Carbon flow via decomposition of organic matter proceeds through different energy channels depending on the input and turnover rate of plant material. Generally, energy channels in grasslands are considered to be bacteria-dominated, whereas in forests the importance of fungal decomposition increases. Agroecosystems are regarded as intermediate. This hypothesis was tested using indices based on nematode faunal analysis that have been proposed as indicators of decomposition pathways. The ratio of fungal- to bacterial-feeding nematodes and the channel index (Ferris et al., 2001) were calculated from a range of studies, taking into account 131 different sites or sampling times. No distinct differences in the fungal- to bacterial-feeder ratio between grassland, field and forest were observed. However, the channel index indicated a fungal-based energy channel in coniferous forest sites. Generally, the results suggest that soil and climate affect nematode faunal indices more strongly than ecosystem type.
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 coli, 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.
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.
Nutrient availability is a major constraint on plant production and carbon storage in arctic ecosystems but knowledge on the decomposer food web interactions and their effect on nutrient cycling is scarce. We manipulated the soil food web at two contrasting subarctic sites, a low altitude heath and a high altitude fell-field. The influx of nutrients and energy in the soil was increased by addition of fertiliser nitrogen, phosphorus and potassium and of labile carbon (sugar). In addition, two bactericides (penicillin and streptomycin) and a fungicide (benomyl) were applied to manipulate the bacterial and fungal component of the soil. Experiments were carried out in a factorial design with repeated additions over four growing seasons. The present study investigated the nematode fauna and the effects of the manipulations on the abundance of nematode species. Fertilisation resulted in an increase of general opportunists, especially Aphelenchoides. Application of carbon or bactericides had only minor impact on the nematode community. In contrast, the fungicide was very toxic to non-target organisms and greatly reduced the abundance of most nematode species, although Acrobeloides buetschlii showed tolerance and greatly increased in number. At both sites, Eudorylaimus was most affected by the treatments. Generally the manipulations performed resulted in a significant increase in stress tolerant or competitive dominant nematode species.
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.
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 O157:H7 and its vector potential was assessed. Salmonella cells were more readily ingested by the nematodes compared to E. coli O157: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 O157: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.