In this study, hydrochar (HC), a carbon-rich product originated from hydrothermal conversion treatment (HTC), was obtained from wastes of the wine and dairy industries. The effect of mixing secondary char and compost was tested, before and after the aerobic mixing of compost (COM) and HC at increasing doses (from 15 to 75 Mg ha^-1 DM), in an effort to lower the HC phytotoxicity due to potential phytotoxic compounds of secondary char. The results indicated that, after the aerobic stabilization, the mix HC/COM was able to double the plant growth in comparison to COM alone. The presence of easily degradable organic compounds probably led to poor stability of HC, increased microbial activity and, consequently, root anoxia when used at high doses. Chemical, spectroscopic and thermal investigation confirmed this hypothesis. In particular, HC shows a high content of dissolved organic matter, characterized by the presence of small molecules, which is negatively correlated with the growth index of lettuce. Furthermore, thermal characterization suggests a higher proportion of less complex and thermally stable molecular compounds in HC in comparison to COM. Therefore, co-composting of HC allows obtaining a useful amendment to support soil organic matter and fertility.

Hydrochar is a new carbonaceous product obtained via hydrothermal carbonization of wet biomass, such as sludges or digested sludges, which often have disposal problems, also due to the presence of contaminants such as heavy metals. The properties of the hydrochar led to an interest in using it as an amendment, but the agro-environmental properties must be considered for its safe use. Raw hydrochar produced by agro-industrial digestate and relative three acidic post-treated hydrochars (for heavy metals removal) have been assessed considering their effect on phytotoxicity, soil, plant growth, mutagenicity, and genotoxicity. The chemical characterization showed the effect of post-treatment on heavy metals contents reduction, except for Cu content (hydrochar, 650 mg/kg; post-treated hydrochars, 940 mg/kg, 287 mg/kg, and 420 mg/kg). The acidic post-treatment also reduces the phytotoxicity compared to raw hydrochar (the germination index at 16 % of hydrochar concentration was: hydrochar, 61.48 %; post-treated hydrochars, 82.27 %, 58.28 %, and 82.26 %), but the low pH and the impact on N-cycle probably have caused the detrimental effect on plant growth of post-treated hydrochar. No mutagenic activity was observed in bacteria using Ames test, while all the samples induced chromosomal aberrations in plant cells (Allium cepa test). The approach adopted, which considers phytotoxicity, plant growth-soil effects, and mutagenicity/genotoxicity bioassays has been proven effective for a proper evaluation of organic products derived from waste to promote a sustainable and circular recovery of materials.

In intensive fruit growing systems, the recovery and maintenance of soil fertility play a crucial role in both environmental protection and sustainable support to plant productivity. The circular economy approach adopted at the EU level strongly promotes the use of organic products instead of mineral fertilizers. This work focuses on two different soil improvers, compost from the organic fraction of municipal solid waste digestate (CO) and “matured” manure, produced after a fast and controlled aerobic treatment in an aerated pile (MM), which were applied in three apple orchards with different soil tillage. The soil improvers have been characterized for amendment and fertilizing properties. After the amendment, the soils were sampled twice a year (Spring and Autumn) for three years. Each sample has been characterized for texture, pH, cation exchange capacity, nutrients, soil organic matter, and micronutrients. The amendments obtained differed on C, N, P, and K contents, but had similar biological stability. The main effects on soils were the increasing of N and soil organic matter after compost application, while the use of matured manure mainly act on available P and exchangeable K. The treatments showed significant effects among fields with a linear increasing trend only for compost. Matured manure showed more effects in earlier times. The data collected aim to improve the knowledge about sustainable management of soil organic matter and organic nutrients in intensive fruit-growing agriculture by using local products.