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Soil food web structure and function is primarily determined by the major basal resources, which are living plant tissue, root exudates and dead organic matter. A field experiment was performed to disentangle the interlinkage of the root-and detritus-based soil food chains. An arable site was cropped either with maize, amended with maize shoot litter or remained bare soil, representing food webs depending on roots, aboveground litter and soil organic matter as predominant resource, respectively. The soil micro-food web, i.e. microorganisms and nematodes, was investigated in two successive years along a depth transect. The community composition of nematodes was used as model to determine the changes in the rhizosphere, detritusphere and bulk soil food web. In the first growing season the impact of treatments on the soil micro-food web was minor. In the second year plant-feeding nematodes increased under maize, whereas after harvest the Channel Index assigned promotion of the detritivore food chain, reflecting decomposition of root residues. The amendment with litter did not foster microorganisms, instead biomass of Gram-positive and Gram-negative bacteria as well as that of fungi declined in the rooted zone. Likely higher grazing pressure by nematodes reduced microbial standing crop as bacterial and fungal feeders increased. However, populations at higher trophic levels were not promoted, indicating limited flux of litter resources along the food chain. After two years of bare soil microbial biomass and nematode density remained stable, pointing to soil organic matter-based resources that allow bridging periods with deprivation. Nematode communities were dominated by opportunistic taxa that are competitive at moderate resource supply. In sum, removal of plants from the system had less severe effects than expected, suggesting considerable food web resilience to the disruption of both the root and detrital carbon channel, pointing to a legacy of organic matter resources in arable soils.

Soil fungi play an essential role in the decomposition of plant-derived organic material entering soils. The quality and quantity of organic compounds vary seasonally as well as with soil depth. To elucidate how these resources affect fungal communities in an arable soil, a field experiment was set up with two plant species, maize and wheat. Resource availability was experimentally manipulated by maize litter input on one half of these maize and wheat plots after harvest in autumn. Fungal biomass was determined by ergosterol quantification, and community structure was investigated by fungal automated ribosomal intergenic spacer analysis (F-ARISA). An annual cycle was assessed across a depth gradient, distinguishing three soil habitats: the plough layer, rooted soil below the plough layer, and deeper root-free soil. Fungal communities appeared highly dynamic and varied according to soil depth and plant resources. In the plough layer, the availability of litter played a dominant role in shaping fungal communities, whereas in the rooted layer below, community structure and biomass mainly differed between plant species. This plant effect was also extended into the root-free soil at a depth of 70 cm. In winter, the availability of litter also affected fungal communities in deeper soil layers, suggesting vertical transport processes under fallow conditions. These distinct resource effects indicate diverse ecological niches along the soil profile, comprising specific fungal metacommunities. The recorded responses to both living plants and litter point to a central role of fungi in connecting primary production and decomposition within the plant-soil system.

Herbicides containing glyphosate are widely used in agriculture and private gardens, however, surprisingly little is known on potential side effects on non-target soil organisms. In a greenhouse experiment with white clover we investigated, to what extent a globally-used glyphosate herbicide affects interactions between essential soil organisms such as earthworms and arbuscular mycorrhizal fungi (AMF). We found that herbicides significantly decreased root mycorrhization, soil AMF spore biomass, vesicles and propagules. Herbicide application and earthworms increased soil hyphal biomass and tended to reduce soil water infiltration after a simulated heavy rainfall. Herbicide application in interaction with AMF led to slightly heavier but less active earthworms. Leaching of glyphosate after a simulated rainfall was substantial and altered by earthworms and AMF. These sizeable changes provide impetus for more general attention to side-effects of glyphosate-based herbicides on key soil organisms and their associated ecosystem services.

This study analyses trophic interactions between soil fungi, micro- and mesofauna in microcosm experiments. The trophic shift of 15N and fatty acids (FAs) was investigated in different food chains, which comprised either two (fungi and grazers) or three (fungi, nematodes and Collembola) levels. Contrary to the widely accepted assumption of 15N enrichment in trophic cascades the experiments revealed enrichment, depletion or no change in 15N of consumers compared to their diet. Factors responsible for this pattern were suggested to be: (1) the main metabolic pathway used for N excretion in ammonotelic nematodes to be similar or depleted in the heavier isotope, and uricotelic Collembola mostly enriched in the heavier isotope; (2) a higher shift in 15N with a high-protein diet (e.g. for predators); (3) compensation due to low-quality food altering the fractionation of 15N. Analysis of the lipid composition showed phospholipids to be generally unaffected and neutral lipids closely related to the FA pattern of the food source. Dietary routing of FAs into neutral lipids occurred, as evidenced by corresponding frequencies of FAs in host and consumer profiles. Additionally, several FAs were only detected in the grazer when present in the food source. Oleic acid showed a shift over three trophic levels, from fungi to nematodes to Collembola. The assimilation of dietary FAs resulted in a more diverse neutral lipid profile, i.e. animals higher in the food chain contained more individual FAs compared to animals lower in the food chain. The results indicate that monoenoic C18 and monoenoic C20 FAs have the potential to act as tools for the bioindication of feeding strategies in belowground systems. We suggest that primary consumers will have no or only trace amounts of monoenoic C20 acids in their neutral lipid profile, whereas consumers feeding on a eukaryote diet will show a considerably higher frequency.

Top–down control by nematodes over soil microorganisms – considered to be stronger over bacteria than over fungi – may dampen microbial responses to global changes in tundra. To test whether large grazers alter the responses of belowground trophic networks to global changes, we employed factorial warming and nitrogen fertilization treatments in adjacent sites with different reindeer grazing intensities for the past 50 years. Lightly grazed tundra is dominated by dwarf shrubs and a more fungal‐based microbial community, while in heavily grazed tundra the high reindeer densities during autumn migration have induced a shift into graminoids and a more bacterial‐based microbial community. We analysed the soil micro‐food web (i.e. the nematode density, trophic structure and species composition) as well as fungal, bacterial and total phospholipid fatty acids (PLFAs) after four growing seasons of warming and fertilization both before and during reindeer migration. We predicted that bacterivore densities are higher and fungivore densities lower under heavy than light grazing (i.e. nematode populations before migration reflect grazing effects via the base of the food web), whereas reindeer migration induces a negative impact on nematode densities under heavy grazing (disturbance by trampling is the driving factor). We further predicted that nematodes negate treatment effects on microbial biomass to a stronger extent in the bacterial‐based heavily grazed tundra than in the fungal‐based lightly grazed tundra. Fungivore densities were higher under light than under heavy grazing, but nematodes did not respond to trampling. Warming increased fungivores and the fungal PLFAs irrespective of grazing and timing but, under heavy grazing, it increased bacterivores while the bacterial PLFAs remained steady. Fertilization increased carnivores and influenced nematode species composition, diversity and maturity interactively with warming. Our data suggest that large grazers affect tundra soil nematodes via bottom–up effects through microbial community composition and biomass, which in turn may alter the strength of their top–down control over soil bacteria under climate warming. [ABSTRACT FROM AUTHOR]

Soil food webs are important drivers for key ecological functions in terrestrial systems such as carbon and nutrient cycling. However, soil food web models generally lack quantitative data, mainly due to the shortage in high-throughput methods to describe energy flows. In marine environments, multivariate optimization models (Quantitative Fatty Acid Signature Analysis) and Bayesian approaches (source-tracking algorithm) were established to predict the proportion of predator diets using lipids as tracers. A premise for the application of such models to soil systems is to acquire the fatty acid pattern of a broad range of resources and to reveal potential overlap in their signatures. We present a comprehensive comparison of lipid pattern across widespread taxa of plants (leaves and roots, n = 48), algae (n = 59), fungi (n = 60), and bacteria (n = 62) as basal food web resources. Lipid profiles from microorganisms and algae were assessed from laboratory cultures, whereas plant tissue was derived from an arable field. A lipid library was constructed and multivariate data analyses (hierarchical clustering, nMDS) was used to assess the extent of separation in lipid pattern by species or resource type. The performance of the lipid library was tested by leave-one-prey-out (LOPO) analysis, giving the distinctiveness of the resource (prey) groups. Fungi and plant leaves were correctly assigned based on their lipid pattern with more than 98%, while plant roots and bacteria achieved 88 and 85%, respectively. However, algae were only correctly classified by 60%, pointing to a bias in the herbivore food chain. Fatty acids most important for separation of algae and plant leaves were of the omega 3 type, i.e. 16:3ω3 and 18:3ω3. In plant roots 18:1ω9 was most important, whereas bacteria were distinguished predominantly by methyl-branched fatty acids. Overall, the lipid pattern of major soil food web resources are sufficiently differentiated to allow for qualitative (biomarker) analyses as well as quantitative modelling, yet with precaution in the case of algae. [ABSTRACT FROM AUTHOR]

The complexity of soil food webs and the cryptic habitat hamper the analyses of pools, fluxes and turnover rates of carbon (C) in organisms and the insight into their interactions. Stable isotope analysis has been increasingly used to disentangle soil food web structure, yet it has not been applied to quantitatively characterize C dynamics at the level of the entire soil food web., The present study employed 13CO2 pulse labelling to investigate the incorporation of maize root-derived C into major soil compartments and food web players in an arable field for 25 days. Bulk tissue and compound-specific (lipids) C isotope ratios were used to quantify pool sizes and 13C incorporation in bacteria and fungi as primary decomposers, nematodes as key drivers of the microfood web and decomposers and predators among the meso- and macrofauna., About 20% of the C assimilated by maize was transferred to below-ground pools. 13C was predominantly incorporated into rhizosphere micro-organisms rather than in those of the bulk soil. 13C in phospholipid fatty acid biomarkers revealed that root-derived C was incorporated into the soil food web mainly via saprotrophic fungi rather than via bacteria. Only small amounts of 13C were transferred to higher trophic levels, predominantly into fungal-feeding nematodes and macrofauna decomposers., Most importantly, C pool size and 13C incorporation did not match closely. Although the fungal C stock was less than half that of bacteria, C transfers from fungi into higher trophic levels of the fungal energy pathway, that is fungal-feeding nematodes and meso- and macrofauna decomposers, by far exceed that of bacterial C. This challenges previous views on the dominance of bacteria in root C dynamics and suggests saprotrophic fungi to function as major agents channelling recent photoassimilates into the soil food web. [ABSTRACT FROM AUTHOR]

Abstract: Positive relationships between biodiversity and ecosystem functioning are reported in many studies. The underlying mechanisms are, however, only little understood, likely due to the focus on the aboveground system and neglecting species-specific traits. We used different clusters of tree species composition to investigate how tree species diversity and tree species identity affect belowground nematode communities. Since soil nematodes comprise different trophic groups and are strongly linked to the microbial community, results can provide insight on how soil food webs are structured. In addition, data on leaf litter and fine root biomass were included to account for different effects of aboveground and belowground resources. Clusters of three trees of one, two and three species were identified within a temperate deciduous forest. Target tree species were European beech (Fagus sylvatica), common ash (Fraxinus excelsior) and lime (Tilia cordata, T. platyphyllos) differing in physiology, leaf litter quality and type of mycorrhiza. Tree species identity strongly affected nematode trophic structure, whereas tree species diversity had no impact. Ash beneficially affected bacterial-feeding nematodes, whereas fungal feeders were suppressed, likely caused by ash litter increasing soil pH. Fostering of the bacterial food chain by ash additionally could be related to rhizodeposition gaining importance after disappearance of high quality ash litter in spring, highlighting seasonal shifts in root and leaf litter-derived resources. The negative effect of ash on fungal-feeding nematodes is suggested to be due to the lack of ectomycorrhizal fungi as ash roots only form arbuscular mycorrhiza. In contrast, beech benefited fungal feeders and detrimentally affected bacterial feeders due to more acidic soil conditions that increase the competitive strength of fungi. Lime tended to negatively impact total nematode density but positively influenced plant-feeding nematodes. Generally, beech and ash strongly but opposingly influenced the trophic structure of nematode communities suggesting that changes in tree species identity result in major shifts in the channeling of energy through decomposer food webs. The results indicate that the structure of soil food webs varies markedly with tree species and point to the importance of basal resources, i.e., leaf litter and rhizodeposits. This suggests bottom-up forces mediated by individual tree species to control major decomposition pathways rather than tree diversity. [Copyright &y& Elsevier]

Abstract: Stable isotope analysis has been used as a powerful tool in food web studies in terrestrial ecosystems. In addition the occurrence and abundance of fatty acids may serve as indicator for feeding strategies of soil animals. Here we combine both approaches and investigate the fatty acid composition, δ 13C values of bulk tissues and individual fatty acids in soil organisms. The fungi Chaetomium globosum and Cladosporium cladosporioides were isotopically labelled by fructose derived from either C3 or C4 plants, and the fungal-feeding nematode Aphelenchoides sp. was reared on C. globosum. Fungi and nematodes were used as diet for the Collembolan Protaphorura fimata. The sugar source was fractionated differently by fungal lipid metabolism in a species-specific manner that points to a sensitivity of physiological processing to the non-random distribution of 13C/12C isotopes in the molecule. As a general trend stearic acid (18:0) was depleted in 13C compared to the precursor palmitic acid (16:0), whereas its desaturation to oleic acid (18:1 ω9) favoured the 13C-rich substrate. Fatty acid profiles of P. fimata varied due to food source, indicating incorporation of dietary fatty acids into Collembolan tissue. Individuals feeding on fungi had lower amounts in C20 fatty acids, with monoenoic C20 forms not present. This pattern likely separates primary consumers (fungivores) from predators (nematode feeders). The isotopic discrimination in 13C for bulk Collembola ranged between −2.6 and 1.4‰ and was dependent on fungal species and C3/C4 system, suggesting differences at metabolic branch points and/or isotope discrimination of enzymes. Comparison of δ 13C values in individual fatty acids between consumer and diet generally showed depletion (i.e. de novo synthesis) or no changes (i.e. dietary routing), but the fractionation was not uniform and affected by the type of ingested food. Fatty acid carbon isotopes were more variable than those of bulk tissues, likely due to both the distrimination by enzymes and the different lipid origin (i.e. neutral or polar fraction). [Copyright &y& Elsevier]

The fatty acid composition of 16 different soil fungi (ascomycetes, basidiomycetes, mitosporic fungi) and a fungal-feeding nematode Aphelenchoides sp. reared on seven fungal species was investigated. Additionally, fatty acid profiles of Aphelenchoides sp. and A. saprophilus grown on the same fungal food source,Agrocybe gibberosa, were compared. Thirteen predominant fatty acids were detected in the fungi. Most of them occurred in each of the 16 species, but relative quantities of individual fatty acids differed, in particular those of unsaturated ones. Most fungal species could be differentiated from each other on this basis. Our study revealed convergence, but also, differences, in the fatty acid composition of systematically related fungi, i.e. a taxonomic or phylogenetic relationship was not necessarily accompanied by similarity in fatty acid profiles. Nematodes comprised a wider fatty acid spectrum than fungi, with 17 predominant fatty acids, and a higher amount of long-chain, polyunsaturated acids than their fungal diet. Fungal host tissue may have supplied palmitic, oleic and linoleic acid present, whereas most of the long-chain unsaturated fatty acids were synthesized by the nematodes. Unsaturated fatty acids mainly belonged to the ω6 and 9 family, indicating a carboxyl-directed desaturation as a major metabolic pathway. The fungal host significantly affected the fatty acid profile of the nematodes. However, we could not assign individual fatty acids as biomarkers reflecting the dietary source, likely due to the considerable convergence within tested fungal species. Of the basidiomycetes analyzed Laccaria laccata, was distinctly separated from the others and this difference could also be detected in the nematodes showing the influence of food type. We conclude that the lipid composition of nematodes is controlled by both the nematode and its diet and that monitoring fatty acid patterns of soil animals may therefore provide a way to detect trophic interactions in belowground food webs. [Copyright &y& Elsevier]

Abstract: The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat+maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat+maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (Corg), extractable organic C (EOC), microbial biomass (Cmic) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to Corg, EOC, Cmic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass. [Copyright &y& Elsevier]

Soil organisms constitute multitrophic food webs, which are important determinants for nutrient and carbon flow through terrestrial systems. However, these complex interactions are not well understood, particular in the most cryptic compartment – the link between microorganisms and microfauna. The present study investigates binary trophic links between microbial prey and nematodes in a controlled semi-natural soil environment. A microcosm system with circulating airflow was filled with γ-irradiated arable soil (Luvisol), inoculated with 13C labelled microbial diet and nematode grazers, and incubated for 72 h. Signatures of 13C and CO 2 evolved from microcosms, soil phospholipid fatty acids and nematode densities were determined. Except for the model nematode Caenorhabditis elegans, all organisms joint in the 12 different investigated binary links represent dominant taxa at the arable field site. Studied were the fungal grazers Aphelenchoides saprophilus and Aphelenchus sp. in combination with the fungi Chaetomium globosum, Aspergillus terreus or Cryptococcus terreus as well as the bacterial grazers C. elegans and Acrobeloides buetschlii in combination with the bacteria Pseudomonas putida , Kitasatospora sp. or Sphingomonas trueperi. The results assigned trophic and non-trophic interactions and considerable variation between binary links. The biomass decline of S. trueperi in presence of C. elegans as well as the increased release of 13C in links of C. globosum and A. terreus with Aphelenchus sp. and S. trueperi with A. buetschlii assign nematode grazing and thus a classical predator-prey system. Such direct trophic interactions can induce distinct C fluxes within a binary link as assigned by the increased density (i.e. biomass) of C. elegans when feeding on S. trueperi. In contrast, the numbers of Aphelenchus sp. declined when feeding on A. terreus , and thus reduced C flow to the next trophic level. Grazing can also foster C allocation in microbial biomass via compensatory growth, as indicated e.g. for the link of Aphelenchus sp. with the fungi C. globosum and A. terreus. While these diverse responses to grazing were to be expected, the use of CO 2 and 13C measurements further revealed that non-trophic interactions shape C flux in a binary link, particularly in the bacterial channel. Besides the fungal link of C. terreus with Aphelenchus sp., non-trophic interactions were obvious for the bacterial links Kitasatospora sp. with C. elegans or A. buetschlii, and P. putida with A. buetschlii. The presence of nematodes caused a reduction in respiration (evolved CO 2), thus a decline in microbial activity. The formation of chemical and/or morphological defense is likely a reason for that reduced prey activity. For further studies the inclusion of such non-trophic interactions is necessary to achieve realistic models for C flow through soil food webs. • Microbial traits cause variation in palatability for nematode grazers. • Predator-prey interactions of nematodes within a functional guild differ. • Impact of non-trophic interactions on C flux depends on the specific binary link. [ABSTRACT FROM AUTHOR]

Abstract: Fatty acid (FA) analysis is used as a promising tool to investigate trophic interactions in soil food webs. The FA profile of neutral lipids in consumers is affected by the diet, and the occurrence and amount of certain FAs can reflect feeding strategies. We investigated the lipid composition of the Collembola Folsomia candida, Heteromurus nitidus and Protaphorura fimata with the fungus Chaetomium globosum as food source. The impact of environmental temperature and life stage was assessed, with special respect to linoleic acid (18:2ω6,9) as a marker FA for fungal feeding. In all Collembola species the ratio of C16/C18 in neutral lipid fatty acids (NLFAs) increased with decreasing temperature. In the NLFAs of F. candida and H. nitidus the Unsaturation Index and the amount of 18:2ω6,9 decreased with temperature, whereas in P. fimata effects were the opposite. The composition of phospholipid fatty acids (PLFAs) differed between species, but was little affected by temperature. The degree of unsaturation in NLFAs increased with the age of Collembola, mainly due to higher amounts of 18:2ω6,9 and a lower proportion of 18:1ω9. The biomarker linoleic acid represented over 20% of FAs in all fungal feeding Collembola. Despite considerable influence of temperature and life stage on its proportion, the amount was always higher than in individuals reared on other diets. This suggests that linoleic acid can serve as marker for fungal feeding independent of such physiological variations in Collembola. [Copyright &y& Elsevier]

Summary: Naturally occurring stable isotopes of carbon and nitrogen are powerful tools to investigate food webs, where the ratio of 15N/14N is used to assign trophic levels and of 13C/12C to determine the food source. A shift in δ 15N value of 3‰ is generally suggested as mean difference between two trophic levels, whereas the carbon isotope composition of a consumer is assumed to reflect the signal of its diet. This study investigates the effects of food quality, starvation and life stage on the stable isotope fractionation in fungal feeding Collembola. The fractionation of nitrogen was strongly affected by food quality, i.e. the C/N ratio of the fungal diet. Collembola showed enrichment in the heavier isotope with increasing N concentration of the food source. Δ15N varied between 2.4‰, which assigns a shift in one trophic level, and 6.3‰, suggesting a shift in two trophic levels. Starvation up to 4 weeks resulted in an increase in the total δ 15N value from 2.8‰ to 4.0‰. Different life stages significantly affected the isotope discrimination by Collembola with juveniles showing a stronger enrichment (Δ15N=4.9‰) compared to adults (Δ15N=3.5‰). Δ13C varied between −2.1‰ and −3.3‰ depending on the food quality, mainly due to compensational feeding on low quality diet. During starvation δ 13C value decreased by 1.1‰, whereas the life stage of Collembola had no significant effect on isotopic ratios. The results indicate that the food resource and the physiological status of the consumer have important impact on stable isotope discrimination. They may cause differences in fractionation rate comparable to trophic level shifts, a fact to consider when analysing food web structure. [Copyright &y& Elsevier]