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.     

Short-term responses of two contrasting species of earthworms in an agricultural soil amended with coal fly-ash

With the renewed interest in the use of coal fly-ash for amendment of agricultural soils in Australia, we assessed how earthworms, as indicators of soil health, responded to this ameliorant. We assessed survival, weight, burrowing and elemental concentrations for earthworms of a native unnamed Megascolecid species and of exotic Aporrectodea trapezoides in intact soil cores treated with an alkaline fly-ash at rates equivalent to 0, 5 and 25 t/ha over 6 weeks. Fly-ash did not affect survival, growth, number of burrows created or phosphorus solubilisation. Transfer of the earthworms to the new environment having vastly different pH from where they were collected, and possibly overcrowding, caused mortality in the soil cores for all treatments. A. trapezoides that had smaller individuals suffered mortality of 12% compared with 23% for the larger earthworms of Megascolecids. Earthworms of Megascolecids each increased their weight by 0.24g (25% of their original weight) while those of A. trapezoides lost 0.18g each (21% of their original weight). The difference in growth between the two earthworms was associated with grazing habit and probably with the large difference in the pH between source soil and that of the core soil. Megascolecids appeared to minimize grazing on ash-tainted soil and so ingested less Zn, which was more abundant in the fly-ash than in the soil, compared with A. trapezoides that had elevated concentration of this metal. Extractable P in the soil was increased with both species of earthworms, more so with the exotic species that solubilized 11% more P than the native Megascolecids. The benign influence of fly-ash on survival and growth of worms was associated with the pH of soil remaining unchanged during the six weeks of incubation.     

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     

Tillage effects on compaction, earthworms and other soil quality indicators in Hungary

The philosophy toward tillage throughout the last century in Hungary can be characterized as a fight against extreme climatic and economic situations. The ‘Hungarian reasonable tillage’ approach that was promoted by Cserháti at the end of the 1800s was aimed at reducing tillage without increasing the risk of crop failure in arable fields. Recently, new tillage trends and systems have been introduced because of the rise in energy prices and because of the need to cut production costs, conserve soil and water resources and protect the environment. In Hungarian relation, the rationalized plowing, loosening and mulching systems are counted to the new tillage solutions. There are new steps in the sowing methods too, such as seedbed preparation and plant in one pass, till and plant, mulch-till and plant and direct drilling, which are environment capable, throughout improving soil condition and avoiding the environment harms. The applicability of various soil conservation tillage methods is currently being tested in research projects and discussed in workshops throughout the country. In this paper, soil quality problems such as compaction, trends in soil tillage, and factors affecting soil quality or condition as well as improvement and maintenance are summarized. The data show that annual disking and plowing causes subsoil compaction at the depth of tillage within 3 years and that the compacted layer expanded both in surface and deeper layers after the 5th year. Soil quality deterioration by tillage-pans was improved by subsoiling and maintained by direct drilling and planting soil-loosening catch crops. Within a loam and a sandy loam soil there were close correlations between earthworm activity and soil quality. Earthworm numbers increased on undisturbed but noncompacted soils and soils that included stubble residues remaining on the surface, but did not increase on soils that were deteriorated by tillage-pans or left bare by the absence of mulch. Our goal for the new millennium, is to use only enough tillage to create and maintain harmony between soil conservation, soil quality and crop production.     

Self-organization in a simple consumer-resource system, the example of earthworms

Classical predator-prey or host-parasitoid models often lead to spatial self-organization due to local interactions and limited dispersal ability of the resource (prey or host) and consumers (predator or parasitoid). We hypothesized that self-organization may also arise in soil organisms when the resource is passive and has a constant renewal rate. Earthworm density is correlated with soil properties, but soil heterogeneity only explains a small proportion of spatial variations in earthworm densities. We hypothesized that this could be partially due to self-organization. These two hypotheses were tested with an original model parameterized for a savannah earthworm population. The model simulates an earthworm population divided in 1 m² cells. It is based on the assumption that fine soil aggregates constitute the only limiting resource influencing mortality, fecundity and dispersal and that this resource is renewed according to a constant rate independent of earthworm dynamics. Simulations lead to aggregated spatial distributions when the sensitivity of mortality or fecundity to the availability of the limiting resource is high, and when earthworm mobility is low. Such parameters values are consistent with what is known about earthworm biology. Applicability to different ecological systems and resulting population dynamical properties are discussed.     

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.     

乙草胺、甲胺磷及其复合污染对黑土土壤真菌的影响

Using plate counting and ergosterol assay, single and joint effects of acetochlor and methamidophos on the dynamics of soil fungal population and total fungal biomass in the black soil zone of Northeast China were investigated. The results demonstrated that acetochlor at high concentration levels （150 and 250 mg kg^-1） had an acute and mostly chronic toxicity on both the soil fungal population and total fungal biomass, but at a low concentration （50 mg kg^-1） generally had a stimulating effect that was stronger with total fungal biomass than with the soil fungal population. Methamidophos at a high concentration level （250 mg kg^-1） alone and almost all of its combinations with various dosages of acetochlor increased the soil fungal population, whereas at most sampling dates with 250 mg methamidophos kg^-1 soil, total fungal biomass increased, but in combination with acetochlor it was decreased in the early period of incubation and then increased 28 days after incubation. Thus, through measuring the number of colony forming unit of the soil fungal population along with the total fungal biomass, a better understanding on effects of agrochemicals on soil fungi could be made.     

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.     

Changes in active microbial biomass by earthworms and grass amendments in agricultural soil

The influence of the earthworm Aporrectodea caliginosa on the biomass and the proportion of active and dormant soil microorganisms after the addition of cut perennial ryegrass (Lolium perenne) to upper soil from agricultural field was studied in a microcosm experiment. During a 2-month period, soil samples were taken 1, 8, 22, 36, 50, and 64 days after cut grass addition. A substrate-induced respiration (SIR) method was used to analyse the samples for total microbial biomass and the distribution of active and dormant microbial biomass. It was found that the addition of grass increased the microbial biomass (SIR) because of an increase in the active microbial biomass. After the initially high values, the active microbial biomass decreased slowly, and at day 64, it was still higher in the grass-amended soils than in the control treatment without grass addition. After 1 day, the active microbial biomass was higher in the soil with A. caliginosa than without the earthworm. At the subsequent samplings, there were no differences in microbial biomass or the proportion of dormant vs active microorganisms between the grass-amended soils. The average from all sampling occasions of SIR was higher in earthworm-treated soil.     

电子垃圾影响区多氯联苯污染农田土壤的生物修复机制与技术发展

电子垃圾中含有大量多氯联苯等有毒有害物质,对电子垃圾的不当拆解可造成土壤、水体和大气的污染,进而对生态环境和人体健康构成潜在的威胁。生物修复是利用生物对环境污染物的吸收、代谢、降解等功能,加速去除环境中污染物质的过程。根据修复所用的主体,生物修复又可分为植物修复、微生物修复、动物修复及其联合修复等。本文结合笔者的研究工作,综述了我国东南沿海某典型电子垃圾拆解区土壤多氯联苯的污染特征,并介绍了当前国内外对多氯联苯污染土壤的微生物修复、植物修复和植物-微生物联合修复技术及其机理研究的现状,并提出未来研究趋势,旨在为促进污染区土壤环境生物修复的深入研究、保障污染区的农产品质量安全和人体健康提供理论参考。     

A method of extracting earthworms from cores of soil with minimum damage to the soil

 Lumbricid earthworms have often been shown to increase the growth of plants. The earthworms and plants were generally grown together in the same soil, although sometimes earthworms were reluctant to enter the soil. It was not possible to isolate the mechanism for the increased growth, as no method was available to extract the earthworms with no damage to the soil before the plants were grown. A method is described which enabled Aporrectodea caliginosa, but not A. trapezoides, to enter all cores, and which extracted A. caliginosa from the cores probably with minimum damage to the soil. Received: 20 October 1998     

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.     

蚯蚓活动对金发草修复土壤菲芘污染的强化作用

采用盆栽试验法,研究了蚯蚓(Pheretimasp.)对金发草(Pogonatherum paniceum L.)修复菲、芘污染土壤的强化作用。结果显示,在试验浓度范围(20.1~322.1 mg kg-1)内,蚯蚓活动促进了菲、芘污染土壤中金发草的生长,其根冠比明显增大。添加蚯蚓70d后,种植金发草的土壤中菲、芘的平均去除率高达77.0%、70.6%,平均去除率分别较无蚯蚓作用时(68.7%、61.2%)增长了8.3%、9.5%;相同污染水平下,蚯蚓活动对芘的强化去除程度总是大于菲。各修复因子中,植物-微生物交互作用对菲、芘去除的平均贡献率(46.6%、42.8%)最为突出,分别较无蚯蚓活动时(41.6%、36.6%)增长了5.0%、6.1%。说明蚯蚓活动可强化土壤-植物系统对土壤中菲、芘污染的去除作用。     

Microbiological characterization of the structures built by earthworms and ants in an agricultural field

The genesis and architecture of the structures built by ants and earthworms differ markedly, suggesting that—in addition to having different physical and chemical properties—the resident microbial community should also have unique properties. We characterized the inorganic N, biomass C, C mineralization rate, and functional diversity of the microbial communities of earthworm casts, earthworm burrow soil, ant mounds, and bulk soil from an agricultural field. Mound soil was most enriched in inorganic N and had the lowest pH, moisture content, and C mineralization rate. Functional diversity was evaluated by determining the ability of microorganisms to grow on 31 substrates using Biolog^®EcoPlates in combination with a most probable number (MPN) approach. Casts had MPNs that were one to two orders of magnitude higher than burrow, mound and bulk soil for most substrates. Casts also had the highest MPNs for particular substrate guilds relative to bulk soil, followed by mound and burrow soil. Indices of substrate diversity and evenness were highest for casts, followed by burrow, mound, and bulk soil. Differences in the type of habitat provided by the structures built by ants and earthworms result in the differential distribution of nutrients, microbial activity, and metabolic diversity of soils within an agricultural field that affect soil fertility and quality.     

Quantitative estimates of burrow construction and destruction,by anecic and endogeic earthworms in repacked soil cores

Although the role of earthworms in soil functioning is often emphasised, many important aspects of earthworm behaviour are still poorly understood. In this study we propose a simple and cost-effective method for estimating burrow system area and continuity, as well as a new and often neglected parameter, the percentage of burrow refilling by the earthworms own casts. This novel parameter is likely to have a huge influence on the transfer properties of the burrow system. The method uses standard repacked soil cores in PVC cylinders and takes advantages of clay shrinkage and the fact that earthworms were previously shown to prefer to burrow at the PVC/soil interface. In this way, after removing the PVC cylinders off dry cores, the external section of the burrow system made by earthworms along the soil walls could be easily described. We applied this method to characterise the burrow systems of four earthworms species: two anecics (Aporrectodea caliginosa nocturna and Aporrectodea caliginosa meridionalis) and two endogeics (Aporrectodea caliginosa icaliginosa and Allolobophora chlorotica). After one month the burrow's area generated by both anecic species were much larger (about 40 cm²) than the endogeic burrow's area (about 15 cm²). A. nocturna burrow system continuity was higher than that of A. meridionalis and both anecic burrow systems were more continuous than those made by the endogeic earthworms. This was partly explained by the far larger proportion of the burrow area that was refilled with casts: approximately 40% and 50% for Al. chlorotica and A. caliginosa, respectively compared with approximately 20% for the anecic burrows. We discuss whether these estimates could be used in future models simulating the dynamics of earthworm burrow systems by taking into account both burrow creation and destruction by earthworms.     

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.     

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     

试论生态效益在水土保持中的地位

本文用归纳法从理论和实践两个方面分析论述了水土保持生态、经济、社会效益之间的关系和地位。作者认为生态效益是经济效益的前提和基础;经济效益是生态效益的体现和保障;而社会效益则是随前两者的产生而产生。三者之中,生态效益是第一位的。     

冶炼厂周边土壤重金属污染范围的界定与不确定性分析

在浙江省富阳市某冶炼厂周边以网格法采集了147个表层土壤样品,测定了铜、锌、铅和镉总含量。运用组合预测法(普通块状克里格法和三角网格插值法)预测了研究区各种重金属含量的空间分布,又运用指示克里格法预测了研究区各种重金属的污染概率;再分别依据土壤污染临界值(土壤环境质量标准)和污染概率的临界值(50%)来界定各种重金属的污染范围。本研究将基于两种方法的界定结果相对比来综合界定污染范围,并将污染土壤重金属污染状况分为污染、未污染和不确定。综合界定结果表明,研究区分别有71.7%、80.0%、3.0%、83.1%的区域土壤被重金属Cu、Zn、Pb、Cd所污染,不能被确定是否被重金属Cu、Zn、Pb、Cd所污染的区域分别占20.8%、10.5%、12.5%和10.9%。