Hydrochar applications to an agroecosystem soil to improve its functionality

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 (C_org), extractable organic carbon (C_ext), mineralizable carbon (C_min), total microbial activity (as soil potential respiration, mg CO₂-C kg^-1 d.w. d^-1), microbial biomass (C_mic), microbial percentage of total C_org (C_mic%C_org), quotient of mineralization (qM, C_min%C_org) and metabolic quotient (qCO_2, CO₂-% C_mic 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.