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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]

Numerous lab and field studies have reported the potential of soil predatory mites for the biological control of plant-parasitic nematodes and arthropods pests. Most of these studies have utilized biocontrol agents in augmentative releases, essentially controlling the pest with the released predators. While this may be a valid approach, we hypothesize that conservation of soil mite predators with available, suitable, and accessible free-living nematodes as prey, will provide better agricultural ecosystem performance and long-range sustainability. In this manuscript, we review the relevant studies on soil predatory mite–nematode interactions and highlight their potential for conservation biological control of soil-borne pests. Additionally, we emphasize the importance of implementing environmentally sound soil management practices for the sustainability and conservation of functional soil food webs. [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]

The objective of the present study was to assess if Acrobeloides buetschlii, an opportunistic species common across many soils, can be employed as a suitable model for interactions between free-living soil nematodes and enteric human pathogens. Acrobeloides buetschlii was exposed to mCherry-tagged Salmonella enterica and Escherichia coli 0157:H7 and its vector potential was assessed. Salmonella cells were more readily ingested by the nematodes compared to E. coli 0157:H7. Adult nematodes ingested more bacteria compared to juveniles. Salmonella survived internally for at least 7 days without affecting the viability of nematodes. Bacterial ingestion by A. buetschlii did not vary for three tested Salmonella serovars but was significantly lower for E. coli 0157:H7. Considering the ubiquitous nature of pathogen and vector, these findings suggest that A. buetschlii can serve as a relevant model for studying nematode-Salmonella interactions in an agricultural setting and as potential transport for food-borne pathogens from soil to crops. [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]

Background: Changes in plant diversity may induce distinct changes in soil food web structure and accompanying soil feedbacks to plants. However, knowledge of the long-term consequences of plant community simplification for soil animal food webs and functioning is scarce. Nematodes, the most abundant and diverse soil Metazoa, represent the complexity of soil food webs as they comprise all major trophic groups and allow calculation of a number of functional indices Methodology/Principal Findings: We studied the functional composition of nematode communities three and five years after establishment of a grassland plant diversity experiment (Jena Experiment). In response to plant community simplification common nematode species disappeared and pronounced functional shifts in community structure occurred. The relevance of the fungal energy channel was higher in spring 2007 than in autumn 2005, particularly in species-rich plant assemblages. This resulted in a significant positive relationship between plant species richness and the ratio of fungal-tobacterial feeders. Moreover, the density of predators increased significantly with plant diversity after five years, pointing to increased soil food web complexity in species-rich plant assemblages. Remarkably, in complex plant communities the nematode community shifted in favour of microbivores and predators, thereby reducing the relative abundance of plant feeders after five years. Conclusions/Significance: The results suggest that species-poor plant assemblages may suffer from nematode communities detrimental to plants, whereas species-rich plant assemblages support a higher proportion of microbivorous nematodes stimulating nutrient cycling and hence plant performance; i.e. effects of nematodes on plants may switch from negative to positive. Overall, food web complexity is likely to decrease in response to plant community simplification and results of this study suggest that this results mainly from the loss of common species which likely alter plant - nematode interactions. [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]

This study assessed the carbon (C) budget and the C stocks in major compartments of the soil food web (bacteria, fungi, protists, nematodes, meso- and macrofauna) in an arable field with/without litter addition. The C stocks in the food web were more than three times higher in topsoil (0–10 cm) compared to subsoil (>40 cm). Microorganisms contained over 95% of food web C, with similar contributions of bacteria and fungi in topsoil. Litter addition did not alter C pools of soil biota after one growing season, except for the increase of fungi and fungal feeding nematodes in the topsoil. However, the C budget for functional groups changed with depth, particularly in the microfauna. This suggests food web resilience to litter amendment in terms of C pool sizes after one growing season. In contrast, the distinct depth dependent pattern indicates specific metacommunities, likely shaped by dominant abiotic and biotic habitat properties. [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]