Testing the influence of digestate from biogas on growth and volatile compounds of basil (Ocimum basilicum L.) and peppermint (Mentha x piperita L.) in hydroponics

In this study, a digestate was evaluated for the first time as a sustainable alternative to the conventional growing medium and the nutrient solution in the hydroponic production of aromatic species. Basil (Ocimum basilicum L.) and peppermint (Mentha x piperita L.) were assessed at crop vegetative stage with five fully expanded leaves. Four combinations, i.e. agriperlite (AG) + standard solution (SS), AG + liquid digestate (LD), solid digestate (SD) + SS and SD + LD, were compared. The SD used as the growing medium increased all the investigated agronomical traits, apart the harvest index and shoot dry weight height ratio, that showed the same values compared to agriperlite, on basil and peppermint, respectively. The LD used as the nutrient solution performed as well as the SS on basil and peppermint production, except for the percentage of emergence and total dry weight of basil, showing lower and higher values compared to SS, respectively. Shoot dry weight was the most important agronomical parameter and both basil and peppermint displayed high values using the SD as the growing medium. However, basil recorded the highest dry matter value using SD + LD, while peppermint showed similar values using SD + SS and SD + LD. As regards the volatile compounds of both species, the percentage relative amount of sesquiterpenes was found to be higher under SD + LD and SD + SS conditions in comparison to AG + SS and AG + LD, where the content of monoterpenes and allyl phenol derivatives was higher. SD and LD appear to be sustainable and suitable growing medium and nutrient solution for basil and peppermint grown in hydroponics. Further studies are needed to investigate the influence of the treatments also in advanced developmental stages of the crops, appropriate for the production of plants with high yield and quality.