Quantifying surface and subsurface cast production by earthworms under controlled laboratory conditions

Earthworm casts, formed when organic substrates and soil minerals pass through the digestive tract, may protect soil organic matter from biological degradation if they persist in the soil. Yet, the stability of casts is affected by their location in the soil profile because surface casts are exposed to more disruptive forces (wetting-drying, freezing-thawing) than subsurface casts. It is not known whether environmental conditions affect the proportions of surface and subsurface casts produced by earthworms. This study investigated how surface and subsurface cast production by juveniles of Aporrectodea spp. and Lumbricus spp. was affected by temperature. Two juveniles of Aporrectodea spp. or Lumbricus spp. were added to plexiglass chambers filled with soil, and five replicate chambers were incubated in the dark at 5°C, 10°C, 15°C or 20°C for 1 week. Most of the casts produced by Aporrectodea spp. and Lumbricus spp. were surface casts, with <10% of casts deposited below the soil surface. The earthworms studied produced more casts, and a greater proportion of surface casts, as the soil temperature increased. These results can be used to estimate the quantity of surface and subsurface casts produced by earthworm populations under field conditions and determine the susceptibility of cast-associated organic matter to decomposition in the medium- to long-term.     

Uptake and bioaccumulation of heavy elements by two earthworm species from a smelter contaminated area in northern Kosovo

As, Cd, Cu, Pb, Sb and Zn concentrations were determined in two earthworm species (Allolobophora rosea and Nicodrilus caliginosus) from a mining and industrial area in northern Kosovo and compared with their contents in the bulk soil and the main soil fractions. Earthworm specimens were collected at fifteen sites located at different distances from a Pb–Zn smelter along a gradient of decreasing contamination. Individuals of A. rosea and N. caliginosus showed similar tissue levels of As, Cd, Cu, Pb, Sb and Zn, suggesting that earthworm species belonging to the same eco-physiological group have a similar propensity to uptake and bioaccumulate heavy elements. Cd, Pb, Sb and Zn concentrations in both earthworm species were positively correlated with the respective total soil contents and generally decreased with distance from the smelter. The bioaccumulation factor (BAF) revealed that Cd and Zn were the only elements bioaccumulated by earthworms. The rank order of BAF values for both species was as follows: Cd > > Zn > > Cu > As = Pb = Sb. The absorption of Cd, Pb, Sb and Zn by earthworms mostly depended on the extractable, reducible and oxidable soil fractions, suggesting that the intestine is likely the most important uptake route. The extractable soil fraction constantly influenced the uptake of these heavy elements, whereas the reducible fraction was important mainly for Pb and Zn. The water soluble fraction had an important role especially for the most mobile heavy elements such as Cd and Zn, suggesting that dermal uptake is not negligible. As a whole, the analytical data indicate that soil fractionation patterns influence the uptake of heavy elements by earthworms, and the extractable fraction is a good predictor of heavy element bioavailability to these invertebrates in soil.     

The fate of catechol in soil as affected by earthworms and clay

The effect of endogeic earthworms (Octolasion tyrtaeum) and the availability of clay (Montmorillonite) on the mobilization and stabilization of uniformly ¹⁴C-labelled catechol mixed into arable and forest soil was investigated in a short- and a long-term microcosm experiment. By using arable and forest soil the effect of earthworms and clay in soils differing in the saturation of the mineral matrix with organic matter was investigated. In the short-term experiment microcosms were destructively sampled when the soil had been transformed into casts. In the long-term experiment earthworm casts produced during 7 days and non-processed soil were incubated for three further months. Production of CO₂ and ¹⁴CO₂ were measured at regular intervals. Accumulation of ¹⁴C in humic fractions (DOM, fulvic acids, humic acids and humin) of the casts and the non-processed soil and incorporation of ¹⁴C into earthworm tissue were determined.Incorporation of ¹⁴C into earthworm tissue was low, with 0.1 and 0.44% recovered in the short- and long-term experiment, respectively, suggesting that endogeic earthworms preferentially assimilate non-phenolic soil carbon. Cumulative production of CO₂-C was significantly increased in casts produced from the arable soil, but lower in casts produced from the forest soil; generally, the production of CO₂-C was higher in forest than in arable soil. Both soils differed in the pattern of ¹⁴CO₂-C production; initially it was higher in the forest soil than in the arable soil, whereas later the opposite was true. Octolasion tyrtaeum did not affect ¹⁴CO₂-C production in the forest soil, but increased it in the arable soil early in the experiment; clay counteracted this effect. Clay and O. tyrtaeum did not affect integration of ¹⁴C into humic fractions of the forest soil. In contrast, in the arable soil O. tyrtaeum increased the amount of ¹⁴C in the labile fractions, whereas clay increased it in the humin fraction.The results indicate that endogeic earthworms increase microbial activity and thus mineralization of phenolic compounds, whereas clay decreases it presumably by binding phenolic compounds to clay particles when passing through the earthworm gut. Endogeic earthworms and clay are only of minor importance for the fate of catechol in soils with high organic matter, clay and microbial biomass concentrations, but in contrast affect the fate of phenolic compounds in low clay soils.     

Effects of earthworms on nitrogen mineralization

The influence of earthworms (Lumbricus terrestris and Aporrectodea tuberculata) on the rate of net N mineralization was studied, both in soil columns with intact soil structure (partly influenced by past earthworm activity) and in columns with sieved soil. Soil columns were collected from a well drained silt loam soil, and before the experiment all earthworms present were removed. Next, either new earthworms (at the rate of five earthworms per 1200 cm³, which was only slightly higher than field numbers and biomass) were added or they were left out. At five points in time, the columns were analyzed for NH ₄ ⁺ , NO ₃ ^– , and microbial biomass in separate samples from the upper and lower layers of the columns. N mineralization was estimated from these measurements. The total C and N content and the microbial biomass in the upper 5 cm of the intact soil columns was higher than in the lower layer. In the homogenized columns, the C and N content and the microbial biomass were equally divided over both layers. In all columns, the concentration of NH ₄ ⁺ was small at the start of the experiment and decreased over time. No earthworm effects on extractable NH ₄ ⁺ were observed. However, when earthworms were present, the concentration of NO ₃ ^– increased in both intact and homogenized cores. The microbial biomass content did not change significantly with time in any of the treatments. In both intact and homogenized soil, N mineralization increased when earthworms were present. Without earthworms, both type of cores mineralized comparable amounts of N, which indicates that mainly direct and indirect biological effects are responsible for the increase in mineralization in the presence of earthworms. The results of this study indicate that earthworm activity can result in considerable amounts of N being mineralized, up to 90 kg N ha^–1 year^–1, at the density used in this experiment.     

Regenwürmer als Bioindikatoren für Schwermetallbelastungen von Böden unter Freilandbedingungen

The use of earthworms in monitoring soil pollution by heavy metals Total heavy metal contents (HNO₃-soluble) and exchangeable fractions (Ca(NO₃)₂-soluble) of Pb, Zn and Cd were measured in soils, litter layers and earthworms (dry masses) from forest, arable and pasture sites in a transect of the main wind direction and varying distances (1.5, 5.4, 11.4, 15.6 km) to a lead smelter near Bad Ems, Germany. Additionally, cast materials of Lumbricus terrestris-individuals were collected from the surface of the pasture sites. In the observed area total soil contents of Pb and Cd exceeded the C-level and total contents of Zn the B-level of the “Netherland-list”. Heavy metal contents in soils and earthworms decreased with increasing distance to the smelter. Pb showed the best correlation. Correlation between total contents of Pb and Cd in soils and earthworms were significant (rs = 0.66; p < 0.05 and rs = 0.67; p < 0.01, respectively). The uptake of heavy metals by Lumbricus rubellus, L. terrestris and Aporrectodea caliginosa was metal specific rather than species specific with factors of accumulation being <1 (Pb), 2.7–7.6 (Zn) and 19.5–85.5 (Cd). The heavy metal contents of the observed cast materials signified the different transfer of elements from soil material via earthworm individuals to the faeces. In the cast materials the amounts of Pb were high and the amounts of Zn and Cd were low. This indicates a high accumulation rate for Zn and especially for Cd in the tissues of the observed earthworm individuals. The present data support the necessity of ecotoxicological threshold levels.     

Effects of heavy metals on the litter consumption by the earthworm Lumbricus rubellus in field soils

Aim of this study was to determine effects of heavy metals on litter consumption by the earthworm Lumbricus rubellus in National Park the “Brabantsche Biesbosch”, the Netherlands. Adult L. rubellus were collected from 12 polluted and from one unpolluted field site. Earthworms collected at the unpolluted site were kept in their native soil and in soil from each of the 12 Biesbosch sites. Earthworms collected in the Biesbosch were kept in their native soils. Non-polluted poplar (Populus sp.) litter was offered as a food source and litter consumption and earthworm biomass were determined after 54 days. Cd, Cu and Zn concentrations were determined in soil, pore water and 0.01 M CaCl₂ extracts of the soil and in earthworms. In spite of low available metal concentrations in the polluted soils, Cd, Cu and Zn concentrations in L. rubellus were increased. The litter consumption rate per biomass was positively related to internal Cd and Zn concentrations of earthworms collected from the Biesbosch and kept in native soil. A possible explanation is an increased demand for energy, needed for the regulation and detoxification of heavy metals. Litter consumption per biomass of earthworms from the reference site and kept in the polluted Biesbosch soils, was not related to any of the determined soil characteristics and metal concentrations.     

Simultaneous effects of plants and earthworms on mineralisation of 15N-labelled organic compounds adsorbed onto soil size fractions

 The simultaneous impact of three successive crops of wheat (Triticum aestivum L.) and of the earthworm (Lumbricus terrestris L.) on the mineralisation of ¹⁵N-labelled organic compounds adsorbed to different soil size fractions (sand and organic residues >50 μm; silt 50–2 μm; coarse clay 2–0.2 μm and fine clay <0.2 μm) was studied under controlled conditions in the greenhouse. Unplanted soils (UPS) were used as controls. In planted soils without earthworm (PS) total plant biomass decreased with each cropping by up to 50%. However, in planted soils with earthworms (PES) the total plant biomass loss was only 17%. This pattern was explained by the earthworm effect. Compared to the unplanted soils, the planted soils had an increased (mean +37%) mineralisation of ¹⁵N adsorbed onto fine clays and a partial transfer of ¹⁵N to silt and coarse clay. The quantities of ¹⁵N mineralised and transferred were higher in the planted soils with earthworms, indicating an amplification of the phenomenon in the presence of earthworms. The simultaneous effect of the rhizosphere and the drilosphere did not lead to increased mineralisation of N adsorbed onto coarse clays and silts but instead a greater transfer of N associated with the fine fractions towards the coarser fractions. Received: 25 April 2000     

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.     

Behavior and respiration responses of the earthworm Eisenia fetida to soil arsenite pollution

Soil arsenic (As) pollution from mining and industrial sources is a serious issue in China. Earthworms are considered ecosystem engineers and contribute to soil fertility development and maintenance of soil physico-chemical properties. In this study, earthworms were exposed to soils with different sodium arsenite concentrations (0, 5, 20, and 80 mg As kg^-1) for 60 d to investigate the changes in soil properties and the responses of the earthworms (e.g., burrowing activity and respiration). Earthworm burrowing activity decreased with increasing arsenite concentrations, and earthworm respiration was significantly lower in soils with 20 and 80 mg As kg^-1 compared to 0 mg As kg^-1. Changes in soil properties were also observed after incubation of As-amended soil with earthworms. Specifically, soil pH decreased, while soil electrical conductivity and contents of soil NH₃^--N, Olsen-P, and available K increased. Our results suggest that arsenite negatively impacts the metabolic activity of earthworms, leading to reduced burrowing activity, which in turn modifies the effects of earthworms on soil fertility and remediation.     

Effect of passage through the intestinal tract of detritivore earthworms (Lumbricus spp.) on the number of selected Gram-negative and total bacteria

Laboratory experiments were carried out to investigate the fate of bacteria during and after passage through the intestinal tract of detritivore earthworms. Earthworms (Lumbricus spp.) were fed with cattle dung inoculated 7 days previously with one of five different Gram-negative bacteria. Bacterial concentrations were determined 2 days later in dung and soil, and in gut material from different parts of the earthworm intestinal tract. A high percentage (28–82%) of the total bacteria (epifluorescence direct counts) in the earthworm gut content was culturable. The concentration of total heterotrophic aerobic bacteria did not vary significantly among the five different bacterial additions and the non-inoculated control. In earthworm casts the number of total heterotrophs per gram dry matter (2.1×10⁹) was higher than in soil (1.7×10⁸), but lower than in the dung (1.5×10¹⁰). The test-bacteria, however, showed different survival patterns along the earthworm intestinal tract. The concentrations of Escherichia coli BJ 18 and Pseudomonas putida MM 1 and MM 11 in earthworm casts were lower than in the ingested dung, while concentrations of Enterobacter cloacae A 107 and Aeromonas hydrophila DMU 115 in dung and casts were similar. Ent. cloacae, and to aminor extent E. coli, were reduced in numbers by several orders of magnitude in the pharynx and/or crop. In the hind gut, however, the concentration of Ent. cloacae had increased to the same level as in the ingested dung, while the concentration of E. coli remained low. Our observations indicate that the bacterial flora of ingested food materials changes qualitatively and quantitatively during gut transit.     

Significance of earthworms in stimulating soil microbial activity

 The stimulatory effect of earthworms (Lumbricus terrestris L.) on soil microbial activity was studied under microcosm-controlled conditions. The hypothesis was tested that microbial stimulation observed in the presence of a soil invertebrate would be due to the utilization of additional nutritive substances (secretion and excretion products) that it provides. Changes in microbial activity were monitored by measuring simultaneously CO₂ release and protozoan population density. The increase in CO₂ released in the presence of earthworms was found to result from both earthworm respiration and enhanced microbial respiration. The stimulation of microbial activity was confirmed by a significant increase in protozoan population density, which was 3–19 times greater in the presence of earthworms. The respiratory rate of L. terrestris was estimated to be 53 μl O₂ g^–1 h^–1. Earthworm respiration significantly correlated with individual earthworm weight, but there was no correlation between the increase in microbial respiration and earthworm weight. This finding does not support the hypothesis given above that enhanced microbial respiration is due to utilization of earthworm excreta. A new hypothesis that relationships between microbial activity and earthworms are not based on trophic links alone but also on catalytic mechanisms is proposed and discussed. Received: 26 August 1997     

Effects of biomass and size of Millsonia anomala (Oligochaeta: Acanthodrilidae) on particle aggregation in a tropical soil in the presence of Panicum maximum (Gramineae)

Summary The present study assessed the effect of the tropical geophagous earthworm, Millsonia anomala, on the aggregate-size distribution of a sieved (2 mm), tropical ferruginous soil in the presence and absence of the perennial tropical grass Panicum maximum. The effect of two size groupings and graded biomass densities of M. anomala on soil aggregation was analysed in time and with depth in the containers within which the plants were grown. In the absence of earthworms, aggregation was rapid although limited (13.5% of soil as aggregates >2.0 mm), and probably arose from a combination of microbial activity and physical processes (interparticle bonding due to clays and other colloids). The roots of the test species contributed little to aggregation. In contrast, the effect of earthworm activity on soil aggregation was rapid and important. The effects of both biomass and, to a lesser extent, size were significant. After 79 days, aggregation reached a maximum with 3 g per container of small earthworms (ca. 59% of soil as aggregates >2.0 mm) and a minimum with 1 g per container of large earthworms (ca. 35% of aggregates >2.0 mm). Aggregation was considered to occur through three different mechanisms: (1) A rapid aggregation due to the interactions of colloids; (2) an intermediate aggregation due to a combination of unquantified processes related to earthworm activity (mucus secretion, development of fungus hyphae); (3) egestion of soil as earthworm casts which are stable macroaggregates.     

Earthworm populations are stable in temperate agricultural soils receiving wood-based biochar

The application of decomposable organic residues such as manure and crop litter is generally beneficial to earthworms. There is an emerging interest in applying biochar, a carbonaceous product of pyrolysis, to temperate agricultural soils. The slow decomposition rate of biochar, which also contains ash and combustion byproducts, could be detrimental to earthworms. The objective of this study was to describe the earthworm populations in biochar-amended soils on a dairy farm in the St. Francis River watershed, Quebec, Canada. Earthworms were collected from replicated field plots under cereal production. Site A received three wood-based biochar types at two application rates(5 and 10 t ha~(-1) biochar) plus an unamended control, while Site B received wood-based biochar(7.5 t ha~(-1) biochar), dairy cattle slurry(10 t ha~(-1) manure), or a combination of the biochar and manure rates plus an unamended control. Earthworms were collected by hand sorting and formaldehyde expulsion from soil pits. Three species, Aporrectodea turgida, Aporrectodea tuberculata, and Lumbricus rubellus, were found at the sites, and Aporrectodea was the dominant genus. Biochar sources, rates, and application with dairy slurry did not affect the earthworm population, which had 52–218 individuals m~(-2) in 2010 and 4–96 individuals m~(-2) in 2011. The seasonal variation in earthworm population may be due to flooding in the spring of 2011, which apparently interfered with earthworm reproduction. The similarity in earthworm abundance and biomass in plots with and without wood-based biochar leads to the conclusion that earthworm populations are stable in biochar-amended soils in this cold, humid temperate region.     

The effect of earthworms (Lumbricidae) on nitrogenase activity in soil

Summary Nitrogenase activity associated with earthworms, their faeces and activity in soil was measured by the acetylene reduction technique. A clear increase in nitrogenase activity was found in field-deposited casts of Aporrectodea caliginosa in comparison with surrounding soil, although potential nitrogenase activity was significantly higher in soil than in casts. Nitrogenase activity associated directly with earthworms (Lumbricus rubellus ) was detected, indicating the presence of active N₂-fixing bacteria on the body surface and/or in the gut. Laboratory experiments showed that nitrogenase activity in the casts of L. rubellus was higher than in unmodified soil, and that nitrogenase activity in soil was significantly increased by the burrowing and feeding activity of these worms. This paper discusses the possible causes of these earthworm effects on soil nitrogenase activity and some methodological problems of determining the nitrogenase activity.     

Enhanced microbial degradation of pentachlorophenol from soil in the presence of earthworms: Evidence of functional bacteria using DNA-stable isotope probing

This study investigated the effect of two earthworm species (Amynthas robustus Perrier and Eisenia fetida Savigny) on the soil microbial degradation of pentachlorophenol (PCP). PCP-degrading microbes were identified using DNA-stable isotope probing (SIP). The results showed that adsorption and fixing to soil particles and organic fractions dominated the fate of PCP in soil without any amendments. The inoculation of both earthworm species significantly enhanced soil PCP disappearance and basal respiration. The DNA-SIP results revealed that Klebsiella, Cupriavidus, Aeromonas, and Burkholderia spp. were present at higher relative abundances in [¹³C]-labeled-PCP-amended soil microcosms than [¹²C]-PCP-amended soil in the presence of A. robustus, indicating that these bacterial species were responsible for PCP assimilation. Cupriavidus and Aeromonas sp. were also detected in the earthworm gut before inoculation, and their relative abundance was affected by earthworms. These results demonstrated that earthworms can introduce functional bacteria into soils and increase the population of PCP-degrading bacteria, thereby accelerating soil PCP degradation.     

Earthworm bioturbation stabilizes carbon in non-flooded paddy soil at the risk of increasing methane emissions under wet soil conditions

Studies on earthworms in rice-based ecosystems tend to focus on some pest species, while the potential of these important soil engineers for beneficially affecting carbon storage and cycling is widely ignored. We carried out a microcosm experiment to quantify the impact of the tropical earthworm Pheretima sp. on the C turnover in paddy soils under different conditions of water saturation and N fertilization. The soil was sampled at the lowland farm of the International Rice Research Institute (Philippines). In the absence of earthworms, soil respiration showed a distinct hump-shaped maximum at intermediate levels of water saturation (4-fold higher than in hand-dry soil) and increased 1.5-fold with increasing amounts of N fertilization. Amounts of CH₄ emitted, in contrast, were small at low to moderate soil humidity and became very high under conditions of water saturation (80-fold higher than hand-dry soil). No response to nitrogen addition was observed. Earthworms suppressed both the respiration maximum at intermediate saturation levels (by a factor of 1.4) and the stimulating impact of N fertilization (1.7-fold at maximum fertilizer level). On the other hand, earthworms strongly increased CH₄ release under conditions of high water saturation (3-fold). No consistent response of the soil microflora (bacterial abundance, soil enzymes) to earthworm activity could be established. Our findings suggest that the stabilization of soil organic C via earthworm bioturbation is confined to the range of soil humidity that allows high activity of Pheretima sp. Under conditions of intensive agriculture, the stabilizing effect of the worms may even be augmented by the fact that they offset the positive effect of N fertilization on microbial respiration. Earthworms may thus play a vital role in reducing the CO₂ flush from paddy soils after the conversion to non-flooded crops such as aerobic rice or maize. Acceleration of methane emission in very humid soils nevertheless points to a certain risk that is associated with increasing earthworm abundance in production systems that are still exposed to temporary flooding during the wet season.     

不同土地利用方式下的蚯蚓种群特征及其与土壤生物肥力的关系

采用样方法对华北平原(河北曲周)盐渍化改造区7种土地利用方式下的蚯蚓种群进行详细调查,并通过培养实验研究了蚯蚓种群特征对若干土壤生物学指标的影响。结果表明:(1)在7种土地利用调查样地中共存在蚯蚓有3个科,5个属,5个种,其中赤子爱胜蚓(Eisenia fetida)占调查样地总个体数的60%以上,梯形流蚓(Aporrectodea trapezoides)和赤子爱胜蚓两个种在本地区广泛分布,样点出现频率分别为74%和44%,为该地区的优势种;(2)不同土地利用方式的蚯蚓种群密度及生物量变化趋势是:庭院菜地>直立免耕>清茬免耕>商品菜地>传统玉米地>果园>原貌地。其中庭院菜地蚯蚓种群的平均密度和生物量分别达到272 Ind.m-2和68.04gm-2;(3)蚯蚓种群密度和物种数等种群特征与土壤基础呼吸强度、微生物生物量碳含量成显著正相关(p<0.01),与土壤基础呼吸商成显著负相关(p<0.01);(4)不同土地利用方式下,蚯蚓的种群密度、生物量等种群特征对土壤中微生物群落的影响作用显著。蚯蚓生物量越大、种群越丰富的土壤有机质、氮、磷、钾等有效成分越高,反之则相反。室内培养实验表明,随着蚯蚓个体数量增加土壤原生动物总丰度、微生物生物量碳、氮也存在升高的趋势,与用土壤生物学特性指标及土壤化学特性指标评价的结果基本一致。     

Effect of Hormogaster elisae (Oligochaeta; Hormogastridae) on the stability of soil aggregates

The production and stability of soil aggregates produced by laboratory cultures of the endogeic earthworm Hormogaster elisae was studied using three different techniques: the determination of the soil mean weight diameter (MWD), the aggregate tensile strength, and by the Blanchart method, which involves three different tests. The MWD index of soils was higher in microcosms containing H. elisae. Tensile strength was significantly higher in earthworm casts than in naturally formed aggregates. The Blanchart method also showed aggregates produced by earthworms to be more stable. The results of all three methods concur in that aggregates produced by H. elisae are larger and more stable than those produced in control microcosms without earthworms.     

Peat amendment and production of different crop plants affect earthworm populations in field soil

A field experiment was conducted to study the effects of peat amendment and crop production system on earthworms. The experiment was established on a field previously cultivated with oats and with silt as the main soil type. Perennial crops strawberry, timothy and caraway, and annual crops rye, turnip rape, buckwheat, onion and fiddleneck were cultivated with conventional methods. All the crops were grown with and without soil amendment with peat. Earthworms were sampled twice: 4 and 28 months after establishment of the experiment. In the former case part of the experimental plots were soil sampled and hand sorted for estimation of earthworms. In the latter case all experimental plots were sampled and both soil sampling and mustard extraction was carried out. Soil organic carbon and microbial biomass was measured at 14 and 28 months. Peat increased the abundance of juvenile Aporrectodea caliginosa by 74% in three growing seasons, but had no effect on adult numbers. Lumbricus terrestris numbers were not increased by peat treatment. Three season cultivation of caraway favoured both A. caliginosa and L. terrestris. An equal abundance of A. caliginosa was also found in plots cultivated with turnip rape and fiddleneck. Total earthworm and especially A. caliginosa numbers were very small in plastic-mulched strawberry beds. This was mainly attributed to repeated use of the insecticide endosulfan. With the strawberry plots omitted there was a significant correlation between soil microbial N measured at 14 months and juvenile Aporrectodea spp. and Lumbricus spp. numbers measured at 28 months. Adult earthworm numbers were not associated with either soil organic C or microbial biomass.     

Experimental evidence for the role of earthworms in compacted soil regeneration based on field observations and results from a semi-field experiment

In laboratory experiments, earthworms are often observed to burrow through compacted soil layers, leading to the general assumption that these animals play a significant role in regenerating compacted soils in agricultural plots. To demonstrate this role under field conditions, the abundance of earthworm macropores inside compacted zones was estimated on plots under reduced (RT) or conventional tillage (CT). Then, different types of compacted zones typically found in CT (plough pan and compacted clods) and RT plots (compacted volume under wheel tracks) were experimentally simulated in wooden boxes, buried in the field and inoculated with different earthworm species. After 6 weeks of incubation, the number of macropores inside the compacted zones was examined. Field observations showed that approximately 10% and 30% of the compacted zones were colonised by at least one macropore in CT and RT plots, respectively. A significantly greater number of anecics was found in RT plots, but we could not conclude that this ecological type of earthworm plays a more major role in the regeneration process in these plots since there were fewer compacted zones and these covered a smaller area in CT. The semi-field experiment provided evidence that earthworm-mediated regeneration of compacted zones is possible and its nature varies between ecological types of earthworm. Lumbricus terrestris, which makes individual burrows that are vertical and deep, was the main species to cross through the plough pan. The other three earthworm species (Aporrectodea giardi, A. caliginosa and A. rosea) did burrow inside the other types of compacted zones (“wheel tracks” and “compacted clods”). In every case, however, macropore density was far greater in non-compacted zones, illustrating that avoidance of compacted soil by earthworms is important and should be taken into account when extrapolating results from laboratory studies.