Alexei V. Tiunov

Microbial respiration, biomass, biovolume and nutrient status in burrow walls of Lumbricus terrestris L. (Lumbricidae)

Abstract Chemical characteristics and microbial activity were studied in burrow walls of the anecic earthworm species Lumbricus terrestris and in control soil of a lime ( Tilia cordata ), oak ( Quercus robur ) and beech ( Fagus sylvatica ) forest. Samples were taken in June and October at distances of 0–4 (drilosphere), 8–12 and 50–60 (control soil) mm from earthworm burrows. The following variables were measured: organic C, total N, moisture, pH, basal respiration, microbial biomass (SIR method), fungal and bacterial volume (epifluorescence microscopy) and nutrient (C, N and P) limitation of microbial growth. Organic C and total N contents increased in the burrow walls by factors of 1.8–3.5 and 1.3–2.2, respectively, compared to the control soil. The moisture content and pH (up to 1.2 units) was higher. Basal respiration, microbial biomass and bacterial volume in the drilosphere exceeded those in control soil significantly by factors of 3.7–9.1, 2.3–4.7 and 2.1–5.4, respectively. Changes in fungal volume with vicinity to burrows differed between forest sites. Fungal volume was increased significantly by factors of 1.9–3.4 in the earthworm burrow walls in the oak and beech forest, but was similar to that in control soil in the lime forest. Microbial growth in the control soil was limited by N in the oak forest and by N and P in the lime and beech forest. The nutrient status of the microflora changed little in vicinity to burrows. However, microbial N and P demand in earthworm burrow walls exceeded that in soil. The specific respiration ( q O 2 ) was increased and the growth response to nutrient additions was faster in the burrow walls suggesting that the microbial community in the burrow walls contains a larger fraction of metabolically-active microorganisms, adapted to continuous resource additions by earthworm faeces and mucus. Enrichment in organic matter, but also other mechanisms, particularly the activity of microbivorous soil animals, are presumably responsible for the formation of a specific microbial community in earthworm burrow walls. It is concluded that L. terrestris burrow walls are stable microhabitats which sustain a large and active microbial community and are likely to play an important role in the soil system by regulating microbial-mediated chemical processes.

Microbial biomass, biovolume and respiration in Lumbricus terrestris L. cast material of different age

EAects of the gut passage, the age of cast material and the type of ingested substrate on the microbial community in Lumbricus terrestris faeces were studied in laboratory microcosms containing four soil‐litter combinations: lime forest soil+lime litter, beech forest soil+lime litter, beech forest soil+beech litter and beech forest soil without litter. Microbial biomass (SIR method), basal respiration and biovolume of bacteria and fungi were measured in earthworm casts and reference material after 1, 5, 10 and 100 d of incubation. To separate eAects of mixing of litter and soil in the gut of L. terrestris from specific gut or cast associated processes, actual faecal properties were compared with ‘expected’ values, calculated on the basis of corresponding measurements in soil and litter ingested by L. terrestris. Microbial respiration, biomass and fungal volume in fresh (1 d old) faeces strongly depended on the type of soil and litter material consumed by L. terrestris. The comparison with ‘expected’ values indicated that microbial biomass and volume changed little during the passage through the gut of L. terrestris. In contrast, even a short incubation of casts caused marked changes in the microbial properties studied: microbial biomass sharply declined during the first 5 d of incubation; basal respiration which exceeded ‘expected’ values by 30‐120% in fresh casts, decreased steadily and was significantly below ‘expected’ values by d 10 of the incubation; biovolume of bacteria and fungi in fresh casts was 20‐60% higher than expected and then steadily declined in lime litter but not in beech litter treatments. Generally, the fungal-to-bacterial volume ratio in cast material was not significantly aAected by the gut passage nor by the incubation of casts. # 2000 Elsevier Science Ltd. All rights reserved.

Effects of the presence and community composition of earthworms on microbial community functioning

Abstract. Earthworms are a major component of many terrestrial ecosystems. By modifying decomposition processes and soil structure, they function as driving factors of the soil microbial community. Using microcosms, we investigated the effects of the presence and community composition of earthworms on the in situ respiratory response of a microbial community to an array of organic substrates including carbohydrates, amino acids, a polymer and an amide. Both the actual in situ catabolic response of non-growing microorganisms and the potential response of growing microorganisms were investigated. Three questions were studied: (1) does the presence of one of the main functional groups of earthworms (endogeic species) affect microbial community functioning; (2) does the presence of two functional groups (endogeic and epigeic species) alter microbial community functioning; (3) does the number of species within functional groups matter. The presence of endogeic earthworms significantly reduced microbial biomass and affected the physiological profile and functioning of the microbial community. In contrast, in the presence of endo- and epigeic species microbial biomass was not reduced significantly, indicating that epigeic species counteracted the effect of endogeic species. The physiological profile of the microbial community significantly differed between the treatment with endogeic species only and the treatment with both endo- and epigeic species. Also, the physiological profile of the microbial community was significantly affected by the number of species per functional group, which at least in part may have been caused by a sampling effect. Overall, the actual in situ catabolic response of non-growing microorganisms appears to be more sensitive than the potential response of growing microorganisms. In addition, the direction of the actual response (negative) was diametrically opposed to that of the potential response (positive). We conclude that the catabolic response of growing microorganisms does not reflect the actual case in situ. For earthworms, loss in both species number and functional group number has the potential to change soil microbial community functioning.

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 Cladosporiumspp., 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. ©2000 Elsevier Science B.V. All rights reserved.

Microflora, Protozoa and Nematoda in Lumbricus terrestris burrow walls: a laboratory experiment

Summary Anecic earthworms Lumbricus terrestris L. were kept in laboratory microcosms containing soil and litter from a lime ( Tilia cordata ) and beech ( Fagus sylvatica ) forest. Nutrient (mineral nitrogen and phosphorus) contents, microbial activity and densities of protozoa and nematodes were determined in burrow walls and control soils after 165 days. Well-developed burrow linings consisting of earthworm faeces were formed in the “ Tilia ” treatment, but not in the “ Fagus ” treatment, presumably because beech litter was not an adequate food resource for L. terrestris . Consequently, increases in microbial biomass, basal respiration and microbial volume in burrow walls compared to surrounding soil were significant in the “ Tilia ” treatment only. However, in both treatments burrow walls were strongly enriched in mineral nitrogen and phosphorus. The density and biomass of protozoa were significantly greater in burrow walls compared to the control soil. The numbers of naked amoebae increased by similar factors of 4 and 3.5 in burrow walls of the “ Tilia ” and “ Fagus ” treatment, respectively. Flagellate density increased more than tenfold in burrow walls of the “ Tilia ” treatment but only twofold in the “ Fagus ” treatment. In addition, a comparatively large ciliate population was present in burrow walls in the “ Tilia ” treatment. The total abundance of protozoa was significantly correlated with the contents of inorganic N and P in the samples ( r = 0.68 and 0.63 respectively, P Tilia ” treatment (mostly due to bacterivorous nematodes), but was not affected in the “ Fagus ” treatment. In both treatments density of fungivorous nematodes increased, while that of plant parasites decreased in burrow walls compared to the control soil. It is concluded that the grazing pressure of protozoa and nematodes may control the dynamics of the microbial succession in earthworm burrow walls, strongly affecting nutrient cycling processes in these microhabitats.

Fungal and bacterial communities in Lumbricus terrestris burrow walls: a laboratory experiment

Summary The taxonomic composition of microbial communities in earthworm burrow walls was studied in a laboratory experiment lasting for 165 days. Anecic earthworms Lumbricus terrestris L. were kept in microcosms containing soil and litter from a lime (Tilia cordata) and beech (Fagus sylvatica) forest. Well-developed burrow linings consisting of earthworm faeces were formed in the “Tilia” treatment, but not in the “Fagus” treatment. The numbers and dominance structure of saprophytic bacteria were determined by the plate count method. Burrow walls in the “Tilia” treatment had tenfold higher CFU numbers than the control soil. The most abundant taxa were Cellulomonasand Promicromonospora, whereas Bacillus and Streptomyces dominated in the control soil. CFU numbers did not differ between burrow walls and control soil in the “Fagus” treatment, but the dominance of Bacillus was significantly decreased in burrow walls. The structure of fungal communities was analysed by modified washing and plating method. In both treatments the dominance structure of the fungal community in burrow walls differed from that in the control soil. The fungal community in burrow walls included many litter-associated taxa (Mucor hiemalis, Trichoderma koningii, Mortierella gamsii), but its taxonomic composition was soil-specific and depended on the quantity and quality of organic matter translocated down the soil profile. Mechanisms responsible for the formation of the specific burrow wall microflora are discussed. It is concluded that L. terrestris burrow walls contribute to the maintenance of high microbial diversity in mineral soil.

Isotopic niche (δ13С and δ15N values) of soil macrofauna in temperate forests

RATIONALE Stable isotope analysis (SIA) is used widely for reconstructing trophic links of vertebrate and invertebrate animals. Soil macrofauna form a substantial food source for a range of predators including amphibians, reptiles, birds and mammals. SIA-based estimations of their trophic niches require knowledge on the full range of isotopic signatures of potential preys. Considering the extremely high diversity of soil animals, this information is not easy to obtain. METHODS We estimated a typical range of the isotopic signatures of soil macrofauna by compiling published and original data on soil macroinvertebrates in 11 temperate forests. We examined whether the baseline correction (i.e. subtracting δ(13)C or δ(15)N values of local litter) would decrease the between-site variability in the δ(13)C and δ(15)N values of soil animals. The dataset was subsequently used to estimate the frequency distribution of δ(13)C and δ(15)N values in saprophagous and predatory soil animals. RESULTS The baseline correction reduced the between-site variability in δ(15)N, but not in δ(13)C values of soil animals. Over 95% of the taxa or individuals examined fell into an isotopic space with uncorrected δ(13)С values ranging from -27.9 to -22.5‰, and litter-normalized δ(15)N values from 0.8 to 9.6‰. Saprophagous and predatory soil animals were on average enriched in (13)C relative to plant litter by 3.5 and 2.7‰, respectively, which is likely to reflect the importance of saprotrophic microorganisms as the main energy source in soil food webs. The difference in δ(15)N values between saprophages and predators averaged 2.8‰, which fits the anticipated trophic enrichment per trophic level. CONCLUSIONS Our results indicate that the range of possible δ(15)N values of soil macrofauna in temperate forest ecosystems can roughly be predicted based on the δ(15)N values of plant litter. On the other hand, no site-specific normalization is usually required when predicting the range of δ(13)C values of soil macroinvertebrates.

High Niche Overlap in the Stable Isotope Space Of Ground Beetles

Classic theories on assembly rules and food-web structure are species-centered, so they largely ignore intraspecific variation. Intraspecific trophic variation, however, might be of key importance in understanding community organization. Here we study the variability of isotopic niche spaces of ground beetles and its consequences for the trophic structure of beetle assembly. Stable isotopes ratios (&dgr;13C and &dgr;15N) were measured in 1156 specimens of carabids belonging to 59 species inhabiting 20 island and two mainland sites of the Masuria Lakeland in northern Poland. Carabid species belonged to three different trophic guilds (named “phytophages”, “decomposer feeders” and “generalist predators”). However, this division is not sharp due to high intraspecific variability of isotopic signatures, which indicates the use of very different types of resources by conspecific individuals inhabiting different sites. As a consequence, most species studied did not differ significantly in the isotopic niche space. This high niche overlap corroborates the view that resource competition is not a major factor shaping the composition of ground beetles communities. Future studies should take into account the complex trophic structure of beetle assemblages and explore the intraspecific niche variability of ground beetles.

Soil fauna: key to new carbon models

Soil organic matter (SOM) is key to maintaining soil fertility, mitigating climate change, combatting land degradation, and conserving above- and below-ground biodiversity and associated soil processes and ecosystem services. In order to derive management options for maintaining these essential services provided by soils, policy makers depend on robust, predictive models identifying key drivers of SOM dynamics. Existing SOM models and suggested guidelines for future SOM modelling are defined mostly in terms of plant residue quality and input and microbial decomposition, overlooking the significant regulation provided by soil fauna. The fauna controls almost any aspect of organic matter turnover, foremost by regulating the activity and functional composition of soil microorganisms and their physical-chemical connectivity with soil organic matter. We demonstrate a very strong impact of soil animals on carbon turnover, increasing or decreasing it by several dozen percent, sometimes even turning C sinks into C sources or vice versa. This is demonstrated not only for earthworms and other larger invertebrates but also for smaller fauna such as Collembola. We suggest that inclusion of soil animal activities (plant residue consumption and bioturbation altering the formation, depth, hydraulic properties and physical heterogeneity of soils) can fundamentally affect the predictive outcome of SOM models. Understanding direct and indirect impacts of soil fauna on nutrient availability, carbon sequestration, greenhouse gas emissions and plant growth is key to the understanding of SOM dynamics in the context of global carbon cycling models. We argue that explicit consideration of soil fauna is essential to make realistic modelling predictions on SOM dynamics and to detect expected non-linear responses of SOM dynamics to global change. We present a decision framework, to be further developed through the activities of KEYSOM, a European COST Action, for when mechanistic SOM models include soil fauna. The research activities of KEYSOM, such as field experiments and literature reviews, together with dialogue between empiricists and modellers, will inform how this is to be done.

The microfungal community of Lumbricus terrestris middens in a linden (Tilia cordata) forest

Summary Large anecic earthworms Lumbricus terrestris create distinctive microhabitats called middens, which strongly affect the spatial distribution of litter and litter-dwelling animals on the soil surface. The community of filamentous microscopic fungi was analysed in L. terrestris middens and control samples using the washing and plating technique. Four substrates were analysed: mineral soil, control litter, litter from L. terrestris middens, and L. terrestris casts from the middens. The composition of microfungal communities in studied substrates was compared using multidimensional scaling and discriminant analysis. No difference was found between dominance structure of fungal communities in control litter and litter from earthworm middens. In both types of litter the most frequent fungi were Alternaria alternata, Cladosporium herbarum and C. cladosporioides . In contrast, the fungal community in L. terrestris casts strongly differed from those in soil and both litter types (P Trichosporiella hyalina, Humicola grisea and Monilia sp., which were much less abundant in soil and litter. It is concluded that at a given spatial scale the feeding activity of L. terrestris has a larger impact on the community of filamentous fungi in the litter layer than the creation of favourable microhabitats for specialized fungivorous animals like collembolans.