Hydrothermal Carbonization of Municipal Woody and Herbaceous Prunings: Hydrochar Valorisation as Soil Amendment and Growth Medium for Horticulture

In this study, we investigate the suitability of hydrochar, produced at industrial scale by hydrothermal carbonization of municipal woody and herbaceous prunings, to be used as soil amendment and peat substitute in organic growth medium for horticulture. Fresh hydrochar and the products of two different hydrochar post-treatments (i.e., washing and aging) were compared in terms of potential phytotoxicity throughout physicochemical characterization and germination tests, performed with a sensitive species (Lactuca sativa). The results showed that the fresh hydrochar obtained from municipal green wastes complies with the Italian regulated parameters for the use as soil amendment. Moreover, hydrochar exhibits biological activity and a high content in organic C, Ca, and other micronutrients (Mg, Zn, Cu, Na, Cl). On the other hand, post-treatments are needed before application of hydrochar as peat substitute in potting mix, since appreciable phytotoxic effects on lettuce seed germination and radicle length of plantlets were observed (e.g., germination percentage of 56% and 54%, with 5 and 10 wt % of hydrochar in the blend, respectively). The inhibition of germination could be mainly attributed to the presence of polyphenols (tannins) and volatile fatty acids, which were most effectively removed through the aging post-treatment.