Phytotoxicity of hydrochar in comparison and in mixture with compost

Hydrothermal carbonization (HTC) is a thermochemical conversion process that is used to treat biomass and organic waste. During HTC, that occurs in a range of temperature between 180 and 250°C, the biomass is transformed into a lignite-like product called hydrochar (HC), a solid, hydrophobic product that, due to the chemical reactions that occur during HTC, can contain potentially toxic compounds. For these reasons, directly contact between HC and plants could cause phytotoxicity effects. Recent works have suggested that aerobic treatment can influence the reduction of this phytotoxicity. The aim of this experiment was to test the performance as an amendment of HC produced in a pilot plant fed with an agro-industrial sludge coming from wine and dairy industry. A compost (C) and a mix HC/C, before and after a rapid aerobic stabilization, were also tested for comparison. A phytotoxicity test was performed, and results have been related to the characteristics of these biomasses. Dissolved organic matter decreased both when the HC was mixed with the compost and, even more, after the composting as consequence of the degradation of simpler organic compound deriving from HC, e.g., sugars and organic acid resulting from HTC conversion. On the contrary, no significant differences were observed for recalcitrant organic compounds. The results obtained by phytotoxicity test indicated that high doses of HC induced toxic effect in comparison to the control. On the contrary, the compost did not show adverse effect independently of the doses. The MIXs, both at the beginning (MIX0) than after 30 days of stabilization (MIX30) were able to double up the plant growth in comparison to C alone, showing. Obtained results indicate that when HC is mixed to C and stabilized it is able to improve the positive effects provided by the compost alone. The higher content of dissolved organic matter in HC can be the main reason causing the partially toxicity observed. As known, labile organic matter shows, generally, higher respiratory activity that, causing root anoxia in the pots and can results in a plant growth reduction.