Journal of Environmental Management xxx (xxxx) xxx Please cite this article as: Silvia Celletti, Journal of Environmental Management, https://doi.org/10.1016/j.jenvman.2020.111635 0301-4797/© 2020 Elsevier Ltd. All rights reserved. Research article Phytotoxicity of hydrochars obtained by hydrothermal carbonization of manure-based digestate Silvia Celletti a, * , Alex Bergamo a , Vittoria Benedetti a , Matteo Pecchi a , Francesco Patuzzi a , Daniele Basso b , Marco Baratieri a , Stefano Cesco a , Tanja Mimmo a a Faculty of Science and Technology, Free University of Bolzano-Bozen, I-39100, Bolzano, Italy b HBI S.r.l., Bolzano, Italy A R T I C L E INFO Keywords: Cress germination test Digestate Growing medium Hydrothermal carbonization Soilless culture Waste conversion technology ABSTRACT The management of digestate, the main by-product of the anaerobic digestion (AD) process, is one of the most serious environmental issues. Although digestate is used on arable land as a fertilizer, it can have a negative impact on the environment due to nitrate leaching into the groundwater and ammonia volatilization into the atmosphere, with high economic and environmental disposal costs. Therefore, hydrothermal carbonization (HTC), a thermochemical biomass conversion process, could represent a sustainable and effcient alternative for digestate management. Hydrochar, the solid product of the HTC process, has been recently proposed as a plant growing medium in soilless culture systems (SCS). Here, using cow manure digestate as feedstock, we investi- gated the infuence of the HTC process reaction temperature (180, 220 and 250 ◦ C) and residence time (1 and 3 h) on the physical-chemical properties (pH, electrical conductivity, and mineral element concentrations) of the resulting hydrochars. Furthermore, in order to fully valorize hydrochar as a growing medium, their possible phytotoxic effects and those of their water extracts (prepared at two different concentrations and at different pHs) were tested in germination tests with cress seeds (Lepidium sativum L.). Concentrations of nutrients, heavy metals, organic acids, sugars and furan compounds were determined in the water extracts. Characterization analysis of these hydrochars revealed that they can be distinguished from each other by their physical-chemical properties, which were signifcantly affected by the two process parameters. Specifcally, the HTC temperature had a greater effect on the composition of hydrochars than the residence time. Germination tests found hydrochar water extracts to show signifcantly lower phytotoxicity than the hydrochars themselves. Notably, the phytotoxic effect of the extracts decreased with increasing extraction ratio and decreasing pH. The chromato- graphic characterization of extracts identifed the presence of potential phytotoxins, such as furan compounds (i. e., hydroxymethylfurfural and furfural). However, before using hydrochars as potential and innovative growing media for plants, their phytotoxicity should be limited, for example through their dilution with other substrates. Overall, AD-HTC coupling could represent a valuable eco-sustainable expedient in the feld of biomasses, green economy and waste conversion and, therefore, further investigations in this direction are necessary. 1. Introduction Anaerobic digestion (AD) is the process of microbial decomposition of organic matter in the absence of oxygen. The application of this process to the treatment of agricultural, urban and/or industrial waste generates considerable production of commercially-valuable biogas and other AD by-products such as digestate (Masebinu et al., 2019). Diges- tate is widely used as an organic soil amendment or fertilizer, being a source of nutrients and organic matter (Nkoa, 2014; Tiwary et al., 2015), and is a more environmentally-friendly alternative to chemical fertil- izers, as it promotes closed-loop nutrient recycling and, due to its low carbon content, can contribute to reduce atmospheric carbon dioxide emissions (Masebinu et al., 2019; Tiwary et al., 2015). However, despite the advantages and continuous technological developments, there are still some challenges that AD systems must face (Masebinu et al., 2019). Inappropriate handling and management of digestates on agricultural land can lead to increased air pollution, due to greenhouse gas emis- sions, and water pollution due to the leaching of various mineral * Corresponding author. Faculty of Science and Technology, Free University of Bozen-Bolzano Piazza Universit` a 5, I-39100, Bolzano, Italy. E-mail address: silvia.celletti@unibz.it (S. Celletti). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: http://www.elsevier.com/locate/jenvman https://doi.org/10.1016/j.jenvman.2020.111635 Received 9 July 2020; Received in revised form 29 September 2020; Accepted 25 October 2020