The effects of arbuscular mycorrhizal fungi and essential oil on soil microbial community and N-related enzymes during the fungal early colonization phase

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.

Instantaneous responses of microbial communities to stress in soils pretreated with Mentha spicata essential oil and/or inoculated with arbuscular mycorrhizal fungus

“…Non synchronizing changes were found in the current experiment but before the initiation of essential oil stress (Stamou et al, ). Three days post stress, NIPALS analysis showed (Figure ) that specific enzymes were ordinated together with microbial ratios in samples inoculated, acclimated and stressed.…”

“…The oil was supplied by Etherio, Research and Commerce, Eratera, Greece and was pure essential oil produced after distillation of M. spicata plants. The major compounds of M. spicata oil were carvone 63.9% and limonene 13.3%, followed by 1,8 cineole, β ‐pinene, myrcene and α‐pinene in percentages of 7.1, 2.8, 2.4 and 1.4%, respectively (Stamou et al, ). Following a 1‐month acclimation, a “stress event” was simulated by applying a 10‐fold amount of M. spicata oil (13.3 mL per pot) in six of the nine inoculated pots.…”

“…Samples exposed to a combination of both pretreatments and were subsequently stressed exhibited increased activity of asparaginase and glutaminase and reduced activity of arylamidase compared to samples exposed to stress only. Previously, Stamou et al (2017) suggested a synergistic effect of inoculation and acclimation for asparaginase and glutaminase activity before stress exposure. This effect appears to continue post stress exposure and is explained by the fact that stress on its own caused no significant effect on enzyme activities.…”

“…According to previous work by Stamou et al (2017), the two different treatments (AMF inoculation or application of low doses of M. spicata oil) acted selectively on members of the soil microbial community in a diverging way. We hypothesized that "selected" microbial communities (via AMF inoculation and/or M. spicata oil acclimation) will respond differently when exposed to a subsequent stress episode (higher-level exposure to M. spicata oil).…”

Soil functionality produced by soil mixing: The role of inoculum and substrate

“…The plates were incubated at 20°C for 18 hr. Ammonium released by the reaction was quantified using colorimetric salicylate and cyanurate reagent packages from Hach (at 570 nm in spectrophotometer) (Stamou et al, ).…”