Earthworm density and biomass in relation to plant diversity and soil properties in a Palouse prairie remnant

Earthworms are important soil animals in grassland ecosystems and are considered to be important to soil quality. The overall impact of earthworms on soil properties and plant diversity, however, depends on earthworm species, functional group and the type of ecosystem. The primary purpose of this study was to document the relationship among earthworms, key soil properties and native and exotic plant diversity in the little studied, Palouse prairie grassland (Idaho, USA). A secondary objective was to determine the effectiveness of three methods commonly used to sample earthworms. A hillslope characterized by Palouse prairie vegetation, well-expressed, hummocky (mounded) topography and known to support both exotic and native earthworm species was selected for study. The hillslope was divided into three zones [annual-dominated (AD), mixed (MX) and perennial-dominated (PD)] based on characteristics of the inter-mound plant communities described in previous research. Total earthworm biomass in the MX zone (53.5 g m⁻²) was significantly greater than in the PD zone (14.7 g m⁻²) (P = 0.0384), but did not differ from the AD zone. Earthworm density ranged from 52 to 81.1 individuals m⁻² but was not significantly different across zones. Total C and N at 0 to 10 and 30 to 50 cm depths were significantly greater in the AD and PD zones as compared to the same depths in the MX zone. Soil textural class was silt loam within all zones and the soil silt fraction was positively correlated with total exotic earthworm density (R = 0.783, P = 0.0125) and biomass (R = 0.816, P = 0.0072). Native earthworms were only found in the zone with the greatest total and native plant diversity (PD). Total soil C and N were not correlated to earthworm density, but soil total C and N were significantly negatively correlated with exotic plant density, which indicates that invasive plants may be decreasing soil total C (R = −0.800) and N (R = −0.800). Calculated earthworm densities using data from the electroshocker were generally lower than those based on the hand-sorting method. Electroshocking, however, created lower disturbance and was the only method that resulted in the collection of the deep-burrowing, native species Driloleirus americanus.     

Identifying earthworm's organic matter signatures by near infrared spectroscopy in different land-use systems in Tabasco,Mexico

In the state of Tabasco, South-eastern, Mexico, land-use changes such as the conversion of natural into agricultural systems, modify soil quality and the abundance of soil macrofauna, including earthworms. The aim of this study was to characterize by near-infrared spectroscopy (NIRS) the earthworms’ fingerprint in soil, in six sites including natural and agricultural ecosystems with low and high earthworm biomass and low and high earthworm diversity, in order to identify specific wavelengths that discriminate the presence/abundance of earthworm species and functional groups. The spectral region of 1860–1870 nm was significantly correlated with total earthworm density, particularly at one of the sites (Cedar polyculture; r = 0.8, p < 0.05). Earthworm biomass had a specific NIRS wavelength according to the earthworm species and feeding category: 1820 and1860–1870 nm wavelengths were significantly correlated with Polypheretima elongata (r² = 0.7, p < 0.05; mesohumic species) biomass and 2090 nm for biomass of all Lavellodrilus species (polyhumics). Two species had a much wider spectral range: L. bonampakensis and Dichogaster saliens (an epigeic worm; 1690–2300 nm, r² = 0.7, p < 0.05). Biomasses of Periscolex brachysistis and Diplotrema murchiei were not significantly correlated with any near infrared wavelength spectra analyzed. Combining a maximum of 4 species per wavelength, mesohumic earthworms had a wider wavelength spectrum than polyhumics. Therefore, earthworm species diversity, biomass and abundance are associated with soil quality (as measured by NIR spectra) and this relationship varies with species and ecological category. Sites with lower and higher earthworm diversity have lower and higher soil organic matter quality, respectively, as observed by the wider or narrower spectral range with which earthworm biomasses are correlated.     

Deep-burrowing earthworm additions changed the distribution of soil organic carbon in a chisel-tilled soil

We investigated the influence of earthworms on the three-dimensional distribution of soil organic carbon (SOC) in a chisel-tilled soil. By burrowing, foraging, and casting at the surface and throughout the soil, anecic earthworms such as Lumbricus terrestris L. may play a major role in regulating the spatial distribution of organic matter resources both at the surface and within the soil. In the fall of 1994, we manipulated ambient earthworm communities, which were without deep burrowing species, by adding 100 earthworm individuals m⁻² in spring and fall for 3 years. Overall, the biomass of L. terrestris was increased with earthworm additions and total earthworm biomass declined compared with ambient control treatments. To investigate the spatial variability in soil organic carbon due to this shift in earthworm community structure, we sampled soil on a 28×24 cm grid from the surface to 40 cm in four layers, 10 cm deep. Samples were analyzed for total carbon. We found that additions of anecic earthworms significantly increased average soil organic carbon content from 16.1 to 17.9 g C kg⁻¹ for the 0–10 cm soil, and from 12.4 to 14.7 g kg⁻¹ at 10–20-cm depth, and also changed the spatial distribution of soil organic carbon from uniform to patchy, compared with the ambient treatment.     

The effects of earthworm functional diversity on microbial biomass and the microbial community level physiological profile of soils

A novel experiment design based on the axial simplex design was used to study the effects of earthworm functional diversity on microbial biomass and microbial community level physiological profiles in soils. This mesocosm experiment was carried out at two densities of earthworm and at two levels of food supply. Soil mesocosms were maintained for 7 months and watered each week with 300 ml of distilled water. At the end of this period the soil columns were divided into upper and lower halves and microbial biomass carbon and functional diversity assessed. Greater microbial biomass and activity was found in the upper layer of soil. Food supply was the major driver leading to increased microbial activity and biomass. Increased earthworm biomass had the opposite effect. In the lower soil layer, the presence of anecic worms had the greatest effect on microbial biomass and activity. This effect was enhanced with increased food supply. Overall, the relationship between earthworm functional diversity and microbial community level physiological profile appeared to be idiosyncratic and the effect of the earthworms varied with soil layer, initial biomass and food supply. The loss of any earthworm functional group might lead to significant alterations in the microbial biomass, distribution and physiological profile.     

Earthworms in Chromolaena odorata (L.) King and Robinson (Asteraceae) fallows along a chronosequence: Changes in community structure and identification of persistent and indicator species

The species Chromolaena odorata (Asteraceae) is a notorious invasive shrub spreading throughout West and Central Africa and as such, there is a need to determine its environmental impact, particularly on soil biodiversity and functioning. Indeed, soil organisms such as earthworms are known to strongly influence soil properties and biogeochemical cycles. This study, conducted in Central Côte d’Ivoire, aims to investigate the temporal dynamics of earthworm communities in C. odorata fallows of different ages and to identify associated indicators and persistent species. Three distinct classes of fallows identified by local farmers, were considered: young (1–3 years, C1), medium-aged (4–8 years, C2) and old (>9 years, C3). Each of the classes included four plot replicates where earthworms were sampled using the Tropical Soil Biology and Fertility (TSBF) 25 cm × 25 cm × 30 cm soil monolith method. The study of earthworm communities was focused on density, biomass, diversity and complementarity. Indicator values (IndVals) were used to identify indicator species of the classes of fallows. The shrub exerted a mixed influence on earthworms depending on the functional group, with litter feeders and polyhumics declining over time as a result of a reduction of the litter availability on the soil surface. The species richness was significantly greater in C1 than in the other classes although the Shannon–Weaver's index did not vary significantly. However, a cluster analysis performed on densities highlighted marked differences between C2 and the two other classes in terms of community composition. Indicator species were found for C1 and C2. The geophagous Millsonia omodeoi has emerged as a persistent species as its density and biomass steadily increased so that it became the dominant species in old fallows. The roles of litters and soil parameters in influencing earthworm communities are discussed.     

Factors influencing earthworm communities in post-industrial areas of Krakow Soda Works

Application of earthworm in soil re-cultivation and re-creation in post-industrial ecosystems make a big challenge for temporal applied zoology. The sediments of the Krakow Soda Works “Solvay” have undergone land reclamation in different ways: older sediments traps were left without any re-cultivation practices; meanwhile the newest ones were reclaimed using standard method (new soil cover planted with combination of grass and leguminous plants). The effect of different treatments on community and population structure of earthworm was estimated during consecutive years 1999–2000. Six localities differing in time of establishment, reclamation processes, vegetation type and soil properties were chosen. Nine species were recorded, among which Aporrectodea caliginosa occurred in all localities, being also the most abundant. Two other species, Lumbricus rubellus and Dendrobaena octaedra, which are epigeic species, become most important in forest assemblages and were characteristic for communities of older succession processes. Abundance of adult forms as well as total biomass were significantly affected by soil depth (r = 0.75, P < 0.05, r = 0.917, P < 0.001, respectively). Species richness however was connected with higher amount of macroelements and average plant height. Shannon diversity index and its evenness negatively correlated only with forestation (r = –0.67, P < 0.05, r = –0.68, P < 0.05, respectively). Niche overlap (α Pianka) for all analysed species extracted two groups differing in environmental requirements. First contained epigeic earthworms, closely related to plant succession (PCA results), the other one grouped endogeic and anecique species correlated significantly with soil depth and plant density. Community structure of earthworms do not reflects succession changes in post-industrial habitats, but is strongly affected by microhabitat factors in local scale (mainly soil depth and plant density).     

Indication of soil heavy metal pollution with earthworms and soil microbial biomass carbon in the vicinity of an abandoned copper mine in Eastern Nanjing,China

Mining activities can cause severe soil pollution in mining area and its surroundings. Nevertheless, very little is known about the local environmental risk after the mining activities are ended in China. Earthworms and soil microbial biomass carbon (SMBC), which are often used as bioindicators of soil pollution, were studied in order to support chemical analyses in assessing the status of soil heavy metal pollution around an abandoned copper mine in eastern Nanjing, China. Seven earthworm species belonging to three families (Megascolecidae, Moniligastridae, and Lumbricidae) were present. Correlations between earthworm densities or biomass and a range of soil physical and chemical parameters were generally poor; however, several linear regression models based on the soil physicochemical characteristics and metal concentrations in earthworm bodies were established for each metal (Cu, Cd, and Zn) and each earthworm family. Therefore, metal bioaccumulation by soil-dwelling earthworms can be used as an ecological indicator of metal availability for this area. The SMBC, which varied from 83.9 to 499 g kg^?1, did not correlate with the soil heavy metal concentrations, and SMBC is not proposed as a sensitive indicator for evaluating the environmental effects of soil heavy metal pollution in this area.     

Earthworm composition,diversity and biomass under three land use systems in south-eastern Australia

In south-eastern Australia, strips of planted native trees and shrubs (shelterbelts) are frequently established to restore ecosystem services altered by agriculture. Despite their wide use, little is known about the effects of establishing shelterbelts on soil macro invertebrates, especially earthworms, which are of major importance in soil processes. We assessed earthworm composition, diversity and biomass in three land use systems: native shelterbelts dominated by Acacia and Eucalyptus species, agricultural pastures and native remnant woodland fragments dominated by Eucalyptus blakelyi and/or Eucalyptus melliodora. Earthworm communities differed significantly among systems, with abundance, biomass and diversity greatest under pasture. Within shelterbelts we saw a shift from high earthworm biomass and density to low with increasing time after establishment. Soil edaphic variables did not correlate strongly with earthworm biomass or density, but were correlated with earthworm community composition. Overall the introduction of native woody vegetation was associated with a decline in density and biomass of earthworms, including a decrease in the relative abundance of exotic species. As such shelterbelts can be used to promote native earthworm relative abundance, which may be important for local diversity, soil function and landscape connectivity.     

蚯蚓对赤红壤草被恢复影响的试验研究

生态修复是南方红壤丘陵区土地退化治理的有效手段之一,但现有研究较少关注土壤动物在生态修复过程中的作用。以退化的赤红壤为研究对象,基于盆栽试验,采用全因子设计方法模拟多种生态系统(裸土vs.黑麦草)×蚯蚓(不接种vs.接种蚯蚓)×水分(湿润vs.干旱),以揭示蚯蚓对干旱条件下退化土壤植被修复前后土壤理化性质和植被生产力的调控作用。结果表明：蚯蚓显著提高了湿润和干旱条件下黑麦草生物量。蚯蚓对土壤有机碳影响不显著,蚯蚓活动提高了裸土生态系统土壤总氮、硝态氮、铵态氮含量,以及黑麦草生态系统土壤速效磷含量。干旱胁迫降低黑麦草生物量,对土壤有机碳影响不显著,但显著改善蚯蚓存在土壤的pH值。黑麦草生物量与土壤总磷、硝态氮、铵态氮和速效磷显著正相关。偏最小二乘路径分析表明蚯蚓活动显著提高土壤全量和速效养分含量,湿润条件下速效养分对植被生物量具有显著正效应,干旱条件下其作用不显著。综上,蚯蚓活动改善土壤肥力状况,促进植被生长,蚯蚓活动可缓解干旱对植被生长的不利影响。研究结果对深入认识蚯蚓对生态系统作用具有重要意义,为退化土地生态修复管理提供科学依据。     

Earthworm collection from agricultural fields: Comparisons of selected expellants in presence/absence of hand-sorting

The role earthworms play in soil fertility is under increasing scientific scrutiny, especially in light of the fact that farmers are seeking to reduce soil tillage. However, there are many discrepancies in earthworm sampling methods. The aim of this study is to compare the efficiency of three chemical expellants (formaldehyde, commercial ‘hot’ mustard and allyl isothiocyanate, abbreviated AITC), with two sampling methods: (i) a simple method that consisted in spreading an expellant solution on the soil and retrieving earthworms that emerged at the soil surface, and (ii) a combined method that consisted in applying a chemical to expel earthworms and then hand-sorting the remaining earthworms from the block of soil. Sampling efficiency was measured in terms of earthworm density and biomass, for endogeic and anecic earthworms.With the simple method, a higher density of earthworms was sampled using formaldehyde and AITC than with mustard. Formaldehyde, AITC and mustard expelled not significantly different biomasses of 47.7, 31.9 and 20.5 g m^?2, respectively, on average over the three plots. The combined method did not yield a significantly different density or biomass with the different chemicals.Formaldehyde is toxic and commercial ‘hot’ mustard is difficult to standardise and inefficient when used without hand-sorting. Accounting for the accuracy of the sampling methods as well as the toxicity of the chemicals to users and soil organisms, AITC appears to be a reliable and promising chemical expellant whether or not in combination with hand-sorting. Its use would be a step towards standardizing earthworm sampling methods.     

Earthworm colonisation of abandoned arable soil polluted by copper

The distribution, density and biomass of earthworms were investigated at the copper polluted site, Hygum (Denmark). In 1994, shortly after farming of the area was abandoned, only four earthworm species were present and their distribution was restricted to areas where copper concentration did not exceed 200 mg kg^?1 dry soil. Sixteen years later (in 2010), without any agricultural activity, ten species of earthworms were found, in particular, epigeic species were present where soil copper concentrations reached >1000 mg kg^?1 dry soil.     

The significant contribution of mycorrhizal fungi and earthworms to maize protection and phytoremediation in Cd-polluted soils

Mycorrhizal fungi and earthworms can individually or interactively influence plant growth and heavy metal uptake. The influence of earthworms and arbuscular mycorrhizal (AM) fungi either alone or in combination on maize (Zea mays L.) growth and cadmium (Cd) uptake was investigated in a calcareous soil artificially spiked with Cd. Soils were contaminated with Cd (10 and 20 mg Cd kg⁻¹), inoculated or un-inoculated with the epigeic earthworm Lumbricus rubellus and two AM fungal species (Rhizophagus irregularis and Funneliformis mosseae) for two months of growth under greenhouse conditions. Generally, earthworms alone increased both shoot P uptake and biomass but decreased shoot Cd concentration and root Cd uptake. AM fungi individually often increased total maize P uptake, declined shoot Cd concentration, and consequently produced higher total biomass. However, R. irregularis enhanced shoot Cd uptake at low Cd level and root Cd uptake at high Cd level. In plants inoculated with F. mosseae species, earthworms increased shoot biomass and Cd uptake, decreased root biomass and Cd uptake at all Cd levels, and increased shoot Cd concentration at low Cd level. In plants colonized by R. irregularis species, however, earthworm addition decreased maize biomass only at high Cd level and root Cd concentration and total maize Cd uptake at both Cd levels. Earthworm activity decreased Cd transfer from the soil to maize roots at low Cd level, but this was counterbalanced in the presence of F. mosseae. Mycorrhizal symbiosis significantly reduced the transfer of Cd from roots to shoots, independence of earthworm effect. Overall, it is concluded that L. rubellus and AM fungi, in particular F. mosseae isolate, improved maize tolerance to Cd toxicity both individually and interactively by increasing plant growth and P nutrition, and restricting Cd transfer to the aboveground biomass. Consequently, the single and interactive effects of the two soil organisms might potentially be important not only in protecting maize plants against Cd toxicity, but also in Cd phytostabilization in soils polluted by this highly toxic metal.     

Diversity and abundance of earthworms in land use systems in central-western Colombia

The neotropical landscapes of Colombia's Andean region are characterized by a mosaic of agroecosystems presenting a range of vegetational cover for which soil fauna adaptation is still unknown. To analyze the diversity and abundance of earthworm species in relation to changes in selected soil physical and chemical parameters (bulk density, C stock, N stock, %C, %N, ¹³C, ¹⁵N, C:N ratio), these systems were classified into five categories according to land use intensity: non-intensive (NI), low intensity (LI), medium intensity (MI), high intensity (HI), and maximum intensity (IN). The influence of livestock production was confirmed in the significant differences observed in bulk density and ¹⁵N between NI, HI, and IN (P < 0.05). The C and N contents of IN systems (pastures) differed significantly (P < 0.05) in comparison with the other categories. Average δ ¹³C at the three soil depths evaluated (0–10, 10–20, and 20–30 cm) ranged between ?24.9 for LI and ?22.87 for IN, indicating that soil organic carbon was related to C3-type vegetation for all land uses and vegetation covers. Overall, 26 earthworm species were recorded, of which 16 showed a high capacity to adapt to natural, NI, LI, and MI systems. Depending on land use intensity, significant differences were also observed (P < 0.05) in the origin and ecological category of earthworms, mainly in terms of the following variables: diversity of native species, diversity of endogeic species, abundance of native species, biomass of native species, and abundance of exotic species. Based on the results obtained, the most favorable mosaic systems for agrobiodiversity conservation were identified as well as those subsystems that require special management to solve problems of habitat degradation.     

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.     

What are the limits of the drilosphere? An incubation experiment using Metaphire posthuma

The near infrared reflectance spectroscopy (NIRS) method was used in the present study to compare earthworm-made soil aggregates to aggregates found in the surrounding bulk soil. After initially assessing the daily cast production of Metaphire posthuma, boxes with soil incubated with M. posthuma and control soils were subjected to wetting in order to reorganize the soil structure. After two months of incubation, soil aggregates produced by earthworms (casts and burrows), soil aggregates that were appeared to be unaffected by earthworms (bulk soil without visible trace of earthworm bioturbation from the earthworm treatment) and soil aggregates that were entirely unaffected by earthworms (control – no earthworm – treatment) were sampled and their chemical signatures analyzed by NIRS. The production of below-ground and surface casts reached 14.9 g soil g worm^?1 d^?1 and 1.4 g soil g worm^?1 d^?1, respectively. Soil aggregates from the control soils had a significantly different NIRS signature from those sampled from boxes with earthworms. However, within the earthworm incubation boxes the NIRS signature was similar between cast and burrow aggregates and soil aggregates from the surrounding bulk soil. We conclude that the high cast production by M. posthuma and the regular reorganization of the soil structure by water flow in and through the soil lead to a relatively homogenous soil structure. Given these results, we question the relevance of considering the bulk soil that has no visible activity of earthworm activity as a control to determine the effect of earthworms on soil functioning.     

The efficiency of soil hand-sorting in assessing the abundance and biomass of earthworm communities. Its usefulness in population dynamics and cohort analysis studies

Pit digging and manually revising soil blocks is a frequently used method used for field studies of earthworm communities. The aim of this study was to compare the efficiency of hand-sorting (HS) to extract small earthworms, ca. 0.2 g, and the usefulness in studies of population dynamics and cohort analysis. Many earthworms are not recovered when revising manually the soil. Factors include soil characteristics, i.e. moisture, texture, etc. and also a human factor, which is more relevant if the study is conducted in the long-term. We used data collected in a field study of earthworm communities during 2 years in the savannas of Colombia. Small soil blocks (20 × 20 × 20 cm) were dug out in order to collect the smallest earthworms by washing-sieving (WS) and compare the results with the standard HS of large monoliths (100 × 100 × 50 cm). In fact, this methodology has rarely been addressed in earthworm population field studies. Our results showed that HS efficiency varied owing to the species and ranged from 31.4% up to 100% in the savanna and from 44% to 80% in the pasture, for two small species, i.e Aymara n. sp. (epigeic) and Ocnerodrilidae sp. (endogeic). In the case of the Glossodrilus n. sp. (endogeic) these values were similar, i.e. 51.7% and 58.1%, in the savanna and pasture, respectively. We also used frequency tables to calculate the average efficiency of HS 1 m² soil cores for each weight class in each species in order to obtain a population density correction factor. This allowed us to make corrections in earthworm density in the histograms for population dynamics analysis. We conclude that this method should be the modus operandi in long-term earthworm demography studies.     

Earthworms in a 15 years agricultural trial

Alternative cropping systems have been proposed to enhance sustainability of agriculture, but their mid and long-term effects on soil biodiversity should be studied more carefully. Earthworms, having important agro-ecological functions, are regarded as indicators of soil biological health. Species composition, abundance, and biomasses of earthworms were measured in autumn 2005–2007 (period 1) and 2011–2013 (period 2) in a trial initiated in 1997 near Paris, France. A conventional, an organic and a direct seeded living mulch-based cropping systems were compared. Earthworms were sampled in a wheat crop by combining the application of a chemical expellant and hand-sorting.In period 1, earthworm abundance did not usually differ in the three cropping systems, but sometimes it was higher in the conventional system. Mean total abundance was 122, 121 and 149 individuals m⁻² in period 1 and 408, 386 and 216 in period 2 in the organic, living mulch and conventional systems respectively. While earthworm abundance and biomass increased slightly in the conventional system between the two periods, they at least tripled in the other two systems. This was mainly due to the species Aporrectodea caliginosa and Aporrectodea longa in the living mulch cropping system, and to A. caliginosa, Lumbricus castaneus, Lumbricus terrestris and A. longa in the organic system.After at least 14 years, organic and living mulch cropping systems contained between 1.5 and 2.3 times more earthworms than the conventional system. Considering the inter-annual variations in earthworm communities due to climatic conditions and cultural practices, earthworm communities should be assessed over several years before conclusions can be drawn. Moreover, since changes in cultural practices may take a long time to affect earthworm communities, mid and long-term trials are needed to assess the effects of cropping systems on soil biodiversity.     

The combined effects of earthworms and arbuscular mycorrhizal fungi on microbial biomass and enzyme activities in a calcareous soil spiked with cadmium

Earthworms and arbuscular mycorrhizal fungi (AMF) are known to independently affect soil microbial and biochemical properties, in particular soil microbial biomass (SMB) and enzymes. However, less information is available about their interactive effects, particularly in soils contaminated with heavy metals such as cadmium (Cd). The amount of soil microbial biomass C (MBC), the rate of soil respiration (SRR) and the activities of urease and alkaline phosphatase (ALP) were measured in a calcareous soil artificially spiked with Cd (10 and 20 mg Cd kg⁻¹), inoculated with earthworm (Lumbricus rubellus L.), and AMF (Glomus intraradices and Glomus mosseae species) under maize (Zea mays L.) crop for 60 days. Results showed that the quantity of MBC, SRR and enzyme activities decreased with increasing Cd levels as a result of the elevated exchangeable Cd concentration. Earthworm addition increased soil exchangeable Cd levels, while AMF and their interaction with earthworms had no influence on this fraction of Cd. Earthworm activity resulted in no change in soil MBC, while inoculation with both AMF species significantly enhanced soil MBC contents. However, the presence of earthworms lowered soil MBC when inoculated with G. mosseae fungi, showing an interaction between the two organisms. Soil enzyme activities and SRR values tended to increase considerably with the inoculation of both earthworms and AMF. Nevertheless, earthworm activity did not affect ALP activity when inoculated with G. mosseae fungi, while the presence of earthworm enhanced urease activity only with G. intraradices species. The increases in enzyme activities and SRR were better ascribed to changes in soil organic carbon (OC), MBC and dissolved organic carbon (DOC) contents. In summary, results demonstrated that the influence of earthworms alone on Cd availability is more important than that of AMF in Cd-polluted soils; and that the interaction effects between these organisms on soil microorganism are much more important than on Cd availability. Thus, the presence of both earthworms and AMF could alleviate Cd effects on soil microbial life.     

Role of soil water content in the carbon and nitrogen dynamics of Lumbricus terrestris L. burrow soil

We evaluated the role of soil water content in controlling C and N dynamics within the drilosphere created by the anecic earthworm Lumbricus terrestris (L.). Mesocosms (volume = 3.1 l) were each amended with corn litter and three earthworms. Control treatments received no earthworms and no other earthworm species were present in the soil. WET and DRY treatments received a total of 9.25 cm and 3.25 cm of water, respectively. Water was added on weeks 1, 3, 7, and 10 at a rate of 2.0 cm per mesocosm for WET treatments and 0.5 cm per mesocosm for DRY treatments. Mesocosms were sampled destructively after incubation at 18–20 °C for 0, 3, 7, and 13 weeks. The water content of WET burrow soil ranged from 0.12 g g⁻¹ to 0.18 g g⁻¹ and was significantly higher than in the DRY treatment throughout the incubation period. The live weight of earthworms was significantly higher in the WET treatment only on week 13, whereas litter consumption was significantly lower in the DRY treatment for week 13. Carbon mineralization, measured as CO₂ evolved after a 24-h incubation, was consistently higher in WET than in DRY burrow soil. Effects of differences in soil water content were also apparent for biomass C and metabolic quotient. Soil water content did no affect the total C concentration of burrow soil. DRY burrow soil had consistently lower levels of nitrate than WET soil throughout the experiment. Lower levels of ammonium and inorganic N were observed for WET burrow soil on weeks 3 and 7. Water content did not have a significant effect on burrow soil total N. We concluded that the water content of the drilosphere affects both C and N dynamics and can affect the speciation of inorganic N; yet, the effects of soil water content do not appear to result from differences in the feeding activities of anecic earthworms.     

Earthworm invasion alters enchytraeid community composition and individual biomass in northern hardwood forests of North America

European earthworms are invading many ecosystems worldwide and fundamentally transform habitats by acting as dominant ecosystem engineers. However, there is little knowledge of the consequences of earthworm invasion on the composition and diversity of native soil organisms. Particularly functionally similar groups, such as enchytraeids (Annelida: Enchytraeidae), may be affected through changes in the chemical and physical properties of the soil, but also due to competition for resources. In 2010–2011, we studied the impact of earthworm invasion on enchytraeids at two sites in the northern hardwood forests of North America: one site within the Chippewa National Forest in northern Minnesota and one site in the Chequamegon-Nicolet National Forest in northern Wisconsin, USA. At each site, three plots were sampled along a transect, representing (1) a non-invaded or very slightly invaded area, (2) the leading edge of earthworm invasion and (3) a heavily invaded area with an established population of the anecic earthworm Lumbricus terrestris (among other species). In total, 29 enchytraeid (morpho)species were identified (some yet to be formally described, several first or second records for the continent); of those 24 occurred at the Minnesota site and 17 at the Wisconsin site. The structure of enchytraeid assemblages differed significantly among the three invasion stages, although this was not equally pronounced at the two sites. Each stage was characterized by one or several indicator species. Mean enchytraeid densities (10,700–30,400 individuals/m²) did not differ significantly among the invasion stages, but were lowest at the leading edge of earthworm invasion at both sites. In the heavily invaded plot at the Minnesota site, the mean enchytraeid density and biomass in L. terrestris middens were significantly higher than in soil in-between the middens. This was due to a pronounced effect of L. terrestris middens in the uppermost 3 cm of soil. Differences in biomass among earthworm invasion stages were most apparent for mean individual biomass. This was significantly higher in the heavily invaded area than at the leading edge or in the non-invaded area at the Minnesota site. Compositional changes of the enchytraeid assemblage are likely to result in changes in the functioning of soil foods webs. Our results suggest that earthworm invasions can cause a loss of native species in soil, including heretofore unknown ones, that might go unnoticed.