Effect of hydrochar and HTC process water on sodic soil reclamation and pulse seedling growth

This study examined the potential of organic fraction of municipal solid waste (OFMSW) derived hydrochar and the process water from hydrothermal carbonization (200 °C; 4h; liquid/solid ratio - 9:1) for the reclamation of sodic soils. The hydrochar had a high carbon and calcium content, while the process water contained easily soluble K⁺, Ca²⁺ and Mg²⁺ and bioactive compounds. The addition of hydrochar to sodic soil reduced the exchangeable sodium percentage (ESP) and improved the soil physicochemical properties by increasing Ca²⁺/Mg²⁺ exchange, promoting Na⁺ desorption, and enhancing nutrient retention and abundance of O-H, N-O, and N = C = S functional groups supported its reactivity for ion exchange. Strong negative relationships between ESP and Ca²⁺, Mg²⁺, K⁺, P, and OC were observed using Pearson correlation analysis, supporting the hydrochar-mediated redistribution of cations. Additionally, a leach-out study shows 42.6% ESP reduction by hydrochar addition compared with gypsum application (21% ESP reduction). HTC process water at low concentrations (1%-3%) exhibited seed priming effects and increased the germination index over control by 39.2% in black gram, 7.7% in green gram and 14.4% in moth bean. Overall, this study demonstrates a circular, cost-effective strategy integrating hydrochar and processing water for sustainable sodic soil reclamation and improved crop establishment.