Comparative effects of hydrochar and hydrothermal humic acid on soil amendment: Is further humification of hydrochar to humic acid necessary?

The efficacy differences between biomass-derived hydrochar and hydrothermal humic acid (HHA) obtained via further hydrothermal humification for soil amendment remain unclear. Using model feedstock chitosan as the primary research focus and cellulose serving as the control, this study examined whether hydrochar requires further hydrothermal conversion to HHA for effective soil improvement. Results showed that the chitosan-derived hydrochar obtained after acid-hydrothermal treatment contained extremely low HHA (<1%), while subsequent alkaline hydrothermal treatment could converted 48.8% of hydrochar to 23.9% HHA. Chemical characterization confirmed that the chitosan-derived HHA had higher aromaticity and more oxygen-containing functional groups. Pot experiments demonstrated that HHA produced from chitosan-derived hydrochar more effectively promoted fresh biomass accumulation in Chinese cabbage seedlings than raw hydrochar, may be partly attributed to the latter's humic substances deficiency. This finding was further supported by the results of pot experiments using cellulose as a control feedstock: the cellulose-derived hydrochar containing 13.3% HHA exhibited a comparable effect to that of HHA obtained from further humification of cellulose hydrochar in promoting the fresh biomass of Chinese cabbage seedlings. Metagenomics revealed significant ecological differences between chitosan-derived hydrochar and its humified components in shaping soil microbial communities and functions. Considering agronomic benefits, process input, and biowaste type, further conversion of hydrochar into HHA could be warranted for enhancing efficacy when using poorly-humified feedstocks, or when the goal is to produce a high-value humic product for broader applications.