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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]

Free-living nematodes harbor and disseminate various soil-borne bacterial pathogens. Whether they function as vectors or environmental reservoirs for the aquatic L. pneumophila, the causative agent of Legionnaires' disease, is unknown. A survey screening of biofilms of natural (swimming lakes) and technical (cooling towers) water habitats in Germany revealed that nematodes can act as potential reservoirs, vectors or grazers of L. pneumophila in cooling towers. Consequently, the nematode species Plectus similis and L. pneumophila were isolated from the same cooling tower biofilm and taken into a monoxenic culture. Using pharyngeal pumping assays, potential feeding relationships between P. similis and different L. pneumophila strains and mutants were examined and compared with Plectus sp., a species isolated from a L. pneumophila-positive thermal source biofilm. The assays showed that bacterial suspensions and supernatants of the L. pneumophila cooling tower isolate KV02 decreased pumping rate and feeding activity in nematodes. However, assays investigating the hypothesized negative impact of Legionella's major secretory protein ProA on pumping rate revealed opposite effects on nematodes, which points to a species-specific response to ProA. To extend the food chain by a further trophic level, Acanthamoebae castellanii infected with L. pneumphila KV02 were offered to nematodes. The pumping rates of P. similis increased when fed with L. pneumophila-infected A. castellanii, while Plectus sp. pumping rates were similar when fed either infected or non-infected A. castellanii. This study revealed that cooling towers are the main water bodies where L. pneumophila and free-living nematodes coexist and is the first step in elucidating the trophic links between coexisting taxa from that habitat. Investigating the Legionella–nematode–amoebae interactions underlined the importance of amoebae as reservoirs and transmission vehicles of the pathogen for nematode predators. [ABSTRACT FROM AUTHOR]

Carbon (C) cycling is crucial to agroecosystem functioning. Important determinants for the belowground C flow are soil food webs, with microorganisms and microfaunal grazers, i.e., nematodes, as key biota. The present study investigates the incorporation of plant-derived C into the nematode micro-food web under two different cropping systems, grassland (ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.)) and agroforest (willow (Salix schwerinii Wolf and Salix viminalis L)). To quantify the C flux from the plant into the soil micro-food web, grass and willow were pulse-labeled with 13CO2 and the incorporation of 13C into the nematode trophic groups was monitored 3, 7, 14 and 28 days after labeling. The natural stable isotope signals (13C/12C, 15N/14N) were analyzed to determine the structure of the nematode micro-food web. The natural isotopic δ15N signal revealed different trophic levels for omnivores and predators in grassland and agroforest soils. The incorporation of plant C into nematode tissue was detectable three days after 13CO2 labeling with the highest and fastest C allocation in plant feeders in grassland, and in fungal feeders in agroforest soil. C flux dynamics between the aboveground vegetation and belowground micro-food web varied with cropping system. This demonstrates that crop-specific translocation of C affects the multitrophic interactions in the root environment, which in turn can alter soil nutrient cycling. [ABSTRACT FROM AUTHOR]

Abstract: Knowledge on food web structure and function provides important information on trophic relationships, energy flux, and ecosystem response to environmental changes. The tracing of trophic marker fatty acids (FAs) to assign predator–prey interactions is well established in food web research. However, the application on the level of entire consumer communities has not been performed yet. Here, we demonstrate that lipid pattern analysis of entire consumer assemblages can provide a first‐line assessment for food web diagnostics. The trophic diverse group of nematodes was used as a model and community level lipid profiling (CLLP) was performed. As prerequisite a lipid library was constructed, assigning FAs to their predominant organismic origin to disentangle phylogenetic (i.e. consumer lipid metabolism) from diet type effects on CLLP. The suitability of CLLP analysis was tested using 150 forest sites differing in management type and intensity across three German regions. The nematode CLLP reflected ecosystem conditions and thereby separated regions as well as forest habitats, that is, conifer and deciduous stands. CLLP further enabled to address if environmental properties acted on the level of consumers or their resource use or both. However, forest management intensity was poorly assigned, likely due to a predominant species‐specific impact, not represented on community basis. We propose CLLP as a fast and robust biochemical method for food web diagnostics in cryptic habitats such as soil or benthos. Provided that organismic assemblages compared are substantially different in lipid metabolism, CLLP give insight into changes in the composition of consumer communities as well as their major diet under field conditions. [ABSTRACT FROM AUTHOR]

The multitrophic interactions in the rhizosphere impose significant impacts on microbial community structure and function, affecting nutrient mineralisation and consequently plant performance. However, particularly for long-lived plants such as forest trees, the mechanisms by which trophic structure of the micro-food web governs rhizosphere microorganisms are still poorly understood. This study addresses the role of nematodes, as a major component of the soil micro-food web, in influencing the microbial abundance and community structure as well as tree growth. In a greenhouse experiment with Pedunculate Oak seedlings were grown in soil, where the nematode trophic structure was manipulated by altering the proportion of functional groups (i.e., bacterial, fungal, and plant feeders) in a full factorial design. The influence on the rhizosphere microbial community, the ectomycorrhizal symbiont Piloderma croceum, and oak growth, was assessed. Soil phospholipid fatty acids were employed to determine changes in the microbial communities. Increased density of singular nematode functional groups showed minor impact by increasing the biomass of single microbial groups (e.g., plant feeders that of Gram-negative bacteria), except fungal feeders, which resulted in a decline of all microorganisms in the soil. In contrast, inoculation of two or three nematode groups promoted microbial biomass and altered the community structure in favour of bacteria, thereby counteracting negative impact of single groups. These findings highlight that the collective action of trophic groups in the soil micro-food web can result in microbial community changes promoting the fitness of the tree, thereby alleviating the negative effects of individual functional groups. [ABSTRACT FROM AUTHOR]

• Nematode communities were shaped by environmental filtering. • Tree type, ground cover and resources were drivers in the organic layer. • Abiotic conditions (water content, soil type) impacted the mineral soil most. • Specific nematode families correlated with forest stand age and density. • Management practice increasing deadwood had no effects. A key to forest productivity is structure and function of the biotic microbiome in the soil, which determines nutrient mineralisation and consequently tree growth and fitness. Soil nematodes are a dominant part of these assemblages, with an important role in the associated micro-food web. However, nothing is known about how forest management and soil properties affect soil nematodes. This study investigated the nematode communities in the organic layer and mineral soil of 150 forests located in three different German regions. Fifty forest stands each were studied in Schwäbische Alb (ALB), Hainch-Dün (HAI) and Schorfheide-Chorin (SCH). The effect of forest management (forest management index – ForMI, silvicultural management intensity indicator – SMI), soil properties (total C, N, S, P, organic C, inorganic C, pH, water content) and microbial food source (phospholipid fatty acids - PLFA) on nematode density, community and ecological indices were assessed. The nematode density in the organic layer of the forests was with a mean of 290 individuals g−1 DW comparable across all three regions, while in the mineral soil nematode density was significant lower in SCH. Here, dry sandy soils are frequently stocked with intensively managed pine forests, and the associated fungal-based decomposition is reflected in a greater proportion of fungal-feeding nematodes and a higher Channel Index. Effects of region and soil layer were mirrored by the structure of the nematode community. Organic layer and mineral soil differed distinctly, while in the latter SCH separated from ALB and HAI. The nematode community was predominantly affected by region, soil properties and resources, with microbial diet as driving factor in the organic layer, whereas soil parameters were more important in mineral soil. In comparison, the influence of forest management was low, with ForMI and SIM together explaining 8% (organic layer) and 4% (mineral soil) of the overall variation. Most remarkable, the nematode community in HAI, a region dominated by broad-leaf forests, was resilient to management intensity in the organic layer. On the other hand, management practice had family-specific impact: In the organic layer of ALB, the density of the bacterial-feeding Teratocephalidae and the fungal-feeding Aphelenchoididae increased with higher Inonat (indicates non-natural tree species) and higher SMIr (addresses stand age and species selection related to the risk of stand loss). In sum, the structure and function of nematode communities were driven by both, biotic factors such food source (i.e. the soil layer inhabited) and abiotic factors such as soil type and climate (i.e. region). Furthermore, forest management can affect nematode communities, mainly by the selected tree species, but also by stand age and density. [ABSTRACT FROM AUTHOR]

Interactions between bacteria and nematode grazers are an important component of soil food webs yet, due to the cryptic habitat, they are almost exclusively investigated in artificial agar substrate. Transport, food choice and foraging experiments were performed in a modular microcosm system with the nematode Acrobeloides buetschlii and bacterial diets (Escherichia coll, Pseudomonas putida and Bacillus subtilis) in gamma-irradiated soil. Bacterial biomass was assessed by soil phospholipid fatty acids (PLFAs). Continuous random foraging of nematodes was affected by soil type. Food choice experiments revealed diet switch and time lag preference responses, suggesting that nematode population fluctuations are driven by multiple factors such as bacterial attractants, defence strategies or food quality. Application of PLFA markers revealed a strong nematode predation pressure, as biomass in P. putida declined by 50%, whereas no transport of bacteria through soil was indicated. Overall, semi-natural experimental systems are an essential prerequisite to gain a realistic picture in microbial-microfaunal interactions. [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]

Nematodes are the most abundant invertebrates in soils and are key prey in soil food webs. Uncovering their contribution to predator nutrition is essential for understanding the structure of soil food webs and the way energy channels through soil systems. Molecular gut content analysis of consumers of nematodes, such as soil microarthropods, using specific DNA markers is a novel approach for studying predator-prey interactions in soil. We designed new specific primer pairs (partial 18S rDNA) for individual soil-living bacterial-feeding nematode taxa ( Acrobeloides buetschlii, Panagrellus redivivus, Plectus velox and Plectus minimus). Primer specificity was tested against more than 100 non-target soil organisms. Further, we determined how long nematode DNA can be traced in the gut of predators. Potential predators were identified in laboratory experiments including nine soil mite (Oribatida, Gamasina and Uropodina) and ten springtail species (Collembola). Finally, the approach was tested under field conditions by analyzing five mite and three collembola species for feeding on the three target nematode species. The results proved the three primer sets to specifically amplify DNA of the respective nematode taxa. Detection time of nematode DNA in predators varied with time of prey exposure. Further, consumption of nematodes in the laboratory varied with microarthropod species. Our field study is the first definitive proof that free-living nematodes are important prey for a wide range of soil microarthropods including those commonly regarded as detritivores. Overall, the results highlight the eminent role of nematodes as prey in soil food webs and for channelling bacterial carbon to higher trophic levels. [ABSTRACT FROM AUTHOR]

Soil animals live in complex and heterogeneous habitats including litter of various types but also microhabitats such as mosses, fungal mats and grass patches. Soil food webs have been separated into a slow fungal and a fast bacterial energy channel. Bacterial-feeding nematodes are an important component of the bacterial energy channel by consuming bacteria and forming prey for higher trophic levels such as soil microarthropods. Investigating the role of nematodes as prey for higher trophic level consumers has been hampered by methodological problems related to their small body size and lack in skeletal structures which can be traced in the gut of consumers. Recent studies using molecular gut content analyses suggest that nematodes form major prey of soil microarthropods including those previously assumed to live as detritivores. Using molecular markers we traced nematode prey in fourteen abundant soil microarthropod taxa of Mesostigmata and Oribatida (both Acari) from three different microhabitats (litter, grass and moss). Consumption of nematodes varied between mite species indicating that trophic niche variation contributes to the high diversity of microarthropods in deciduous forests. Further, consumption of nematodes by Mesostigmata (but not Oribatida) differed between microhabitats indicating that trophic niches vary with habitat characteristics. Overall, the results suggest that free-living bacterial-feeding nematodes form important prey for soil microarthropods including those previously assumed to live as detritivores. [ABSTRACT FROM AUTHOR]

Increased carbon translocation to the rhizosphere via ‘leakage’ induced by low amounts of plant parasitic nematodes can foster microorganisms. The effects of the root-knot nematode Meloidogyne incognita on microbial biomass (Cmic) and community structure (phospholipid fatty acids) in the rhizosphere of barley were studied. Inoculation densities of 2000, 4000, and 8000 nematodes were well below the threshold level for plant damage. A 13CO2 pulse-labelling was performed to assess the distribution of assimilated 13C in the rhizosphere. Infection with M. incognita increased the carbon concentration in shoots, and enhanced root biomass slightly. The presence of nematodes did not affect microbial biomass, but significantly changed the allocation of the recent photosynthate. Less plant carbon was sequestered by microorganisms with increasing nematode abundance. Microbial community structure was distinctly altered in the early stages of the plant–nematode interactions. Both, bacteria and fungi, showed a positive response with 2000, and a negative one with 4000 and 8000 M. incognita added. The results suggest that low-level root herbivory still imposes a considerable carbon demand, and that proliferation of microorganisms due to increased rhizodeposition may be short-termed. The carbon flow to rhizosphere microbial communities is likely dependent on the specific nematode–plant association and the developmental stage of the nematode in the host. [ABSTRACT FROM AUTHOR]

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]

The growth of Aphelenchoides sp. populations was investigated in vitro with 17 different fungal species as food source. Nematode mass cultures were obtained with saprophytic (Agrocybe, Chaetomium) and especially with mycorrhizal fungi (Cenococcum, Hymenoscyphus, Laccaria). Mitosporic species, like Alternaria, Monocillium or Penicillium, were generally meagre or non-hosts. This poor host suitability is likely due to the release of toxic metabolites (e.g. antibiotics) and/or to morphological differences (e.g., forming of conidiophores) by the fungi. Frequent grazing of nematodes on mycorrhizal mycelia may be of major significance for the establishment and maintenance of mycorrhizal associations in the field. Food preference of Aphelenchoides sp. was tested in choice chamber experiments. Nematodes showed a marked preference for particular fungal species. They changed food source with time, indicating a “mixed diet” selection, probably a strategy to avoid the concentration of toxic metabolites. The attractiveness of a fungus was not necessarily correlated with its suitability as a host. That a poor fungal host can be a strong nematode attractant and influence their spatial distribution in the soil has implications for nematode populations in the field. In Laborexperimenten wurde die Vermehrung des Nematoden Aphelenchoides sp. mit 17 verschiedenen Pilzspezies als Nahrungsgrundlage untersucht. Neben saprophytischen Arten (Agrocybe, Chaetomium) eigneten sich insbesondere Mykorrhizapilze (Cenococcum, Hymenoscyphus, Laccaria) für eine Massenvermehrung. Eine schlechte Nahrungsquelle stellten mitosporische Arten, wie Alternaria, Monocillium oder Penicillium, dar. Dies dürfte auf toxische Stoffwechselprodukte (z.B., Antibiotika) und/oder auf morphologische Unterschiede (z.B., Sporenbildung) zurückzuführen sein. Die gute Vermehrung der Nematoden an Mykorrhizapilzen ist von weitreichender Bedeutung für das Freiland. Negative Auswirkungen auf die Ausbildung und Funktion von Mykorrhiza im Boden sind zu erwarten. In Nahrungswahlexperimenten zeigte Aphelenchoides sp. eine ausgeprägte Präferenz für bestimmte Pilzarten. Das Wechseln zwischen den einzelnen Pilzspezies weist auf die Bevorzugung von “Mischnahrung” hin. Dies dürfte eine Strategie zur Vermeidung von hohen Konzentrationen toxischer Nahrungsbestandteile sein. Präferenz und Nahrungsqualität standen nur in geringem Zusammenhang. Somit können auch Pilze, die eine schlechte Nahrungsquelle darstellen, attraktiv auf Nematoden wirken und deren Verbreitung in Boden und Rhizosphäre beeinflussen. [ABSTRACT FROM AUTHOR]

The nematode fauna at two contrasting subarctic sites was studied at Abisko, Swedish Lapland. One site was a dwarf shrub dominated, tree-line heath (450 m a.s.l.) and the other a high altitude fellfield (1150 m a.s.l.). Responses in the composition of the nematode fauna to eight growing seasons with simulated climate change were monitored. The environmental manipulations consisted of temperature increase by using passive greenhouses, NPK fertilization, and a combination of both. A total of 98 species was found at the sites, with twice as many species at the heath compared to the climatically harsh fellfield. Species similarity was low, most likely due to differences in soil type and vegetation cover. The environmental perturbations caused distinct changes in dominance and trophic structure. Taxa common at both sites responded similarly with increases in Aphelenchoides, Filenchus and Plectus , and decreases in Eudorylaimus, Monhystrella and Teratocephalus . Generally, the manipulations changed the trophic composition in favour of fungal and plant feeding species, indicating a shift in the decomposition pathway. Modification provoquee du climat dans des sols subarctiques: reaction de la composition specifique des nematodes et de la structure de la dominance - La faune des nematodes a ete etudiee a Abisko (Laponie suedoise) en deux sites subarctiques contrastes. L'un consistait une vegetation de bruyere dominee par des buissons nains (a 450 m d'altitude) et l'autre etait situe sur un coteau a haute altitude (1150 m). La reaction de la faune nematologique aux modifications provoquees du climat a ete observee. Ces modifications comprenaient une augmentation de la temperature par utilisation de serres passives, une fertilisation NPK et une combinaison des deux. Un total de 98 especes ont ete trouvees dans les deux sites. La similitude des especes etait peu elevee, ce tres probablement du aux differences dans les types de sols et la couverture vegetale. Le nombre d'especes etait deux fois plus eleve sous bruyeres, que pour le coteau au climat rude. Apres huit saisons de croissance ayant comporte des perturbations de l'environnement, de nets changements sont apparus dans la dominance et la structure trophique. Les nematodes communs aux deux sites ont montre une reaction similaire, positive pour les Aphelenchoides, Filenchus et Plectus et negative pour les Eudorylaimus, Monhystrella et Teratocephalus. En general, les modifications ont change la composition trophique en faveur des especes fungivores et herbivores, indiquant par la des changements dans le mode de decomposition des vegetaux. [ABSTRACT FROM AUTHOR]

Free-living nematodes act as reservoirs and vectors of the food-borne pathogen Salmonella enterica , one of the leading causes of gastrointestinal human infections worldwide. Although the transport of this human pathogen through soil by nematodes has major concerns for food safety, little is known about the food preference for S. enterica in nematodes. To disentangle the trophic links between nematodes and bacteria, the food choice of the nematode species Acrobeloides buetschlii , Caenorhabditis elegans and Pristionchus pacificus was examined under semi-natural conditions using a modular soil system. S. enterica serovar Typhimurium LT2 was tested against five common soil bacteria, i.e. Arthrobacter alpinus, Bacillus subtilis, Escherichia coli OP50 , Granulicella arctica and Pseudomonas putida IsoF in binary food choice assays. The tested nematodes showed a general preference for bacterial strains with a low C/N ratio (E. coli), and avoidance of strains with a high C/N ratio (G. arctica and A. alpinus), indicating that food choice is rather linked to nutritional value than to bacterial pathogenicity. Moreover, the enrichment opportunists C. elegans and P. pacificus showed a stronger attraction to a suitable bacterial diet compared to the general opportunist A. buetschlii, which also thrives food-depleted soils. Interestingly, there was no evidence regarding a differentiation between common soil bacteria and the pathogen S. enterica. Overall, S. enterica was used as resource by the tested bacterial-feeding soil nematodes, highlighting their vector potential for food-borne pathogens in agricultural soils. • Tested nematodes fed on Salmonella enterica , suggesting a general vector potential. • Food selection is mainly due to bacterial nutritional value and not pathogen status. • Nematode life-history traits may additionally affect choice behavior. [ABSTRACT FROM AUTHOR]

Abstract: Recent stable isotope analyses indicate that a number of putative detritivorous soil microarthropods is not typical detritivores but rather live as predators or scavengers. Using molecular gut content analyses the present study investigates if nematodes indeed form part of the diet of oribatid mites. First, in a no-choice laboratory feeding experiment two nematode species (Phasmarhabditis hermaphrodita and Steinernema feltiae) were offered to eight species of oribatid mites and one gamasid mite. Second, after feeding for 4 and 48h on each nematode species the detection time of prey DNA in the oribatid mite species Steganacarus magnus was investigated. Third, in a field experiment nematode prey (P. hermaphrodita and S. feltiae) in the diet of microarthropods was investigated distinguishing between scavenging and predation. In the no-choice laboratory experiment not only the gamasid mite but also several of the studied oribatid mite species consumed nematodes. After feeding on nematodes for 4h prey DNA was detectable in S. magnus for only 4h, but after feeding for 48h prey DNA was detectable for 128h, indicating that the duration of feeding on prey is an important determinant for prey DNA detection. The field experiment confirmed that oribatid mite species including Liacarus subterraneus, Platynothrus peltifer and S. magnus intensively prey on nematodes. Interestingly, DNA of dead P. hermaphrodita was detectable to a similar degree as that of living individuals indicating that scavenging is of significant importance in decomposer food webs. Results of our study indicate that predation and scavenging on nematodes by “detritivorous” microarthropods in soil food webs need to be reconsidered. [Copyright &y& Elsevier]

For vertebrates, the adequate supply of polyunsaturated fatty acids (PUFA) by the diet, in particular ω3 long-chain PUFA, is considered essential for neural development, growth and reproduction. In contrast to aquatic ecosystems, ω3 long-chain PUFA apparently are not widely available in the terrestrial food chain. Their de novo synthesis requires the presence of Δ12 and ω3 fatty acid desaturase enzymes, which are absent in vertebrates but present, for example, in the nematode Caenorhabditis elegans (FAT-2 and FAT-1). This raises the question if soil-dwelling nematodes offer substantial supply of these valuable nutritional compounds in terrestrial food webs. BLAST searches in available nematode genomes revealed the existence of fat-2 like genes in almost all clade III–V species, but failed to identify orthologs in clade I–II nematodes. An additional RT-PCR screen across soil-dwelling nematode species identified six novel fat-2 like genes. Hints for the genetic basis of a ω3 (fat-1) desaturase activity was found only in selected clade IV–V species, but not in clades I to III nematodes. Fatty acid pattern analyses following a PUFA-free cultivation and enzymatic characterization of six selected fat-2 or fat-1 like desaturases in yeast confirmed the findings from the genetic approaches. Thus, in similar soil habitats, taxa exist that can synthesize ω3 long-chain PUFA (as Panagrolaimus , Mesorhabditis and Caenorhabditis) whereas others are unable to do so (Acrobeloides , Cephalobus and Oscheius). While these nematodes do not differ in trophic position or major diet, distinction in reproduction mode may have led to the observed variations in desaturase genes. • Six novel delta-12 desaturases in soil-dwelling nematodes were detected. • Heterologous expression in yeast confirmed enzymatic activity of desaturases. • Presence/absence of delta-12/15 desaturases was specific to nematode family. • Nematodes are a source for omega-3 long-chain PUFA in soil food webs. [ABSTRACT FROM AUTHOR]

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]

Nematodes, with functional groups at all trophic levels, play a key role in soil food webs. Their community structure allows to assign food web conditions and soil health, e.g. to address management impact on arable land. A field experiment was set up near Potsdam (Brandenburg, north-east Germany) to investigate the effects of different fertilization and irrigation regimes on the nematode fauna and the micro-food web in a sandy soil. At an arable site cropped with potatoes the following treatments were established in a randomized plot design: surface, drip and no irrigation, combined with/without fertilization. Additionally, fertigated plots (simultaneous application of fertilizer in dissolved drip irrigation water) with/without crops were investigated. Soil samples were collected during one vegetation period in June, July and August. The nematode community structure was determined and a faunal analysis performed resulting in: Maturity Index (MI), Plant Parasite Index (PPI), Channel Index (CI), Enrichment Index (EI), Structure Index (SI) and metabolic footprints (F). In addition, phospholipid fatty acids were determined to identify microbial community structure and biomass. In August, surface irrigation resulted in a higher MI (i.e. lower disturbance) at unfertilized compared to fertilized plots. Furthermore, the fertigated plots showed a higher MI with crops than without. The PPI increased over time, while the CI indicated higher fungal decomposition at dryer conditions in August. The EI points to moderately enriched nutrient conditions, which was underlined by a high metabolic footprint of bacterial feeders. The structure and enrichment footprint showed that the stability of the micro-food web increased with time, indicating a positive influence of crop growth. In sum, effects of irrigation practice on the nematode community were apparent, while no response to the different fertilizer treatments was detected. • The arable soil harbored a disturbed micro-food web at basal status. • Food web stability increased with crop growth. • Linkage between nematodes and microbial resources was strongest at plant senescence. • Fertilization practice had no and irrigation minor impact on nematode communities. [ABSTRACT FROM AUTHOR]

The assemblages of organisms residing at different soil horizons are important drivers for carbon and nutrient cycle in arable systems, yet knowledge on food web dynamics below the plough zone is sparse. The present study investigated the effects of management practice on the nematode community structure and biomass in the top soil, rooted zone below plough layer, and root free deeper soil in two successive vegetation periods. Resource quality was manipulated by crop plant (maize and wheat) and organic amendment (with/without maize litter), to differently fuel the major soil carbon pathways based on roots, bacteria and fungi. Nematode assemblages were used as model to determine changes in the structure, function and carbon allocation of the soil micro-food web. The biomass of nematode families as well as the fungal to bacterial feeder ratio F/(F + B) assigned a predominance of the bacterial carbon channel in the arable soil. The differences in spatial and temporal availability of major plant resources, i.e. litter (recalcitrant substrate) and rhizodeposits (labile carbon), separated the nematode fauna into meta-communities along the depth profile. The seasonal pattern in population density, and the positive effects of litter amendment predominantly in the top soil, indicated low migration between these patches during crop growth. However, mass-flow as resource subsidy for communities at depth takes place under fallow during winter. Crop type was most important for carbon allocation, with wheat generally resulting in a greater biomass build-up of nematodes. The biomass distribution of trophic groups differed with depth, pointing to a strong bottom-up effect in the root channel, whereas predation likely was the primary regulating force in the fungal channel. Overall, combining nematode community composition with biomass revealed spatially separated micro-food web assemblages, differing in structure and carbon allocation. Thereby transport processes cross community boundaries, linking top soil and subsoil, notably in periods without a crop. [ABSTRACT FROM AUTHOR]

The agricultural application of organic fertilizer as well as irrigation with partially treated wastewater is proposed to increase crop yield, but also the incidence of food borne pathogens. Studies have concentrated on the impact of such management practice on soil health, and on spread of pathogens during harvest, transport, and marketing, whereas the role of soil organisms in these processes was largely neglected. Therefore, the objective of the present work was to study microorganisms and nematodes as potential agents in suppression or dissemination of human pathogens. Two experimental field sites were established in semi-arid soils in the West Bank in Palestine. Treatments comprised application of fresh water, wastewater, sewage sludge and chicken manure in three successive crop cycles, and the effects on the microbial and nematode communities were assessed. Microbial biomass was generally fostered by all three organic amendments but the increase was dependent on arable site as well as the type of urban waste. The introduced microorganisms were able to establish in the field environment, indicating that the autochthonous microbial community was not effective in suppressing allochthonous enteropathogens. Nematodes, as potential bacterial vectors, displayed a population increase with organic input across crops and sites. Effects were most distinct by amendment with chicken manure, which additionally resulted in a decrease in diversity, with a distinct increase in bacterial-feeding r -strategists and strongly enriched food web conditions. In contrast, application of treated wastewater and sewage sludge fostered K -strategist, predominantly omnivores. This enhanced biomass at higher trophic levels indicates ample energy flux along the trophic cascade as well as a stable and structured food web, despite possible contaminants introduced via urban residues. The results highlight the potential role that soil biota play in governing soil responses to the application of biosolids and wastes in agricultural management practice. [ABSTRACT FROM AUTHOR]

The synergistic and antagonistic interactions among biotic components in the rhizosphere play a crucial role in plant defence against soil-borne pathogens. We investigated if the rhizosphere helper bacterium Streptomyces sp. AcH 505 (HB) indirectly protects the plant from the parasitic nematode Pratylenchus penetrans by modifying the rhizosphere microbial community structure and whether these interactions are dependent on the growth stage of oaks. Changes in the abundance of Streptomyces sp. AcH 505 and the phospholipid fatty acid (PLFA) composition of the rhizosphere soil as well as oak shoot and root biomass were assessed. Investigated were the bud resting stage A and the bud swelling stage B with maximal root elongation of oak microcuttings at two successive harvest times. The deleterious effect of P. penetrans on oak biomass was dependent on plant development, being limited to oak microcuttings growing at the stage B. In comparison to control and HB inoculated soils, shoot biomass decreased by about 33% and 41%, and root biomass by about 33 and 48%, respectively. The antagonistic effect of Streptomyces against the nematode was linked to shifts in the rhizosphere microbial community. The Streptomyces AcH505 strain promoted growth of oak microcuttings at bud swelling stage B during maximal root elongation and enhanced the abundance of saprophytic and ectomycorrhizal fungi in the rhizosphere by 158% with respect to controls. Our results highlight the importance of Streptomyces for counteracting the damage of nematodes and promoting plant growth in natural ecosystems such as forests. [ABSTRACT FROM AUTHOR]

Soils store ∼80% of global terrestrial organic carbon and alterations in fluxes into and out of this pool may interact with ongoing climate change. Belowground food webs drive soil C dynamics, but little is known about their responses to co-occurring global change agents. We investigated open-air experimental grassland communities at ambient and elevated atmospheric CO 2 concentration, ambient and enriched nitrogen input, and ambient and reduced summer precipitation to evaluate how these agents interactively affect soil nematodes, which are often used as an indicator group for soil food web structure and soil health. The aim of the study was to elucidate the response of the functional diversity of soil nematodes to changing environmental conditions by using nematode functional guilds and indices as indicators. The results suggest that nematode functional guilds surpass nematode trophic groups as soil indicators, suggesting that more detailed data on nematode community structure is essential to capture functional changes in response to environmental change. For instance, the density of opportunistic fungal feeders increased due to N addition with the response being more pronounced at elevated CO 2 , whereas densities of sensitive fungal-feeders were increased at ambient N and elevated CO 2 , illustrating opposing responses within one trophic group. Opportunistic bacterial feeders increased at elevated N, but did not respond to other environmental factors studied. Root-feeding Longidoridae were significantly reduced at elevated CO 2 and elevated N compared to ambient conditions, whereas other plant feeders were little affected by the manipulations. Predacious nematodes were less abundant at elevated N, and the Structure Index (which indicates food web structure) suggested reduced top-down forces and simplified soil food webs, although omnivores did not vary significantly. Elevated CO 2 buffered the effect of reduced precipitation on the Enrichment Index (which indicates increased resource availability) and the Channel Index (which indicates changes in decomposition channel) probably due to reduced stomatal conductance at elevated CO 2 . Further, the results suggest that the decomposer community switched from a bacterial-dominated to a fungal-dominated system at elevated N, indicating shifts in the microbial community as well as in the functioning of belowground food webs. Overall, the studied global change agents interactively and differentially affected functional guilds of soil nematodes, suggesting complex changes in soil processes. We highlight that detailed information on the functional guilds of nematodes is likely necessary to fully understand alterations in soil food webs and related processes due to global environmental change. [ABSTRACT FROM AUTHOR]

Abstract: Soil food webs are important determinants for the carbon flow through terrestrial systems, with the trophic networks between microbes and microfaunal grazers forming the basis for processing plant resources. At an agricultural field site cropped with maize or wheat, plant carbon input to soil was experimentally manipulated by amendment with maize litter. The community structure of dominant micro-food web components, the bacteria, fungi, protozoa and nematodes, was investigated across a depth gradient comprising plough layer, rooted soil below plough horizon, and deeper root free soil. The community composition and diversity within micro-food webs, and the response to resource supply, were assessed in summer, the vegetation period with highest root exudation. In the plough layer amendment with plant residues increased microbial biomass as well as density of fan shaped amoeba morphotypes and of bacterial- and fungal-feeding nematodes. Diversity of food web assemblages was assessed by operational taxonomic units (OTU) for bacteria and fungi, protozoa morphotypes and nematode families. Changes in diversity were either not apparent (fungi, protozoa), negatively related to litter (bacteria) or positively linked to the presence of a specific crop plant (bacteria, nematodes). Based on nematode functional guilds and the related enrichment and structure index, general food web conditions were assigned as nutrient enriched, with a high degree of disturbance, and a dominance of the bacterial energy channel. In sum, litter amendment fostered the abundance but not the diversity of organisms as food webs remained bottom heavy with only small amounts of carbon conserved at higher trophic levels. Food web structure was more affected by the abiotic (soil profile) and biotic (crop plant) environment than by the supply with litter resources. [Copyright &y& Elsevier]

Abstract: A major issue in current ecological research is the effect of biodiversity on ecosystem functioning. Although several studies reported a positive diversity – productivity relationship, the role of soil animals has been largely neglected. Nematodes are among the most widespread and important herbivores causing substantial yield losses in agriculture; however, impacts of nematodes on the diversity – productivity relationship in semi-natural plant communities have not been investigated until today. In the framework of the Jena Experiment (Thuringia, Germany) we established control and nematicide treated subplots to manipulate nematode densities on plots varying in plant species (1–16) and functional group richness (1–4). We explored the interacting effects of nematicide application and plant diversity on the main trophic groups of nematodes and on aboveground plant productivity. Nematicide application reduced the number of nematodes significantly, particularly that of plant feeders and predators. The negative impact of nematicide application on plant and bacterial feeders depended however on the diversity of the plant community. Total plant shoot biomass tended to decrease in the presence of ambient nematode densities. In detail, nematode effects varied however with plant functional group identity by reducing only the shoot biomass of herbs significantly but not that of legumes. Furthermore, the shoot biomass of grasses tended to decrease in the presence of ambient nematode densities. In contrast to total shoot biomass, nematodes decreased grass shoot biomass only in high diverse but not in low diverse plant communities. Thus, the present study for the first time highlights that nematodes likely modify the community structure und functions of semi-natural plant communities by altering the competition between plant functional groups and by attenuating the diversity – productivity relationship. [Copyright &y& Elsevier]

• Anthropogenic forest use and management may affect ecological communities. • Aquatic communities in water-filled tree holes were used as a model system. • Forest management showed strong, often negative and indirect effects on insects. • Nematode and bacterial communities were less affected. • Forest management impacts are complex but should be considered explicitly. Ecological communities in forests have been shown to be strongly affected by forest management but a detailed understanding of how different components of management affect insect communities directly and indirectly via environmental variables, how management influences functional trait diversity and composition, and whether these results can be transferred to other functional groups besides insects (e.g. bacteria or nematodes) is still missing. To address these questions we used water-filled tree holes, which provide habitats for insect larvae and other aquatic organisms in forests, as a model system. We mapped all water-filled tree holes in 75 forest plots (1 ha) under different management intensity in three regions of Germany. We measured structural and climatic conditions at different spatial scales, sampled insect communities in 123 tree holes and bacterial and nematode communities in a subset of these. We found that forest management in terms of harvesting intensity and the proportion of non-natural tree species (species not part of the natural vegetation at the sites) negatively affected tree-hole abundance. An increased proportion of non-natural tree species had a positive direct effect on insect richness and functional diversity in the tree holes. However, a structural equation model showed that increasing management intensity had negative indirect effects on insect abundance and richness, operating via environmental variables at stand and tree-hole scale. Functional diversity and trait composition of insect communities similarly responded to changes in management-related variables. In contrast to insects, bacterial and nematode richness were not directly impacted by forest management but by other environmental variables. Our results suggest that forest management may strongly alter insect communities of tree holes, while nematodes and bacteria seem less affected. Most effects in our study were indirect and negative, indicating that management has often complex consequences for forest communities that should be taken into account in forest management schemes. [ABSTRACT FROM AUTHOR]