Abstract
The arbuscular mycorrhizal fungi (AMF) and the essential oils are both agents of sustainable agriculture, and their independent effects on the community of free-living soil microbes have been explored. In a tomato pot experiment, conducted in a sandy loam mixture, we examined the independent and joint effects of inoculation with the fungus Rhizophagous irregularis and the addition of Mentha spicata essential oil on the structure of the soil microbial community and the activity of soil enzymes involved in the N-cycle, during the pre-symbiosis phase. Plants were grown for 60 days and were inoculated with R. irregularis. Then pots were treated with essential oil (OIL) weekly for a period of a month. Two experimental series were run. The first targeted to examine the effect of inoculation on the microbial community structure by the phospholipid fatty acids analysis (PLFAs), and enzyme activity, and the second to examine the effects of inoculation and essential oil addition on the same variables, under the hypothesis that the joint effect of the two agents would be synergistic, resulting in higher microbial biomass compared to values recorded in singly treated pots. In the AMF pots, N-degrading enzyme activity was dominated by the activity of urease while in the non-inoculated ones by the activities of arylamidase and glutaminase. Higher microbial biomass was found in singly-treated pots (137 and 174% higher in AMF and OIL pots, respectively) compared with pots subjected to both treatments. In these latter pots, higher activity of asparaginase (202 and 162% higher compared to AMF and OIL pots, respectively) and glutaminase (288 and 233% higher compared to AMF and OIL pots, respectively) was found compared to singly-treated ones. Soil microbial biomasses and enzyme activity were negatively associated across all treatments. Moreover, different community composition was detected in pots only inoculated and pots treated only with oil. We concluded that the two treatments produced diverging than synergistic effects on the microbial community composition whereas their joint effect on the activity of asparaginase and glutaminase were synergistic.
Highlights
The involvement of a large number of vascular plants in symbiotic relationships with fungi in diverse habitats, indicates the importance of fungi for the plant soil complex
The AM fungus Glomus intraradices renamed Rhizophagous irregularis belonging to the Phylum Glomeromycota has been found in symbiosis with the majority of terrestrial vascular plants [4,5,6]
In a tomato pot experiment we examined the independent and combined effects exerted by the inoculation with the arbuscular-mycorrhizal fungus (AMF) R. irregularis and the repeated addition of essential oil extracted from M. spicata leaves, on the community profiles of free-living microbes and on soil enzymes, mainly those involved in the N-cycle since this cycle is entirely microbially-mediated
Summary
The involvement of a large number of vascular plants in symbiotic relationships with fungi in diverse habitats, indicates the importance of fungi for the plant soil complex. The AM fungus Glomus intraradices renamed Rhizophagous irregularis belonging to the Phylum Glomeromycota has been found in symbiosis with the majority of terrestrial vascular plants [4,5,6]. Inoculation with this fungus significantly influenced the development of the bacterial community of tomato rhizosphere [7] due to the suppression or stimulation of one or more susceptible microbial populations [8]. The inoculation with arbuscular mycorrhizal fungi alleviates the acidity and aluminium toxicity in Curcubita pepo L. [12] or increase the macronutrient concentration of leaf tissue of a cucumber genotype under alkaline conditions [13]
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