Microfungal communities in soil,litter and casts of Lumbricus terrestris L. (Lumbricidae): a laboratory experiment

The anecic earthworm Lumbricus terrestris L. was kept in laboratory microcosms containing beech forest soil without litter, with beech leaf litter or with lime leaf litter. The structure of microfungal communities in soil, litter and fresh and aged (100 days) earthworm faeces was analysed using the washing and plating technique. The passage of mineral soil through the gut of L. terrestris affected the structure of the fungal community only little. In contrast, in the litter treatments the structure of the fungal community in fresh earthworm casts significantly differed from that in soil and litter. The majority of soil and litter inhabiting fungi survived passage through the gut of L. terrestris and the fungal community in casts consisted of a mixture of soil and litter inhabiting fungi. However, the frequency of Cladosporium spp., Alternaria spp., Absidia spp., and other taxa was strongly reduced in fresh casts. The degree of colonization of litter particles (number of isolates per number of plated particles) also decreased, but some fungi (mainly Trichoderma spp.) benefited from gut passage and flourished in fresh casts. During ageing of cast material the dominance structure of the fungal community changed. Both the degree of colonization of organic particles and the species diversity increased and approached that in soil. However, the structure of the fungal community in casts remained cast specific even after 100 days of incubation. It is concluded that the feeding and burrowing activity of L. terrestris accelerates the colonization of litter by the edaphic mycoflora but also extends the range of occurrence of litter-associated fungi into mineral soil layers.     

Chemical changes of beech litter during passage through the gut of the lumbricid earthworm Eisenia fetida (SAV.)

Casts of the lumbricid earthworm species E. fetida were investigated by means of chemical analyses and ¹³C NMR spectroscopy and compared to the beech litter used as food. The casts are characterized by slightly reduced C-to-N ratios, higher ash contents and lower polysaccharide and lignin concentrations in comparison with the litter material. These differences between casts and litter are very small, ranging from 1 to 7%, and are not supported by any statistical evidence. In addition, NMR and Py-FIMS data complete the picture of E. fetida casts as almost exclusively physically altered (i.e. comminuted) plant residues.     

Characteristics of earthworm casts affecting herbicide sorption and movement

Various physical and chemical characteristics of earthworm casts collected from a laboratory incubation and a field experiment were examined in relation to their effect on the sorption and the movement of three ¹⁴C-labelled ionic herbicides: atrazine, 2,4-dichlorophenoxyacetic acid, and metsulforon methyl. The earthworm casts contained higher levels of fine fractions and total and soluble C. This is attributed to the grinding action of the earthworm gut and selective feeding on zones with higher organic matter and fine size fractions. The earthworm casts had a higher pH than the source soil, resulting in a higher number of surface negative charges. The earthworm casts sorbed higher amounts of herbicides than the source soil, mainly due to the increases in the amount of organic C and fine size fractions. The incrased sorption of herbicides by the casts resulted in decreased leaching.     

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.     

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.     

The interactions between soil type and earthworm species determine the properties of earthworm casts

Earthworms are recognized to increase soil porosity, reorganize soil structure, and stimulate soil microflora and nutrient mineralization. The properties of earthworm casts should depend both on earthworm species or ecological group and on soil properties. Interactions between earthworm species and soil types have been suggested, but only poorly demonstrated. In order to better understand those interactions, two hypotheses led our study: (1) Soil type has a greater influence on cast properties than earthworm; (2) Earthworms from different species influence cast properties differently; (3) The intensity and direction of the impact of each earthworm species on cast properties vary with soil properties. Fifteen physical and chemical variables (N–NH₄⁺, N–NO₃⁻, total organic C and N, C/N ratio, CaCO₃, pH, P, K⁺, Mg²⁺, Mn²⁺, Na⁺, CEC, moisture, wettability) were measured in casts of three earthworm species (Lumbricus terrestris, Allolobophora chlorotica and Aporrectodea rosea) produced in three temperate soils. Univariate and multivariate analyses showed that earthworm species and soil types significantly impacted cast properties. pH, N_t, K and Mg contents were interactively altered by both factors. Multivariate analysis showed that a difference of soil type had a major impact on casts properties (62%) compared to the impact of a difference of earthworm species (10%). Cast properties were most impacted by L. terrestris, then by A. chlorotica and last by A. rosea. The response ratio (ratio of the properties of the casts to the properties of the bulk soil) was used to quantify the effect of earthworm species compared to the control soil. It showed a higher response of variables in casts in nutrient-rich soils, especially in casts of L. terrestris. The interactions between earthworm species and soil types on cast properties were discussed with regards to earthworm ecology, properties of the soil, and earthworm modifications of cast microflora.     

Changes in the chemical composition of soil organic matter after application of compost

Is the composition of soil organic matter changed by adding compost? To find out we incubated biowaste composts with agricultural soils and a humus‐free mineral substrate at 5°C and 14°C for 18 months and examined the products. Organic matter composition was characterized by CuO oxidation of lignin, hydrolysis of cellulosic and non‐cellulosic polysaccharides (CPS and NCPS) and ¹³C cross‐polarization magic angle spinning nuclear magnetic resonance (CPMAS ¹³C‐NMR) spectroscopy. The lignin contents in the compost‐amended soils increased because the composts contained more lignin, which altered little even after prolonged decomposition of the composts in soil. A pronounced decrease in lignin occurred in the soils amended with mature compost only. Polysaccharide C accounted for 14–20% of the organic carbon at the beginning of the experiment for both the compost‐amended soils and the controls. During the incubation, the relative contents of total polysaccharides decreased for 9–20% (controls) and for 20–49% (compost‐amended soils). They contributed preferentially to the decomposition as compared with the bulk soil organic matter, that decreased between < 2% and 20%. In the compost‐amended agricultural soils, cellulosic polysaccharides were decomposed in preference to non‐cellulosic ones. The NMR spectra of the compost‐amended soils had more intense signals of O–alkyl and aromatic C than did those of the controls. Incubation for 18 months resulted mainly in a decline of O–alkyl C for all soils. The composition of the soil organic matter after compost amendment changed mainly by increases in the lignin and aromatic C of the composts, and compost‐derived polysaccharides were mineralized preferentially. The results suggest that decomposition of the added composts in soil is as an ongoing humification process of the composts themselves. The different soil materials affected the changes in soil organic matter composition to only a minor degree.     

Changes in potassium availability and other soil properties due to soil ingestion by earthworms

An incubation experiment was conducted to study the changes that occur in potassium availability and other soil properties with ingestion of soil by earthworms. Two soils were used. Raumai soil with high non-exchangeable K and Milson soil with low non-exchangeable K were incubated with two species of earthworm, Aporrectodea caliginosa and Lumbricus rubellus, for 8 weeks. The casts and soil samples were analysed for exchangeable K, Ca, Mg, Na, and H, pH, organic C, and texture. The results indicated that in Raumai soil, the exchangeable K levels of the casts of both earthworm species were significantly higher than for the control soil, the effect being more marked for L. rubellus than for A. caliginosa. In Milson soil, the exchangeable K levels were significantly lower in the casts of both types of earthworm than in the control soil. The nitric acid-extractable K of the soil and casts was not markedly different for either soil type, but available non-exchangeable K values were significantly higher for the casts of L. rubellus from Milson soil than for the noningested Milson soil. In Raumai soil, the exchangeable Ca was higher in the casts of L. rubellus, exchangeable Mg and H were reduced, and exchangeable Na did not change markedly in the cast compared to the control soil. For Milson soil, the casts contained lower exchangeable Ca and H but higher Na and Mg than the control. The casts of both species of earthworm had significantly higher pH values for both soil types. There was no marked difference in the organic C content of the control soil and cast samples for Milson but a reduction in the casts of A. caliginosa for the Raumai soil. Finer fractions increased in the casts of both earthworm species in both soil types.     

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.     

Application of diffuse reflectance FT-IR spectroscopy and partial least-squares regression to predict NMR properties of soil organic matter

Carbon 13 nuclear magnetic resonance spectroscopy (¹³C NMR) is a powerful technique for studying the structure and turnover of soil organic matter, but is time consuming and expensive. It is therefore worth seeking swifter and cheaper methods. Diffuse reflectance FT‐IR spectroscopy (DRIFT), along with partial least squares (PLS) algorithms, provides statistical models to quantify soil properties, such as contents of C, N and clay. I have applied DRIFT?PLS to quantify soil organic C species, as measured by solid state ¹³C NMR spectroscopy, for several bulk soils and physical soil fractions. Calibration and prediction models for organic C and for particular NMR regions, namely alkyl C, O?alkyl C and carboxyl C, attained R² values of between 0.94 and 0.98 (calibration) and 0.70–0.93 (cross‐validation). The prediction of unknown soil samples, after pre‐selection by statistical indices, confirmed the applicability of DRIFT?PLS. The prediction of aromatic C failed, probably because of superimposition of aromatic bands by signals from minerals. Results from fractions of particulate organic matter suggest that the chemical homogeneity of the material hampers the quantification of its constituting C species by DRIFT?PLS. For alkyl C, prediction of carbon species by DRIFT?PLS was better than direct peak‐area quantification in the IR spectra, but advantageous in parts only compared with a linear model correlating C species with soil C contents. In conclusion, DRIFT?PLS calibrated with NMR data provides quantitative information on the composition of soil organic matter and can therefore complement structural studies by its application to large numbers of samples. However, it cannot replace the information provided by more specific methods. The actual potential of DRIFT?PLS lies in its capacity to predict unknown samples, which is helpful for classification and identification of environmental outliers or benchmarks.     

Near‐infrared spectroscopy can predict the composition of organic matter in soil and litter

The usefulness and limitations of near‐infrared reflectance spectroscopy (NIRS) for the assessment of several soil characteristics are still not sufficiently explored. The objective of this study was to evaluate the ability of visible and near‐infrared reflectance (VIS‐NIR) spectroscopy to predict the composition of organic matter in soils and litter. Reflectance spectra of the VIS‐NIR region (400–2500 nm) were recorded for 56 soil and litter samples from agricultural and forest sites. Spectra were used to predict general and biological characteristics of the samples as well as the C composition which was measured by ¹³C‐CPMAS‐NMR spectroscopy. A modified partial least‐square method and cross‐validation were used to develop equations for the different constituents over the whole spectrum (1st to 3rd derivation). Near‐infrared spectroscopy predicted well the C : N ratios, the percentages of O‐alkyl C and alkyl C, the ratio of alkyl C to O‐alkyl C, and the sum of phenolic oxidation products: the ratios of standard deviation of the laboratory results to standard error of cross‐validation (RSC) were greater than 2, the regression coefficients (a) of a linear regression (measured against predicted values) ranged from 0.9 to 1.1, and the correlation coefficients (r) were greater than 0.9. Satisfactorily (0.8 ≤ a ≤ 1.2, r ≥ 0.8, and 1.4 ≤ RSC ≤ 2.0) assessed were the contents of C, N, and production of DOC, the percentages of carbonyl C and aromatic C and the ratio of alkyl C to aromatic C. However, the N‐mineralization rate and the microbial biomass were predicted unsatisfactorily (RSC < 1.4). The good and satisfactory predictions reported above indicate a marked usefulness of NIRS in the assessment of biological and chemical characteristics of soils and litter.     

Earthworm feeding activity and development of the humus profile

Earthworm casts and digestive tract contents were simultaneously examined, using the same methods, in a recently formed humus profile in a mountain spruce forest. Earthworm species had distinct diets and an earthworm foodweb could be distinguished. Lumbricus terrestris and Aporrectodea icterica were distinct from the other species examined: the former to some extent as a litter consumer, and both species because they excavated mineral material which was deposited within new top layers of the mull humus. Aporrectodea nocturna and Aporrectodea caliginosa both had a non-specific soil feeding mode. Most of the species enriched the humus profile with amorphous organic matter finely incorporated within a mineral matrix. Besides different food selection, a network of burrows was produced as a consequence of the different burrowing behaviour of each earthworm species. Received: 2 January 1997     

Carbon and nitrogen dynamics in ageing earthworm casts in grasslands of the eastern plains of Colombia

 The effects of a large species of anecic earthworm, Martiodrilus carimaguensis Jiménez and Moreno, on soil C and N dynamics were investigated in a native savanna and a man-made pasture of the eastern plains of Colombia. We compared, across time (11 months), the total C, total N, NH⁺ ₄ and NO^– ₃ contents in the earthworm casts, the underlying soil and the adjacent soil. Additional sampling of root biomass and macrofauna was performed. In the two management systems, the total C and N contents were higher in casts (4.33–7.50%) than in the bulk soil (2.81–4.08%), showing that the earthworms selected food substrates with high organic contents. In general, C contents significantly increased during cast ageing (+100%), possibly because of CO₂ fixation processes, dead root accumulation and/or macrofaunal activities in casts. In fresh casts, NH⁺ ₄ levels were very high (294.20–233.98 μg g^–1 dry cast) when compared to the soil (26.96–73.95 μg g^–1 dry soil), due to the intense mineralisation processes that occurred during the transit of soil and organic matter through the earthworm gut. During the first week of cast ageing, NH⁺ ₄ levels sharply decreased, while NH^– ₃ levels showed successive peaks in the casts, the underlying soil and the adjacent soil. These results suggested the rapid production of NO^– ₃ by nitrification processes in the fresh casts, followed by diffusion to the nearby soil, first vertically, then horizontally. After 2 weeks of cast ageing, NH⁺ ₄ and NO^– ₃ levels only showed slight variations, likely because of organic matter protection in stable dry casts. The root biomass was higher (1.6–4.7 times) below the old earthworm casts. The ecological significance of these results is discussed. Received: 22 October 1998     

Effect of long‐term combined nitrogen and phosphorus fertilizer application on 13C CPMAS NMR spectra of humin in a Typic Hapludoll of northeast China

Because of its insolubility, heterogeneity and structural complexity, humin is the least understood among the three fractions of soil humic substances. This research aimed to evaluate the long‐term effect of combined nitrogen and phosphorus (NP) fertilizer addition on the chemical structure of humin under maize (Zea mays L.) monoculture in a Typic Hapludoll of northeast China. Soil samples were collected 12 and 25 years after the initiation of the fertilizer treatment. Soil humin was isolated using NaOH‐Na₄P₂O₇ extraction to remove humic and fulvic acids, which was followed by HF‐HCl treatment to remove most of the inorganic minerals. Solid‐state ¹³C cross‐polarization magic angle spinning nuclear magnetic resonance (¹³C CPMAS NMR) spectroscopy was used to characterize the chemical structure of the humin isolates. Results showed that the organic carbon (C) content of humin increased after NP fertilizer addition, compared with a no‐fertilizer (CK) treatment. ¹³C CPMAS NMR indicated that O‐alkyl C and aromatic C of humin decreased, while alkyl C and the ratios of alkyl C/O‐alkyl C, aliphatic C/aromatic C and hydrophobic C/hydrophilic C all increased in the NP fertilizer treatment. The long‐term application of NP fertilizer changed the molecular structure of soil humin to be more alkyl and hydrophobic, and was thus beneficial to the sequestration and stability of organic C in soil.     

The effect of earthworms and liming on soil microbial communities

The effect of liming and earthworms on the composition and function of soil microbial communities was investigated in an upland soil from the UK in order to understand interactions between the biotic and abiotic components of soil systems. A factorial experiment was established using soils from the Sourhope Farm, near Kelso, with lime or no lime added, with or without earthworms added and a combined treatment of both lime and earthworm additions. The soils were incubated and destructively sampled after 180 days. Measurements of soil microbial biomass, dehydrogenase activity, phenotypic structure (by phospholipid fatty acid analysis (PLFA) and responses to four carbon substrates (d-glucose, l-arginine, α-ketoglutaric acid, α-cyclodextrin) were determined. Statistically significant results were limited to the litter layers, with no significant observations in either the H or Ah horizons. There were significant decreases in the soil microbial biomass and microbial activity in the litter layers caused by the addition of earthworms; liming reduced microbial biomass only. The addition of earthworms caused a significant difference in the PLFA principle component analysis (PCA) profile, as did liming. For the PLFA PCA profile, earthworm plus lime treatment was indistinguishable from the liming result. Addition of earthworms significantly suppressed the response to glucose; this effect was removed by liming. This indicates that liming may significantly alter the ecological interactions between earthworms and the microbial community.     

Impacts of wetting-drying cycles on short-term carbon and nitrogen dynamics in Amynthas earthworm casts

Effects of earthworm casts on soil nutrient dynamics and their responses to changing moisture availability in subtropical ecosystems remain poorly understood. This study aimed to examine short-term carbon(C) and nitrogen(N) dynamics and their interactions with wetting-drying cycles in three different structural forms(i.e., granular, globular, and heap-like) of Amynthas earthworm casts. The rates of C and N mineralization in the earthworm casts were examined under two different wetting-drying cycles(i.e., 2-d and 4-d wetting intervals) using a rainfall simulation experiment. After three simulated rainfall events, subsamples of the earthworm casts were further incubated for 4 d for the determination of CO_2 and N_2O fluxes. The results of this study indicated that the impacts of wetting-drying cycles on the short-term C and N dynamics were highly variable among the three cast forms, but wetting-drying cycles significantly reduced the cumulative CO_2 and N_2O fluxes by 62%–83% and 57%–85%, respectively, when compared to the control without being subjected to any rainfall events. The C mineralization rates in different cast forms were affected by the amount of organic substrates and N content in casts, which were associated with the food preference and selection of earthworms. Meanwhile, the cumulative N_2O fluxes did not differ among the three cast forms. Repeated wetting and drying of casts not only enhanced aggregate stability by promoting bonds between the cast particles, but also inhibited microbial survival and growth during the prolonged drying period, which together hindered decomposition and denitrification. Our findings demonstrated that the interactions between the structural forms, aggregate dynamics, and C and N cycling in the earthworm casts were highly complex.     

Competition between invasive earthworms (Amynthas corticis, Megascolecidae) and native North American millipedes (Pseudopolydesmus erasus, Polydesmidae): Effects on carbon cycling and soil structure

Invasive earthworms can have significant impacts on C dynamics through their feeding, burrowing, and casting activities, including the protection of C in microaggregates and alteration of soil respiration. European earthworm invasion is known to affect soil micro- and mesofauna, but little is known about impacts of invasive earthworms on other soil macrofauna. Asian earthworms (Amynthas spp.) are increasingly being reported in the southern Appalachian Mountains in southeastern North America. This region is home to a diverse assemblage of native millipedes, many of which share niches with earthworm species. This situation indicates potential for earthworm-millipede competition in areas subject to Amynthas invasion.In a laboratory microcosm experiment, we used two ¹³C enriched food sources (red oak, Quercus rubra, and eastern hemlock, Tsuga canadensis) to assess food preferences of millipedes (Pseudopolydesmus erasus), to determine the effects of millipedes and earthworms (Amynthas corticis) on soil structure, and to ascertain the nature and extent of the interactions between earthworms and millipedes. Millipedes consumed both litter species and preferred red oak litter over eastern hemlock litter. Mortality and growth of millipedes were not affected by earthworm presence during the course of the experiment, but millipedes assimilated much less litter-derived C when earthworms were present.Fauna and litter treatments had significant effects on soil respiration. Millipedes alone reduced CO₂ efflux from microcosms relative to no fauna controls, whereas earthworms alone and together with millipedes increased respiration, relative to the no fauna treatment. CO₂ derived from fresh litter was repressed by the presence of macrofauna. The presence of red oak litter increased CO₂ efflux considerably, compared to hemlock litter treatments.Millipedes, earthworms, and both together reduced particulate organic matter. Additionally, earthworms created significant shifts in soil aggregates from the 2000-250 and 250-53 μm fractions to the >2000 μm size class. Earthworm-induced soil aggregation was lessened in the 0-2 cm layer in the presence of millipedes. Earthworms translocated litter-derived C to soil throughout the microcosm.Our results suggest that invasion of ecosystems by A. corticis in the southern Appalachian Mountains is unlikely to be limited by litter species and these earthworms are likely to compete directly for food resources with native millipedes. Widespread invasion could cause a net loss of C due to increased respiration rates, but this may be offset by C protected in water-stable soil aggregates.     

蚯蚓对土壤温室气体排放的影响及机制研究进展

土壤是温室气体的重要源和汇。蚯蚓是土壤物质循环的重要参与者,能够直接或间接影响土壤CO2、N2O和CH4等温室气体的产生和释放。蚯蚓呼吸产生的CO2,是土壤呼吸的重要组成部分;蚯蚓自身肠道、分泌液、消化物和排泄物等微环境促进反硝化过程释放N2O。蚯蚓还通过取食、掘穴、排泄等活动,改变土壤理化性质、微生物组成和活性及其他土壤动物的组成,影响地上植物生长,调节土壤分解、矿化、硝化、反硝化和甲烷生成及氧化等生态过程,间接影响土壤温室气体的排放。蚯蚓对土壤温室气体排放的影响逐渐受到重视,但目前研究仍以室内培养和单因子环境条件的模拟为主,缺少野外原位实验和多环境因子的交互实验研究。长期监测和同位素示踪技术,是深入探讨蚯蚓影响温室气体排放机制的重要手段。温室气体类型上,CO2和N2O是研究热点,CH4研究比较罕见。未来研究,应重视不同生态类群蚯蚓与土壤理化特征、微生物组成、其他类群土壤动物和地上植物间的交互作用,加强机制研究,并关注土壤污染环境下蚯蚓功能性状的变化;综合评价蚯蚓对土壤温室气体排放和土壤碳氮固定的影响,科学评估蚯蚓活动对土壤碳氮释放的促进或减缓作用。     

Effect of earthworm activity (Aporrectodea giardi) on atrazine adsorption and biodegradation

We investigated the influence of earthworm (Aporrectodea giardi) activity on soil properties and on atrazine (AT) adsorption and biodegradation by comparing a coarse‐textured smectite‐free wetland soil (Brittany, France) with the earthworm casts derived from the top horizon of this soil. Casts are characterized by lower pH, are enriched in organic carbon (OC) and clay content, have a larger cation exchange capacity, and a greater exchangeable Ca content. The clay mineralogy of the soil studied and casts is characterized by a muscovite–kaolinite–chlorite association. In addition, the clay fraction of the soil contains lepidocrocite (γ‐FeOOH), which was not found in the casts. Atrazine adsorption isotherms were reasonably well described by the Freundlich equation and were all non‐linear. The mean amounts of adsorbed AT for starting concentrations of 3–30 mg litre^?1 ranged from 8 to 34%, being largest in earthworm casts. Soil AT adsorption capacity was well correlated with OC content. Non‐decomposed organic matter present in the coarse size fractions and specific compounds present in earthworm casts (proteins, mono‐ and polysaccharides, polyphenols, sugars, lignin) and microbial and fungal biomass contribute to AT adsorption. Weak electrostatic (physical) sorption of AT on organic compounds and on mineral surfaces prevails. For casts, the formation of additional hydrophobic interactions between AT and SOM is proposed. We also studied AT biodegradation by the model bacterium Pseudomonas sp. strain ADP in the presence of soils or earthworm casts. An enhancement of the AT disappearance rate was observed in the presence of all the solid matrices tested compared with that obtained in an aqueous medium. The biodegradation rate was shown to be dependent not only on the OC content of the solid matrix, but mainly on its composition and structure.