Enhancing soil health with hydrochar: Improvements in chemical and biological properties

Agricultural activity may cause soil degradation through pollution, erosion and consequent loss of organic matter and nutrients. Restoration of degraded agricultural soils is essential for safeguarding the ecosystem services they provide (like climate regulation, water and nutrient cycling, and food provisioning and security). An innovative strategy to reach this goal is the addition of low-decomposable organic improvers to soil, as hydrochar, which derives from hydrothermal carbonization. Aim of this study was to investigate the hydrochar's effects on soil microbial community, as it plays a key role in nutrient cycling, and on physical and chemical properties. A pot experiment was performed using five treatments: control without addition of hydrochar (C), hydrochar deriving from sewage sludge (HS), or thistle residues of Cynara cardunculus L. (HC) with increasing doses (HS3, HS6 and HC3, HC6, respectively) and two different exposure times (18 and 92 days). After the two exposure times, soils were analyzed for microbial biomass (C_mic), total microbial activity (as potential respiration) and specific activities involved in nutrient cycling (as respiration responses induced by 11 simple substrates as well as activities of 5 specific enzymes), quotient of mineralization (qM), genetic bacterial diversity (as richness). Additionally, pH, electrical conductivity, cation exchange capacity, total organic C, mineralizable C, total N content and C/N ratio were also evaluated. Hydrochar did not have toxic effects on microbial community and generally improved pH, organic C, potential respiration, respiratory responses to most substrates and enzymatic activities. The best results were observed after 92-days exposure time at treatments with higher doses (68 g kg^-1) with hydrochar from C. cardunculus also promoting bacterial diversity. Further studies are needed to confirm positive effects of hydrochar in relation to longer exposure time and different feedstock.