不同秸秆施用方式下接种蚯蚓对土壤团聚体及其中碳分布的影响

通过室内培养试验,研究在施用有机物条件下接种蚯蚓对土壤团聚体的分布、团聚体的水稳性以及不同粒径的水稳性团聚体中有机碳含量的影响.干筛结果表明,不同秸秆施用方式下,蚯蚓接种能显著促进各个处理中＞2 mm团聚体含量的增加,且在秸秆混施的处理中表现得尤为明显,团聚体含量增加了2.95倍;湿筛结果表明,蚯蚓在不施和混施秸秆的处理中能显著降低土壤黏砂粒含量,即增加土壤中水稳性团聚体的含量,但是在表施秸秆的处理中显著降低了0.25～ 0.053 mm粒级团聚体含量,使之分散为黏砂粒.蚯蚓和秸秆对土壤团聚体分布和水稳性的影响都达到显著水平.蚯蚓对水稳性微团聚体的影响极显著,而秸秆的作用更多地表现在水稳性大团聚体上.在秸秆表施和秸秆混施条件下,接种蚯蚓均显著促进了微团聚体碳含量的增加,分别为相应对照的2.1和1.2倍.蚯蚓作用能显著降低黏砂粒有机碳在全碳中含量,增加团聚体有机碳含量,主要是由于蚯蚓的作用能促进黏砂粒黏结为团聚体.     

Earthworm channels in cultivated clayey and loamy Norwegian soils

Studies of earthworm species and their activity, expressed as channels, on cultivated loamy (Humic Cryaquept) and clayey soils (Typic Endoaqualf and Typic Cryaquept) were conducted in southern and central Norway before conversion from conventional to organic cropping systems. At all the three study areas: Landvik (Grimstad), Voll (Ås) and Kvithamar (Stjørdal), the earthworm species Aporrectodea caliginosa, Aporrectodea rosea, Allolobophora chlorotica and Lumbricus rubellus were found. At Landvik, where the deep-burrowing species Aporrectodea longa, A. caliginosa and Lumbricus terrestris dominated, medium (4–6 mm) and coarse (>6 mm) earthworm channels were most numerous below the plough layer. Almost no coarse pores were found at Voll and Kvithamar. The volume of earthworm channels below the plough layer was 0.6–0.8% of total soil volume at Landvik and Voll and 0.3–0.4% at Kvithamar. Earthworm channels >6 mm below the plough layer were useful to identify present and previous activity of deep-burrowing earthworms such as L. terrestris. Below the plough layer, roots were almost entirely restricted to earthworm channels and interaggregate spaces.     

Interactions of an introduced shrub and introduced earthworms in an Illinois urban woodland: Impact on leaf litter decomposition

This study examined an ‘invasional meltdown’, where the invasion of a Midwestern woodland by an exotic shrub (Rhamnus cathartica L.P. Mill) and the invasion by Eurasian earthworms facilitated one another. Using a litterbag approach, we examined mass loss of four substrates (R. cathartica, Acer saccharum, Quercus rubra, and Quercus alba) along a gradient of Eurasian earthworm density and biomass throughout a 40.5 ha oak woodland in Glencoe, Illinois. Earthworm densities and biomass were greatest in patches where R. cathartica prevailed, and populations were lowest in an upland forest subcommunity within the woodland. At each of three points along this earthworm gradient, we placed replicated litterbags constructed either to permit or to deny access to the litter by earthworms. The treatments were, therefore, plot treatments (low, medium and high earthworm density and biomass) and litterbag treatments (earthworm access and earthworm excluded). We found that earthworms promoted a very rapid loss of litter from R. cathartica bags. Within 3 months greater than 90% of this litter was lost from the litterbags. Earthworm impacts on other substrates followed the sequence A. saccharum>Q. alba=Q. rubra. Effects of both litterbag and plot treatments were found within 3 months for A. saccharum but Quercus species were affected only after a year. We propose that the impact of earthworms on litter breakdown creates conditions that promote and sustain invasion by R. cathartica. Previous work has demonstrated that R. cathartica may alter soil properties in a way that promotes and sustains invasion by earthworms. These findings have implications for the restoration management of these systems, since the legacy of R. cathartica on soil properties and earthworm populations may persist even after the plant has been physically removed.     

Role of the anecic earthworm Lumbricus terrestris L. in the distribution of plant residue nitrogen in a corn (Zea mays)–soil system

While the benefits of earthworms to crop production are widely acknowledged, the mechanisms involved are poorly understood. We examined the effects of an anecic earthworm (Lumbricus terrestris) on the distribution of plant residue N in a corn (Zea mays)/soil system. Soil (mixed Ap and B horizons) mesocosms (10 cm diameter, 39 cm deep) were amended with ¹⁵N-labeled corn litter, inoculated with one earthworm per mesocosm (WORM) or none (CTRL), and pre-incubated for 1, 2 or 3 weeks. Earthworms and remaining plant residues were removed and sweet corn grown in the mesocosms in a greenhouse for 3 weeks. Litter, earthworms, shoots, roots and bulk and burrow soil were analyzed for total N and ¹⁵N. Plant and earthworm biomass were also determined. Earthworms had no significant effect on the N content of shoots, roots or bulk soil. Recovery of ¹⁵N ranged from 92.6 to 101.9% in CTRL and 60.2 to 83.2% in the WORM treatment. The ¹⁵N content of bulk soil in the WORM treatment was significantly higher than in CTRL and increased with pre-incubation time. Excess at.% ¹⁵N of burrow soil was 10–100 times higher than in bulk soil. Incorporation of ¹⁵N by shoots and roots was significantly higher in the WORM treatment and increased significantly with pre-incubation time only in the WORM treatment. In WORM mesocosms pre-incubated for 3 weeks, the distribution of added ¹⁵N was 9.8% in litter, 6.5% in plant, 31.5% in soil, 12.0% in earthworms and 39.8% presumably lost as gas; in CTRL mesocosms, the values were 75.7% in litter, 3.2% in plant, 13.7% in soil and 7.4% in presumed gas losses. The activities of L. terrestris altered the distribution of plant residue N significantly, increasing the transfer of N to plants and soil and enhancing losses of N in the gas phase as pre-incubation time increased.     

Indirect effects of earthworms on microbial assimilation of labile carbon

Interactions between earthworms and microorganisms can be important in regulating the rate of soil carbon turnover and maintaining soil fertility in agroecosystems. Despite the significance of earthworms in nutrient cycling in agroecosystems, the indirect influence of earthworms on C assimilation by microorganisms has not been adequately quantified. We assessed microbial assimilation of ¹³C-labeled acetate in earthworm (Lumbricus terrestris) middens and surrounding soil collected from maize agroecosystems. Incorporation of ¹³C into microbial lipids was used as an indicator of microbial growth rates. Earthworm middens had significantly lower concentrations of microbial phospholipid phosphates and lower natural abundance δ¹³C than the surrounding soil. After incubation with ¹³C-labeled acetate, microbial communities in earthworm middens had greater ¹³C/¹²C ratios of microbial lipids than microbial communities from surrounding soil. The ¹³C enrichment per unit of microbial phospholipid was much greater in middens than in surrounding soil indicating that: (i) microbial lipid synthesis was significantly higher in the earthworm middens; (ii) microbial assimilation efficiency for ¹³C-labeled acetate was greater in midden soil; or (iii) assimilation of ¹³C-labeled acetate relative to other C sources was proportionately greater in middens than in the surrounding soil. Our results suggest that there were functional differences between microbial communities in earthworm middens and surrounding soil, probably due to a combination of physical, chemical, and biological changes in the midden microenvironment. The resulting differences in microbial communities or activity increased microbial growth rates and assimilation of readily available C substrates in middens relative to surrounding soil.     

Earthworm influence on carbon dioxide and nitrous oxide fluxes from an unfertilized corn agroecosystem

Earthworms modify the soil environment through their feeding, casting, and burrowing activities, which may lead to more decomposition and respiration in aerobic microsites and more denitrification in anaerobic microsites. The objective of this study was to determine whether earthworms increase CO₂ and N₂O fluxes from an unfertilized corn agroecosystem. Earthworm populations within field enclosures (2.9 m²) were reduced by repeatedly applying carbaryl insecticide, then single and mixed populations of Lumbricus terrestris L. and Aporrectodea caliginosa (Savigny) were added. Gas samples were collected once a week for 14 weeks, from June to September 2005. Carbaryl applications reduced, but did not eliminate earthworms from enclosures. The CO₂ and N₂O fluxes were affected by the sampling date, with peak gas fluxes after rainfall events. Mean CO₂ and N₂O fluxes during the study period tended to be greater from enclosures with added earthworms than the control (no earthworms added), but were not significantly affected by earthworm treatments due to the low survival rate of introduced earthworms. Better control of earthworm populations in the field is required to fully assess the impact of earthworms on CO₂ and N₂O fluxes from temperate agroecosystems.     

Earthworm community structure on five English golf courses

A clear understanding of the size and structure of earthworm communities is important to sports turf facilities managers if they are to control the activity of earthworms within the soil. Earthworms are directly linked to a wide range of biogeochemical nutrient cycles, and are frequently described as ecosystem engineers. In this role they assist land managers in maintaining a healthy turf sward. Despite this, earthworm populations in sports turf were frequently suppressed using organochloride based vermicides prior to such compounds being banned in the UK. A survey of earthworm species diversity was carried out over 1 year, using mustard extraction at five golf courses in Bedfordshire and Buckinghamshire, UK, to investigate how species diversity varied temporally and was influenced by the physical environment within different course surfaces. Rank–abundance analysis indicated that four species of earthworm (Aporrectodea rosea, Lumbricus rubellus, Aporrectodea longa and Lumbricus terrestris) were more dominant in the community than any other. The endemic earthworm population number of the five study sites was found to differ significantly (p < 0.01) related to their geographical location, and the most diverse communities were found where the population was greatest. A very strong linear correlation was found between the number of earthworms recovered and the sand content of the soil (r² = 0.97). Significant differences in the size of the earthworm populations were recorded at different times of the year (p < 0.01). A distinct and non-linear relationship between species diversity and microbial biomass C was shown (p < 0.01).     

Earthworm casts form stable aggregates in physically degraded soils

Topsoils affected by surface mining suffer severe physical degradation and lose most of their earthworm populations. After mining, replaced soils are planted to grassland and managed to improve soil structure. Earthworm inoculation into selected restored areas produced populations similar to those of undisturbed soils within 3 years. Soil properties in inoculated areas were compared with those of controls to evaluate the contribution of casts to bulk soil aggregation, and soil organic matter and root content responses to earthworm activity. Crumb porosity and coarse particle content were measured in water-stable macro-aggregates and earthworm casts to establish whether aggregates were formed by earthworms. Over a 5- to 6-year period, inoculation increased stable aggregation (>2 μm, >60 μm and >3 mm), even at 0- to 5-cm depth where it reduced soil organic matter content. Productivity and root content were also increased by inoculation; roots and organic matter were re-distributed to greater depth. Crumb porosity decreased with casts > aggregates (inoculated plots) > aggregates (control plots). Coarse particle content increased with casts < aggregates (inoculated plots) < aggregates (control plots). Coarse particle and porosity data were consistent with much of the newly aggregated soil being processed and formed by earthworms as casts. Whilst levels of soil organic matter were often closely associated with percentage stable aggregation, root content showed weaker associations. Aggregation percentage was most closely associated with abundance of Aporrectodea longa, although at particular depths significant correlations were also obtained for Aporrectodea caliginosa and Lumbricus terrestris. Results suggest that earthworms, rather than plant roots, initiate aggregation in severely degraded grassland soils.     

Understory vegetation,soil structure and soil invertebrates in Congolese eucalypt plantations,with special reference to the invasive plant Chromolaena odorata and earthworm populations

Earthworm relationships with vegetation have received extensive attention, and earthworm density has been shown to be related to vegetation types or plant species. However, the factors involved are rarely known. In Congo, we studied the effect of Chromolaena odorata (L) R.M. King & H. Robinson, which invades eucalypt plantations, on soil invertebrates, especially earthworms. In order to investigate relationships between vegetation cover and soil invertebrates, four understory species, including C. odorata, were studied. Also, comparisons were made between plots invaded by C. odorata and plots free from it. The addition of leaf litter on experimental plots was made in order to check its influence. Plant remains were observed in the digestive tract of earthworms. An increased earthworm density was observed under C. odorata. The leaf litter and roots of this species had low lignin/N ratio. The size of leaf fragments found in the digestive tract of the earthworms, and the lack of short-term effect of experimentally added leaf litter, suggested that litter quality could influence earthworm through their feeding on fine particulate top soil organic matter. The amount of soil aggregates, in the size classes that fitted the size range of earthworm casts, was increased under C. odorata. More field experiments are needed to establish a causal effect in the relationships observed between earthworm density and C. odorata. If so, the major drawbacks, such as water and nutrient competition, resulting from C. odorata overrunning the plantations, could be somewhat offset by its positive effects through soil improvement.     

Earthworm impacts on soil organic matter and fertilizer dynamics in tropical hillside agroecosystems of Honduras

Earthworms are important processors of soil organic matter (SOM) and nutrient turnover in terrestrial ecosystems. In agroecosystems, they are often seen as beneficial organisms to crop growth and are actively promoted by farmers and extension agents, yet their contribution to agroecosystem services is uncertain and depends largely on management. The Quesungual slash-and-mulch agroforestry system (QSMAS) of western Honduras has been proposed as a viable alternative to traditional slash-and-burn (SB) practices and has been shown to increase earthworm populations, yet the effect of earthworms on soil fertility and SOM in QSMAS is poorly understood. This study examined the role of Pontoscolex corethrurus in QSMAS by comparing their influence on aggregate-associated SOM and fertilizer dynamics with their effects under SB and secondary forest in a replicated field trial. Both the fertilized QSMAS and SB treatments had plots receiving additions of inorganic ¹⁵N and P, as well as plots with no inorganic N additions. Earthworm populations were manipulated in field microcosms at the beginning of the rainy season within each management treatment via additions of P. corethrurus or complete removal of existing earthworm populations. Microcosms were destructively sampled at harvest of Zea mays and soils were wet-sieved (using 53, 250 and 2000 μm mesh sizes) to isolate different aggregate size fractions, which were analyzed for total C, N and ¹⁵N. The effects of management system were smaller than expected, likely due to disturbance associated with the microcosm installation. Contrary to our hypothesis that earthworms would stabilize organic matter in soil aggregates, P. corethrurus decreased total soil C by 3% in the surface layer (0-15 cm), predominantly through a decrease in the C concentration of macroaggregates (>250 μm) and a corresponding depletion of C in coarse particulate organic matter occluded within macroaggregates. Earthworms also decreased bulk density by over 4%, but had no effect on aggregate size distribution. Within the two fertilized treatments, the QSMAS appeared to retain slightly more fertilizer derived N in smaller aggregate fractions (<250 μm) than did SB, while earthworms greatly reduced the recovery of fertilizer N (34% decrease) in both systems. Although management system did not appear to influence the impact of P. corethrurus on SOM or nutrient dynamics, we suggest the lack of differences may be due to artificially low inputs of fresh residue C to microcosms within all management treatments. Our findings highlight the potential for P. corethrurus to have deleterious impacts on soil C and fertilizer N dynamics, and emphasize the need to fully consider the activities of soil fauna when evaluating agroecosystem management options.     

Endogeic and anecic earthworm abundance in six Midwestern cropping systems

Endogeic and juvenile anecic earthworm abundance was measured in soil samples and anecic populations were studied by counting midden numbers at the sites of two long-term cropping systems trials in South-central Wisconsin. The three grain and three forage systems at each site were designed to reflect a range of Midwestern USA production strategies. The primary objectives of this work were to determine if the abundance of endogeic or anecic earthworms varied among cropping systems or crop phases within a cropping system and were there specific management practices that impacted endogeic or anecic earthworm numbers. The earthworms present in the surface soil were: Aporrectodea tuberculata (Eisen), A. caliginosa (Savigny), A. trapezoides (Dugés); and juvenile Lumbricus terrestris (L.). True endogeic abundance was greatest in rotationally grazed pasture [188 m^?2 at Arlington (ARL) and 299 m^?2 at Elkhorn (ELK)], and smallest in conventional continuous corn (27 m^?2 at ARL and 32 m^?2 at ELK). The only type of anecic earthworm found was L. terrestris L. There was an average of 1.2 middens per adult anecic earthworm and the population of anecics was greatest in the no-till cash grain system (28 middens m^?2 at ARL, 18 m^?2 at ELK) and smallest in the conventional continuous corn system (3 middens m^?2 at ARL, 1 m^?2 at ELK). Earthworm numbers in individual crop phases within a cropping system were too variable from year-to-year to recommend using a single phase to characterize a whole cropping system. Indices for five management factors (tillage, manure inputs, solid stand, pesticide use, and crop diversity) were examined, and manure use and tillage were the most important impacting earthworm numbers across the range of cropping systems. Manure use was the most important management factor affecting endogeic earthworm numbers; but no-tillage was the most important for the juvenile and adult anecic groups and had a significantly positive influence on endogeic earthworm counts as well. The pesticides used, which were among the most commonly applied pesticides in the Midwestern USA, and increasing crop diversity did not have a significant effect on either the endogeic or anecic earthworm groups in this study. Consequently, designing cropping systems that reduce tillage and include manure with less regard to omitting pesticides or increasing crop diversity should enhance earthworm populations and probably improve sustainability.     

Earthworm populations and growth rates related to long-term crop residue and tillage management

Conventional tillage creates soil physical conditions that may restrict earthworm movement and accelerate crop residue decomposition, thus reducing the food supply for earthworms. These negative impacts may be alleviated by retaining crop residues in agroecosystems. The objective of this study was to determine the effects of various tillage and crop residue management practices on earthworm populations in the field and earthworm growth under controlled conditions. Population assessments were conducted at two long-term (15+ years) experimental sites in Québec, Canada with three tillage systems: moldboard plow/disk harrow (CT), chisel plow or disk harrow (RT) and no tillage (NT), as well as two levels of crop residue inputs (high and low). Earthworm growth was assessed in intact soil cores from both sites. In the field, earthworm populations and biomass were greater with long-term NT than CT and RT practices, but not affected by crop residue management. Laboratory growth rates of Aporrectodea turgida (Eisen) in intact soil cores were affected by tillage and residue inputs, and were positively correlated with the soil organic C pool, suggesting that tillage and residue management practices that increase the soil organic C pool provide more organic substrates for earthworm growth. The highest earthworm growth rates were in soils from RT plots with high residue input, which differed from the response of earthworm populations to tillage and residue management treatments in the field. Our results suggest that tillage-induced disturbance probably has a greater impact than food availability on earthworm populations in cool, humid agroecosystems.     

Effects of adult Lumbricus terrestris on cocoons and hatchlings in Evans’ boxes

The basic ecology of well-researched earthworm species, such as Lumbricus terrestris, remains relatively unknown. In order to contribute to the limited body of knowledge, the present work aimed to investigate if specific behaviours could be detected between an adult and the cocoons it produced by comparison with a non-parental adult and cocoons. Modified Evans’ boxes (0.8×0.2×0.008 m) with recently mated adults allowed observation of their habits under what was essentially a 2-D system. Experimental manipulation (at 81 days) involved four treatments: (1) L. terrestris removal (LtRm); (2) L. terrestris replacement (LtRp), where individuals were replaced by others; (3) control, with no manipulation and (4) L. terrestris removal control (CLtRm) where individuals were removed and then reintroduced in the same box, to account for any effects of the removal method. Burrowing patterns of adults and later, hatchlings, were recorded throughout the duration of the experiment (191 days). Cocoon production across treatments did not differ significantly (p>0.05) and averaged 26±6.4 cocoons worm⁻¹ over a 26 week period. In the LtRm treatment mean cocoon production was 12.6±2.7 cocoons worm⁻¹ over 11 weeks. Forty one percent of cocoons were deposited in the upper 0.05 m of the soil profile. Cocoons below that depth were found deposited in side burrows (1–4 cm in length) and were coated with earthworm casts. Hatchling number and survival rate did not differ significantly across treatments (p>0.05) with means±s.d. of 2±2.2 and 62±29%, respectively. Hatchling mass in the LtRm treatment (0.4±0.38 g), was found to be significantly greater (p<0.01) than that of all other treatments (0.12±0.12 g). In this experimental set up, the presence of parental earthworms and other con-specifics had no significant effect on cocoon viability and hatchling survival, however, results indicated a negative effect of adult presence on hatchling growth.     

Earthworm activity affects soil aggregation and organic matter dynamics according to the quality and localization of crop residues—An experimental study (Madagascar)

Soil organic matter (SOM) plays a central role in the functioning of ecosystems, and is beneficial from agronomic and from environmental point of view. Alternative cultural systems, like direct seeding mulch-based cropping (DMC) systems, enhance carbon (C) sequestration in agricultural soils and lead to an increase in soil macrofauna. This study aimed at evaluating in field mesocosms the effects of earthworms on SOM dynamics and aggregation, as influenced by residue quality and management.In the highlands of Madagascar, buckets were filled with 2 mm-sieved clayey Inceptisol. The effects of earthworm addition (Pontoscolex corethrurus), residue addition (rice, soybean, and no addition), and localization of the residues (mulched or buried) were studied. After 5 months, soil from mesocosms with earthworms had significantly lower C concentration and higher proportion of large water-stable macroaggregates (>2000 μm) than those without earthworms, because of the production of large macroaggregates by earthworms. Earthworm effect on soil aggregation was greater with rice than with soybean residues. Casts (extracted from mesocosms with earthworms) were slightly enriched in C and showed significantly higher mineralization than the non-ingested soil (NIS), showing that at the time scale of our study, the carbon contained in the casts was not protected against mineralization. No difference in microbial biomass was found between casts and NIS.Complementary investigations are necessary to assess long-term effects of earthworm addition on SOM dynamics, the conditions of occurrence of physical protection, and the impact of earthworms on the structure of the microbial community.     

Organic matter addition, N, and residue burning effects on infiltration, biological, and physical properties of an intensively tilled silt-loam soil

Seventy years of different management treatments have produced significant differences in runoff, erosion, and ponded infiltration rate in a winter wheat (Triticum aestivum L.)–summer fallow experiment in OR, USA. We tested the hypothesis that differences in infiltration are due to changes in soil structure related to treatment-induced biological changes. All plots received the same tillage (plow and summer rod-weeding). Manure (containing 111 kg N ha⁻¹), pea (Pisum sativum L.), vine (containing 34 kg N ha⁻¹), or N additions of 0, 45 and 90 kg ha⁻¹ were treatment variables with burning of residue as an additional factor within N-treatments. We measured soil organic C and N, water stability of whole soil, water stable aggregates, percolation through soil columns, glomalin, soil-aggregating basidiomycetes, earthworm populations, and dry sieve aggregate fractions. Infiltration was correlated (r = 0.67–0.95) to C, N, stability of whole soil, percolation, and glomalin. Basidiomycete extracellular carbohydrate assay values and earthworm populations did not follow soil C concentration, but appeared to be more sensitive to residue burning and to the addition of pea vine residue and manure. Dry sieve fractions were not well correlated to the other variables. Burning reduced (p < 0.05) water stability of whole soil, total glomalin, basidiomycetes, and earthworm counts. It also reduced dry aggregates of 0.5–2.0 mm size, but neither burning nor N fertilizer affected total C or total N or ponded infiltration rate. Water stability of whole soil and of 1–2-mm aggregates was greater at 45 kg N ha⁻¹ than in the 0 and 90 kg N ha⁻¹ treatments. Zero N fertilizer produced significantly greater 0.5–2.0 mm dry aggregate fractions. We conclude that differences in infiltration measured in the field are related to relatively small differences in aggregate stability, but not closely related to N or residue burning treatments. The lack of an effect of N fertilizer or residue burning on total C and N, along with the excellent correlation between glomalin and total C (r = 0.99) and total N (r = 0.98), indicates that the major pool of soil carbon may be dependent on arbuscular mycorrhizal fungi.     

Reducing tillage in cultivated fields increases earthworm functional diversity

Alternative cropping systems such as conservation agriculture have been implemented to limit the harmful effects of intensive conventional cropping systems. Conservation agriculture is known to modify the structural diversity of earthworm communities, but no data have been reported so far on their functional diversity. Structural and functional indices of community were used to study the effects of different soil tillage intensity on earthworm diversity in arable soils.Field data were collected in four agricultural trials across France representing different soiland climatic conditions. Three types of soil tillage were assessed: plowing, superficial tillage and direct seeding. Earthworm abundance, species richness and ecomorphological group abundance were investigated. Seven functional traits, i.e. body length, body mass/length ratio, epithelium type, cocoon diameter, typhlosolis type, carbon preferences and vertical distribution, were selected according to their hypothesized link with mechanisms of tillage impact. Functional diversity indices were then computed. Soil tillage intensity decreased functional diversity and modified the functional trait profile within the earthworm community whereas neither structural diversity (species number) nor abundance changed with tillage intensity. Differences between plowing and direct seeding were significant in each trial, and superficial tillage often showed intermediate trait values. Regarding ecomorphological groups, anecic abundance was positively influenced by a decrease in soil tillage, contrary to epigeic and endogeic earthworms that showed no response. Tillage acts as an environmental filter, and decreasing its intensity caused a lesser convergence of traits and thus higher functional trait diversity. We demonstrated that a trait-based approach better permitted comparisons of community responses across sites than species number or abundance.     

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     

Phospholipid fatty acid profiles and xylanase activity in particle size fractions of forest soil and casts of Lumbricus terrestris L. (Oligochaeta,Lumbricidae)

Earthworms are key agents in organic matter decomposition, as they remove surface plant litter material and mix it with mineral soil. Plant litter material is comminuted in the gizzard of anecic earthworms and this is enhanced if sand particles are available. We hypothesize that this comminution of soil and litter will result in changes in the distribution of soil organic matter and soil microorganisms in the different particle-size fractions. We investigated soil organic matter content, xylanase- and microbial activity and community structure in bulk soil and particle size fractions of Lumbricus terrestris L. casts and in soil with and without the addition of beech litter.Earthworm gut passage did not affect the particle-size distribution but the content of soil organic matter was decreased in the fine sand fraction in treatments without litter (−6.80%) and increased in treatments with litter (+33.23%). The soil organic matter content of the clay fraction tended to be higher in earthworm casts. Xylanase activity was at a maximum in the fine sand fraction, lower in the coarse sand fraction and at a similar minimum in the silt- and clay-sized fraction. In the coarse sand fraction of the cast and litter treatments xylanase activity was increased by 39.1% and 124.8%, respectively. In the silt-sized fraction of casts the addition of litter increased xylanase activity (+58.6%) whereas, in casts without litter it was decreased (−36.25%). In the particle size fractions of casts, the content of bacterial PLFAs was decreased in the fine sand fraction and tended to be decreased in the clay fraction compared to the respective fractions in soil. In the silt fraction the fungal-to-bacterial PLFA ratio was higher in casts than in soil.We conclude that earthworms stabilize soil organic matter in cast aggregates predominantly by increasing the soil organic matter content in the clay fraction where it becomes protected against microbial attack. Organic matter in the coarse and fine sand fractions is decomposed primarily by fungi; xylanase is very active in these sand fractions and incorporation of litter into these fractions by the earthworms increased fungal biomass. Comminution of litter during passage through the earthworm gut increased the biomass and activity of fungi also in the silt fraction. The use of PLFA profiles in combination with other quantitative microbial methods improves the understanding of stabilizing and mobilizing processes in earthworm casts.     

Nitrogen source and earthworm abundance affected runoff volume and nutrient loss in a tilled-corn agroecosystem

A simulated rainfall event was used to determine the influence of different N sources and earthworm density on surface hydrology and quality of runoff in a tilled-corn cropping system. N was added to spring-disked, continuous-corn plots as chopped legume, manure, or NH₄NO₃. Earthworm populations were manipulated in fertility treatments by reductions, additions, or left as ambient populations. We found that runoff volume and the NH₄⁺ and NO₃^- concentration of runoff were greater in inorganic treatments, compared with the organic N sources. Sediment load, and its total C and total N components were lower in manure treatments than chopped legume and inorganic N treatments. Although earthworm manipulations did not affect runoff volume, more sediment and sediment-associated C and N were lost from the reduction earthworm treatment, compared to the ambient treatment. Multiple regression models showed that surface hydrology and sediment flux were associated with incorporated residue levels, earthworm population, macropore and midden density. Our results indicate that water, nutrient and soil conservation could be improved by the use of organic N fertility sources and that earthworm impact varies with earthworm population, species composition, and incorporated residue level.