Hydrochar, carbon-rich material produced during the thermochemical processing of biomass, is receiving increased attention due to its potential value as soil amendment. It can increase agroforestry systems’ productivity through direct and indirect effects on growth and soil quality. Hydrochar may also directly help mitigate climate change by sequestering stable carbon compounds in the soil and perhaps indirectly through increased C uptake by trees. In this research, we aim to evaluate how the application of hydrochar produced by two feedstock types, Cynara cardunculus L. (Hc) residuals and sewage sludge (Hs), and in two different doses (3 and 6 kg m⁻²) could improve the growth and water use efficiency of Populus alba L., a fast-growing tree species largely used in agroforestry as bioenergy crops and in C sequestration. We considered five plants per treatment, and we measured apical growth, secondary growth, leaf area and intrinsic water use efficiency in each plant for the whole growing season from February to October 2022. Our results highlighted that hydrochar applications stimulate the growth and water use efficiency of plants and that the double dose (6 kg m⁻²) of both hydrochars, and particularly Hc, had positive effects on plant performance, especially during extremely hot periods. Indeed, the year 2022 was characterized by a heat wave during the summer period, and this condition allowed us to evaluate how plants, growing in soils amended with hydrochar, could perform under climate extremes. Our findings showed that the control plants experienced severe damage in terms of dried stems and dried leaves during summer 2022, while hydrochar applications reduced these effects.

Soil, an indispensable component of natural capital, provides essential ecosystem services like food production, climate regulation, and nutrient cycling. However, agriculture, spanning roughly 33% of Earth's land surface, poses a significant threat to soil integrity and functionality, because it could cause the loss of soil organic matter, pollution and alteration of physical properties. According to the UN 2030 Agenda for Sustainable Development, resilient agricultural practices that progressively improve land and soil quality must be employed. The use of organic fertilizers, such as biochar and hydrochar, deriving from thermochemical treatments of wastes, instead of mineral fertilizers, may have the advantage of restoring the organic C stock in the soil, also helping to mitigate climate change. However, the use of these organic fertilizers at a large scale requires a comprehensive assessment to exclude any potential adverse effects on the soil biotic community which plays a key role in the provisioning of ecosystem services.Within the interdisciplinary project “CHIMERA”, aimed to assess the hydrochar effects on the soil-plant-atmosphere system, this study evaluated the changes in the chemical and microbial properties after the addition of hydrochar to degraded agricultural soil. A controlled experiment was conducted within a greenhouse using pots (21 cm in diameter, 16 cm in height), each holding 1 kg of soil. Two types of hydrochar, derived from hydrothermal carbonization (250°C and 50 bar in the absence of oxygen) of two distinct sources (residues of thistle - Cynara cardunculus L. - and sewage sludge, labelled as HC and HS, respectively), were introduced into the soil at two different application rates (3 kg m-2 and 6 kg m-2). This resulted in a total of 5 treatments: four with hydrochar and one control without hydrochar addition. Moreover, the experimental design comprised five replicates for each treatment across three exposure times (18, 92 and 146 days). After each exposure time, soil samples were collected and analysed to assess pH, electrical conductivity, cation exchange capacity, total organic carbon content (Corg), extractable organic carbon (Cext), mineralizable carbon (Cmin), total microbial activity (as soil potential respiration, mg CO2-C kg-1 d.w. d-1), microbial biomass (Cmic), microbial percentage of total Corg (Cmic%Corg), quotient of mineralization (qM, Cmin%Corg) and metabolic quotient (qCO2, CO2-% Cmic d-1).The results showed no negative effect of hydrochar on considered variables, while positive effects on some of them were found. The response of soil variables to the addition of hydrochar depended on exposure time, dose and type of hydrochar. Generally, better results were recorded at 92 days of exposure, especially in treatment with thistle-derived hydrochar at the highest dose. Data suggest that hydrochar may be regarded as a promising soil fertilizer, also considering other results from CHIMERA project showing positive effects on the growth of Populus alba L. and, limited to thistle-derived hydrochar, also a reduction of N2O emission from soil, compared to control. However, further studies are needed to ascertain whether the positive effects persist in the long term and whether they are also confirmed at the field scale and for other feedstock types.

Anthropogenic soil degradation undermines essential ecosystem services such as food production, water purification, nutrient cycling and climate regulation. Unsustainable agricultural practices are among the main causes of soil degradation through pollution, soil loss and consequently lowering organic matter and nutrients. Adopting innovative solutions for agriculture soil management by adding low-decomposable organic amendments to the soil, such as hydrochar, can help reverse soil degradation. Hydrochar, derived from the hydrothermal carbonization of organic waste, may have the advantage of restoring the organic C stock in the soil, helping to mitigate climate change and improving soil health. Before using hydrochar at a large scale, a comprehensive assessment to exclude any potential adverse effects on the soil biotic community, playing a key role in the provisioning of ecosystem services, is needed.This study, part of the interdisciplinary project ‘CHIMERA’ evaluating the impact of hydrochar on the soil-plant-atmosphere system, aims to investigate changes in the chemical and microbial properties of degraded agricultural soil following the application of hydrochar. Therefore, a controlled greenhouse experiment was conducted using pots (21 cm diameter, 16 cm height), each containing 1 kg of soil. Two types of hydrochar, produced by hydrothermal carbonization at 250 °C and 50 bar without oxygen, were tested: one derived from sewage sludge (HS) and the other from thistle (Cynara cardunculus L., HC) residues, respectively. Each hydrochar was applied at two doses (3 kg m-² and 6 kg m-²), and the resulting five treatments (four with hydrochar and one control) were assayed in five replicates. At different exposure times (from 18 to 517 days), the following soil properties were analysed: pH, total organic C (Corg) and its extractable fraction (Cext), microbial biomass (Cmic), activity (as respiration), the quotient of mineralization (qM) and genetic bacterial diversity (richness).The results showed no toxicity to the soil microbial community; moreover, a general improvement of microbial biomass, activity and richness was observed, compared to control, at each exposure time, together with a significant decrease in qM, suggesting that C added as hydrochar was at least in part retained in soil. This ability highlights the positive hydrochar's role in improving soil structure and promoting resilience against erosion, drought and other climate-related challenges.Our findings suggest that hydrochar could be a tool for sustainable agricultural practices in restoring degraded soils. However, the application of hydrochar on soils requires further studies to confirm these positive effects and whether these effects can be observed using hydrochar derived from other raw materials and for other soil types.