The instantaneous response of a soil microbial community to a chemical stressor (Mentha spicata essential oil) was studied post acclimation to the same chemical treatment at lower exposure. Acclimation involved the repeated addition of small amounts of the essential oil weekly for a period of 1 month, while for the stress treatment, a 10‐fold exposure level was introduced. We also tested the role of arbuscular mycorrhizal fungus (AMF) in the reponse of microbial community to the same stress exposure by pre‐inoculating plant roots in the soil with the AMF Rhizophagus irregularis. Three days after stress exposure, the structure of the soil microbial community was investigated in addition to the activities of six soil enzymes mainly related to the N‐cycle. The two preselected AMF inoculation and acclimation soil microbial communities responded differently to the subsequent stress. Acclimation enhanced the biomass of G+ bacteria, fungi and microeukaryotes, showing a priming effect of a low‐intensity stimulus when applied repeatedly, while AMF inoculation decreased the biomass of these microbial groups. The relative changes in microbial biomasses in jointly pretreated samples were not different from the control, suggesting opposing effects of the two pretreatments. On the contrary, the jointly pretreated samples responded to stress exposure by exhibiting increased activity of asparaginase and glutaminase and reduced activity of arylamidase. Finally, the relationship between enzyme activities and certain microbial ratios denotes that specific activities depended on the relative abundance of specific functional groups (e.g., G+ or G−) rather than on their biomass per se.

We assessed the abundance and composition of nematode communities in soil under herbaceous vegetation in reclaimed landfill sites at different ages after closure (3, 10 and 14 years) compared to those in neighboring semi-natural grazed grasslands (reference sites). We further applied network analysis based on the co-occurrence patterns of nematodes. Nematode abundance decreased between 3 and10 years of regeneration, but significantly increased from 10 to 14 years of regeneration. The number and identity of genera were comparable along the succession; however, there were dissimilarities in community composition during early- and mid-succession. The diversity, community composition and abundance at the sites after 14 years of regeneration converged with those at the reference sites. Moreover, changes during succession were not accompanied by the maturation of the soil food web, as demonstrated by Enrichment and Channel indices. In all the networks, centrality and modularity metrics differed significantly from those for random networks, whereas cohesion metrics showed no difference. All the networks exhibited Small-worldness indices higher than one, demonstrating that the networks of the interactions among genera at all the sites shared features that matched both random and non-random networks. The succession trajectory in reclaimed landfills was represented by a sequence of changes that differed in relation to the variable under consideration; network parameters tended to converge with those of a relatively resistant reference community, while the Enrichment and Channel indices did not. Additionally, the succession trajectory was not linear or steady; only the Channel index and Worldness index showed linear responses to succession time. However, across all the successional stages, the resource status remained basal or degraded while the nematode communities had an enhanced ability to cope with sudden changes.

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We performed a microcosm soil mixing incubation experiment to assess the functionality and composition of the microbial communities of “new” created soils. Sterilized soils from a deciduous beech forest in Taxiarchis (T) area and an evergreen sclerophyllous formation in Eleochoria (E) area (substrates) were either self‐ or cross‐inoculated with a small percentage (6%) of nonsterilized soils (inoculums) from E or T or an equal mixing of both. The control microcosms were prepared from original soil collected from E or T or an equal mixture of both. Three and 8 weeks after inoculation, the activity of soil enzymes engaged in the C (peroxidase, β‐glucosidase), N (N‐acetylglucosaminidase, urease and leucine‐aminopeptidase) and P (acid phosphomonoesterase) cycles was recorded. Also, the biomasses of different microbial groups were assessed by phospholipid fatty acids (PLFAs) analysis. Concerning functionality, 3 weeks after inoculation, the type of inoculum was the considerable factor but after 8 weeks the substrate exerted the major influence on the enzymatic profile of the “new soils.” Concerning the composition of microbial communities, substrate had the major impact on both samplings, while the effect of inoculum depended on incubation period and type of substrate. Different inoculums in T substrate resulted in different communities after 8 weeks of incubation but in similar communities in E substrate. The control soil produced by the equal mixing of both (E + T) hosted a microbial community with new composition and functional potentials.

Inoculation with beneficial microbes represents a promising solution for sustainable agricultural production; however, knowledge on the effects of inoculants on the indigenous microbial communities remains limited. Here, we evaluated the impact of the arbuscular mycorrhizal fungus Rhizophagus irregularis and the promoting rhizobacterium Bacillus subtilis on the growth of Lactuca sativa. The biomass, the composition, and the enzyme activity (urease, acid phosphatase, and β-glycosidase) of the rhizosphere microbial community at two soil moisture levels (5 and 10% soil water content) were evaluated. Fungal colonization was lower in co-inoculated plants than those only inoculated with R. irregularis. Plant growth was enhanced in co-inoculated and B. subtilis inoculated soils. Bacterial biomass and the composition of the microbial communities responded to the joint effect of inoculant type × water regime while the biomass of the other microbial groups (fungi, actinomycetes, microeukaryotes) was only affected by inoculant type. Co-inoculation enhanced the activity of acid phosphatase, indicating a synergistic effect of the two inoculants. Co-inoculation positively impacted the index reflecting plant–microbial soil functions under both water regimes. We concluded that the interactions between the two inocula as well as between them and the resident rhizosphere microbial community were mainly negative. However, the negative interactions between R. irregularis and B. subtilis were not reflected in plant biomass. The knowledge of the plant and rhizosphere microbial responses to single and co-inoculation and their dependency on abiotic conditions is valuable for the construction of synthetic microbial communities that could be used as efficient inocula.