Sustainable valorization of waste horticultural biomasses to develop a novel hydrochar to improve soil health and modulate the lettuce metabolism

Hydrothermal carbonization (HTC) represents an innovative and sustainable chemical approach based on sub-critical water and useful to valorize waste biomass to develop a novel solid material with a huge potential for soil microbiota and plant productivity. Two hydrochar types were produced, under mild (180 °C, 10 Bar) and severe (215 °C, 20 Bar) HTC conditions (HC180 and HC215, respectively), by processing spinach, red chicory, and escarole wastes resulting from industrial horticultural production. These hydrochars were characterized for elemental composition and molecular composition via advanced techniques (13C CPMAS NMR and ATR-FT-IR) and compared with the values detected for initial biomass types. HTC progressively converted labile into carbon-dense, aromatic materials, with greater severity reducing nutrient availability. Solid hydrochars retained up to 2.82 g of water per g of material, thus representing a tool to make soil more resilient against the drought. Greenhouse pot experiments on baby-leaf lettuce revealed that HC180 strongly promoted plant shoot and root growth, matching conventional mineral fertilization, while HC215 showing only limited effects. 1H NMR metabolomics indicated that HC180 stimulated primary metabolism, increasing the level of sugars and amino acids, linked to energy production and stress responses, highlighting a biostimulant effect mediated by enhanced nutrient availability and soil-microbiome interactions. These findings demonstrate that mild HTC conditions produce safe hydrochars with plant biostimulant activity, characterized by a balanced composition of labile carbon and nitrogen as well as a spectrum of plant-available nutrients. This offers a tunable strategy for sustainable horticultural residue management to produce a product enhancing soil fertility and vitality.