Decoupling salinity and carbon function during hydrothermal carbonization of anaerobic digestate

Anaerobic digestion (AD) of livestock manure produces substantial quantities of digestate characterized by elevated salinity and residual nutrients, limiting its direct agricultural application. Hydrothermal carbonization (HTC) offers a thermochemical pathway for converting wet biomass into hydrochar, a multifunctional carbonaceous material. However, the extent to which HTC can simultaneously mitigate salinity and tailor the functional state of carbon remains insufficiently understood. Here, saline digestate was subjected to HTC (200-270 °C; 30 min) to evaluate how treatment severity influences inorganic partitioning and carbon transformation. Increasing temperature promoted the transfer of monovalent salts (Na⁺, K⁺) into the aqueous phase, reducing hydrochar electrical conductivity by up to 80 %. In contrast, divalent cations (Ca²⁺, Mg²⁺) and phosphorus were largely retained in the hydrochar through precipitation and surface interactions. Concurrently, hydrochar chemistry evolved from oxygen-rich, thermally labile structures at 200-220 °C to more condensed, aromatic, and mineral-associated carbon at 250-270 °C. Soil incubation demonstrated that these distinct carbon-states exert contrasting effects on soil carbon dynamics. Low-temperature hydrochars ("young carbon") stimulated microbial respiration due to higher labile carbon, whereas high-temperature hydrochars ("mature carbon") showed limited mineralization and enhanced carbon stabilization. Strong correlations between thermogravimetric-labile fractions and respiration responses indicate that hydrochar thermal signatures can predict soil behavior. Overall, HTC emerges not only as a salinity mitigation and nutrient recovery strategy, but as a controllable carbon state tuning platform. This dual functionality enables targeted hydrochar design for soil fertility enhancement or long-term carbon sequestration, while supporting circular economy strategies and addressing key planetary boundaries.