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

Abstract

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.

Impacts of Decaying Aromatic Plants on the Soil Microbial Community and on Tomato Seedling Growth and Metabolism: Suppression or Stimulation?

“…It is shown [ 38 ] that soil amendments with anise, parsley, and rucola act as biostimulants, favoring soil microorganisms both in terms of biomass and functionality and that they also enhance the populations of bacterivorous nematodes, enzymatic activities, and tomato root growth. The enhancement of soil enzymatic activity and total microbial biomass by spearmint essential oil is also reported in [ 14 ]. Recently, a study on the impacts of Thymus citriodorus on soil communities and tomato growth showed that different plant parts had different impacts, suggesting the intervention of specific chemical constituents that were included or not in each plant part, and that soil bacterial biomass increased, whereas fungal biomass did not change in its presence [ 29 ].…”

“…Regarding microbes, apart from the well-known inhibitory activity of essential oils against human, animal, and plant pathogens [ 8 , 9 , 10 ], promoting effects to microbial populations and associated soil and/or foliar metabolism have been found. For instance, it has been reported that essential oils increase soil respiration, which is an indicator of soil fertility [ 11 , 12 , 13 , 14 ], that epiphytic bacterial communities on aromatic plants are not less abundant or diverse than communities on non-aromatic plants [ 15 , 16 ], that specific bacterial strains are even able to grow on oregano glandular hairs [ 17 ], and that the sporulation of some fungi may become much higher in the presence of some essential oils or their constituents [ 10 ]. In addition, many bacteria are reported as having the ability to degrade essential oil ingredients [ 18 ], whereas most essential oils are reported as having only weak antimicrobial activity [ 19 ].…”

Living Mulch with Selected Herbs for Soil Management in Organic Apple Orchards

“…Therefore, a higher soil biological activity in the selected mulching plots could be hypothesized, which could result in a more rapid soil organic matter mineralization and nitrogen availability [51]. The incorporation of essential oils or their major constituents into soils has stimulatory effects on bacterial populations [52], stimulatory or depressing effects on specific fungal populations [53,54], and can stimulate soil respiration [55]. Moreover, a weekly addition of a minimum amount of mint essential oil for a month, which could mimic a natural contribution from the plants growing on the soil, enhanced the biomass of Gram-positive bacteria, fungi, and microeukaryotes, showing a priming effect of a lowintensity stimulus when applied repeatedly, which modified some enzymatic activities also linked with N cycle [54].…”

“…The incorporation of essential oils or their major constituents into soils has stimulatory effects on bacterial populations [52], stimulatory or depressing effects on specific fungal populations [53,54], and can stimulate soil respiration [55]. Moreover, a weekly addition of a minimum amount of mint essential oil for a month, which could mimic a natural contribution from the plants growing on the soil, enhanced the biomass of Gram-positive bacteria, fungi, and microeukaryotes, showing a priming effect of a lowintensity stimulus when applied repeatedly, which modified some enzymatic activities also linked with N cycle [54]. The removal of Alchemilla monticola, a species close to Alchemilla vulgaris, from a grassland strongly affected the bacterial community and weakly influenced mycorrhizal fungi, resulting in a decreased rate of plant litter decomposition and soil respiration [56].…”

The Effect of Rhizophagus irregularis, Bacillus subtilis and Water Regime on the Plant–Microbial Soil System: The Case of Lactuca sativa

“…The authors attributed the reduction in fungal abundance to the death of some pathogenic fungi by the development of mycorrhizal mycelium. Alternatively, competition between mycorrhizae and free fungi for nitrogen might exist [67]. Fungal hyphae absorb NH 4 and/or NO 3 to meet their high nitrogen requirements [68] in order to absorb soil phosphorus.…”