Decomposition profiles of leaf essential oils in the soil environment

Abstract

In this study, the changes that essential oils undergo during the decomposition process of the plants producing them are examined. This was done for peppermint, spearmint, and rosemary, the shoots of which were added as soil amendments at a concentration of 4% (w/w). We sampled at 0, 30 and 60 days after the plants’ incorporation into the soil. We distilled the soil mixtures and found the essential oil content to be reduced, in the spearmint and peppermint treatments, by approximately 90% after 30 days. In the rosemary treatment, it was reduced only at the last sampling to about 50% of its initial value. Essential oil composition changed dramatically with time. The relative contribution of monoterpenoids, initially about 90% in spearmint and peppermint, fell to 45% and 20%, respectively, after 60 days. Sesquiterpenes and sesquiterenoids increased both in number and relative contribution. Percent participation of β-caryophyllene increased more than 15-fold in the spearmint and peppermint essential oils, and compounds that were not detected at first appeared at later stages of the decomposition process (after 30 or 60 days). In contrast, monoterpenoids made the bulk of rosemary oil at all sampling times, with only a minor reduction (about 5%) at the end of the experimental period. In a second experiment, where rosemary was let to decompose for one year, quantitative and qualitative analysis confirmed persistence of its oil for long in the soil environment. These results can explain the different effects on crop plants that have been reported for rosemary and spearmint soil mixtures and provide further insight for the potential of aromatic plants to find novel uses in agriculture.