Hydrochar as an effective amendment for enhancing soil aggregation and carbon sequestration: evidence from comparative microcosm experiments

Abstract Enhancing soil organic carbon (SOC) and aggregate stability is pivotal for maintaining soil health and ensuring agricultural sustainability. However, conventional organic amendments often exhibit suboptimal efficiency in achieving these goals. Hydrochar, synthesized via hydrothermal carbonization (HTC), offers a promising solution by integrating labile and recalcitrant carbon fractions to synergistically address these challenges. However, its mechanisms of action remain not fully understood. In the present study, a microcosmic incubation experiment was conducted to evaluate the short-term impacts of hydrochar on SOC sequestration and soil aggregation in comparison with biochar and straw in a purple soil ( Entisol ). Hydrochars derived from maize straw (SH), pig manure (PH), and Zanthoxylum stalks (HH) were also compared to assess feedstock-driven variability. The results demonstrated the superior performance of hydrochars, particularly those derived from Zanthoxylum stalks, which significantly increased the mean weight diameter (MWD) by 70–100% and SOC content by 143–149%, outperforming biochar and straw. Specifically, hydrochar-originated carbon persisted primarily as particulate organic matter (POM) and accumulated in macro-aggregate, while shifts in microbial communities contributed to SOC stabilization. In comparison, soil aggregation was driven by labile carbon fractions (e.g., dissolved organic carbon, DOC) and soil microorganisms, specifically Actinobacteria and Ascomycota . Feedstock properties, such as the C/N ratio and lignin content, modulated the effectiveness of hydrochar as a soil amendment. Notably, stalk-derived hydrochar exhibited superior carbon retention (12% total carbon loss vs. 30–44% for other amendments) and aggregate stability due to its recalcitrant lignin structure. Nutrient content and ratio further influenced these outcomes, with manure-derived hydrochar promoting microbial biomass carbon (845 mg kg −1 vs. 350 mg kg −1 in control), while stalk-derived hydrochar was more effective at optimizing carbon sequestration. These findings highlighted the dual role of hydrochar in enhancing soil structure and SOC sequestration, with feedstock selection critically determining functional priorities. Such insights could provide valuable guidance for tailoring hydrochar production and application to improve agricultural sustainability through soil quality improvement. Graphic Abstract