Soil microbial biomass responses to essential oils extracted from different Mediterranean herbs

Essential oils (EOs) extracted form herbs contain terpenes, terpenoids and polyphenols which show inhibitory effects on seed germination and seedling growth of weeds. Therefore, EOs are potential sources for the development of new bioherbicides. A previous study carried out by the authors has found EOs extracted from Thymbra capitata (L.) Cav., Mentha x piperita L., and Santolina chamaecyparissus L. were able to reduce the emergence and seedling growth of many weeds. However, due to their potential antimicrobial activity, EOs could affect all soil processes mediated by microorganisms. Since EOs impacts seem to be rather complex as they have been found not only to inhibit but also stimulate microbial activities, thus, elucidating the interactions between soil microorganisms and EOs was the goal of this study. To this end, the EOs extracted from T. capitata, M. piperita and S. chamaecyparissus were applied to a soil at different concentrations. Before applying the EOs, soil aliquots were moistened up to 2/3 of 50% of their water holding capacity (WHC). Then, they were brought up to 50% of their WHC by adding a solution containing the EOs, so reaching a concentration of 31, 67, 93 µl/mL 100 g-1 of soil for M. piperita and S. chamaecyparissus, and 93, 123, 154 µl/mL 100 g-1 of soil for T. capitata EO. Fitoil was used to emulsify the EOs at a concentration of 0.05% (v/v). The soil used was the topsoil (0-10cm) collected in a citrus orchard never treated with herbicides. Four replicates per treatment were run. Soil samples were analyzed to determine microbial biomass C and N, and microbial community structure (through ELFAs fingerprints) after one week, one month, and two months of incubation. In parallel, glass jars with 20 g of soil were incubated to measure CO2 released by soil respiration for 60 days. Preliminary results reveal that EOs were able to kill some soil microorganisms, thereby providing substrate easily decomposable by the surviving ones, but also that EOs were assimilated by some soil microorganisms as C and energy sources.