farming. Differing earthworm species or ecological group response to interactions between soil tillage, crop, and organic matter management in conventional and organic farming has implications for management to maximise soil ecosystem functions.     

Contrasting response to cropping of populations of earthworms and predacious nematodes in four soils

The activities of many soil animals make a positive contribution to soil processes and they should be considered for inclusion in indices of ‘soil quality'. To assess the potential use of nematodes and earthworms as indicators, the relationships between populations of earthworms (Lumbricidae), total number of nematodes and predacious nematodes (Mononchoidea) and six soil physical factors, soil carbon and pH were investigated in four New Zealand soils. In each, soil treatments ranged from 5–90 year pastures to cropping with maize or barley for 11–29 years. With increasing cultivation, trends in bulk density, total porosity, aggregate stability and concentration of total carbon were similar in all four soils. In Manawatu (Dystric Fluventic Eutrochrept) and Kairanga (Typic Endoaquept) soils earthworm populations were negligible under continuous cropping while Mononchoidea were abundant (11 600 and 34 100 m⁻²). In contrast, in Moutoa (Fluvaquentic Endoaquoll) and Wakanui (Aquic Ustochrept) soils earthworms persisted under cultivation, while Mononchoidea were less abundant (300 and 2500 m⁻²). At these two latter sites, aggregate stability was higher (1.14 and 0.92 mm mean weight diameter (MWD)) than in Manawatu and Kairanga soils (0.38 and 0.35 mm MWD). These relationships between aggregate stability, earthworm abundance and predacious nematodes show not only that some potential indicators may have a local rather than national application, but also that there are important interactions between soil physical properties and soil fauna which require further investigation.     

Movement of invasive aquatic plants into Minnesota (USA) through horticultural trade

The extent to which invasive aquatic plants move via accidental inclusion in aquatic plant orders or sale of prohibited species is unknown. Forty orders were placed to 34 aquatic plant vendors across the US between May and September 2001 to determine the prevalence of movement into Minnesota via horticultural trade. Federal noxious weeds or Minnesota prohibited exotic species were acquired 92% of the time they were ordered and included Alternanthera sessilis, Butomus umbellata, Hydrocharis morsus-ranae, Potamogeton crispus, and Lythrum salicaria. Ninety three percent of orders received from aquatic plant vendors contained a plant or animal species not specifically requested; 10% of these purchases contained federal noxious weeds or Minnesota exotic species. Lemna minor was the most common incidental receipt found per taxa and per purchase, although Hydrilla verticillata, Lythrum salicaria, Salvinia molesta, and Potamogeton crispus (all prohibited taxa) were also found. Misidentified plants were found in 18% of the orders; unordered seeds in 43%. The sale and transport of prohibited aquatic plants likely presents the greatest risk associated with the aquatic plant trade. Other important factors include misidentification leading to the unintentional sale of invasive plants and the incidental inclusion of species during translocation.     

Landscape-scale species richness of earthworms in the Porongurup Range,Western Australia: influence of aspect,soil fertility,and vegetation type

Indigenous Australian earthworms comprise a patchily collected taxon that is likely to have high diversity and pose difficulties for systems of biodiversity prediction or monitoring that use surrogates. This is because surrogates for earthworm diversity have scarcely been identified at landscape scales in Australian contexts. In the present paper we examine the diversity of earthworms in a 26 km² remnant of native vegetation. Comparison of diversity within the Porongurup Range to diversity of other earthworm faunas demonstrates the sampling intensities required to adequately sample regions of 10⁰–10¹ km². In addition diversity patterns were examined in relation to habitat as described by landscape context, vegetation cover and soil characteristics. Species accumulation in samples from the northern aspect of the Porongurup Range was less than expected from a random distribution of species. Combined Karri-, and Marri- and Karri-vegetated sites supported significantly more earthworm species on the southern aspect of the Porongurup Range than expected from accumulation in randomly ordered samples. Sites carrying Jarrah as the only dominant canopy had fewer earthworm species than would be expected from accumulation in randomly ordered samples. Jarrah overstorey sites and southern aspect Karri sites correspond to extremes in a continuum of soil and landscape characteristics. Earthworm responses to landscape and vegetation units in the region warrant further investigation.     

Alteration of soil labile organic carbon by invasive earthworms (Pontoscolex corethrurus) in tropical rubber plantations

The invasive earthworm (Pontoscolex corethrurus) is commonly found in rubber plantations of Xishuangbanna, southwestern China. To understand the long-term impact of this invasive earthworm on soil labile organic carbon (LOC), we examined changes in LOC that was protected and unprotected by soil aggregates during a worm-exclusion experiment in rubber plantations of Xishuangbanna. We found that the presence of this invasive earthworm for 20 months increased LOC (up to 35%) protected by aggregates in surface soil layer (0–5 cm). In contrast, the presence of this earthworm increased LOC unprotected by the aggregates and the turnover rate of LOC protected by the aggregates in subsurface soil layer (5–15 cm). Soil total LOC did not differ between the control and worm-exclusion treatments. These findings suggest that the invasion of P. corethrurus can redistribute LOC along soil vertical profiles with accumulation of protected LOC on surface soil layer and unprotected LOC on subsurface soil layer. Earthworms' redistribution of LOC between the protected and unprotected forms and along soil profile may affect long-term soil carbon cycling.     

Reproduction and growth of three deep-burrowing earthworms (Lumbricidae) in laboratory culture in order to assess production for soil restoration

Laboratory experiments were conducted to examine the growth and reproduction of three deep-burrowing lumbricids, Aporrectodea longa, Lumbricus terrestris, and Octolasion cyaneum. The reproductive output was recorded as 18.8, 38.0, and 32.3 cocoons per worm per year for A. longa, L. terrestris, and O. cyaneum, respectively. For the same species, maturity was reached at a mean mass of 3.9, 5.0 and 2.4 g, within 3 months from the hatchling stage by L. terrestris and within 4 months by the other two species. The hatching success of cocoons at 15 and 20°C was within the range of 70–80% for each species, except A. longa at the higher temperature, where a viability of 47% was recorded. Twenty percent of viable O. cyaneum cocoons produced twin hatchlings, compared with only one percent for A. longa and L. terrestris. A combination of these results suggests that a complete life-cycle for each species could be achieved within 6 months (L. terrestris and A. longa) or 7–8 months (O. cyaneum). Each species has particular life-cycle strategies that would aid survival and colonisation, under field conditions, if inoculated into restored soils.     

Effects of liming on survival and reproduction of two potentially invasive earthworm species in a northern forest Podzol

During the last several decades, colonization of soil by exotic earthworms and their effects on soil properties and biodiversity have been reported in forests of North America. In some northern hardwood stands, acid soils or harsh climate may have prevented earthworm colonization. However, climatic change and the increasing use of liming to restore the vigor of declining sugar maple (Acer saccharum Marsh.) stands, situated on base-poor soils in USA and Canada, could make many of these sites more suitable for earthworm colonization. We tested survival and reproduction of two exotic earthworm species (Lumbricus terrestris and Amynthas hawayanus) in unlimed and limed soils at the northern limit of the northern hardwood forest distribution in Canada. Improving soil parameters of base-poor, acidic soils by liming positively influenced activity, survivability and reproductive output of L. terrestris in this northern hardwood forest. In contrast, the high mortality and low vigor of L. terrestris observed in the unlimed plots show that soils in this area with a pH of 4.3 are not favorable to this species. Our results suggest that A. hawayanus was very active prior to winter at both soil pHs, but was not able to complete its life cycle during one year at this latitude. Both earthworm species significantly reduced organic C and total N, and increased the C/N ratio of the forest floor. Given that forest liming activities are increasing in proximity to human activities, there is high probability that some earthworm species, such as L. terrestris, will invade limed northern hardwood forests in the next decades, with possible consequences for soil organic matter turnover, nutrient cycling and forest biodiversity and dynamics.