This study compared hydrothermal humification (HTH) of two lignocellulosic biomasses with contrasting compositions (rice straw and tobacco stems) to elucidate how carbon and nitrogen sources govern the yield and structure of artificial humic acid (AHA). Rice straw (high hemicellulose/low lignin) favored lower temperatures coupled with higher alkali concentrations, whereas tobacco stems (low hemicellulose/high lignin) required higher temperatures and stronger alkali for optimal humification. Under optimized conditions, substituting water with biogas slurry increased AHA yield by 4.25 % (rice straw) and 4.06 % (tobacco stems), and raised nitrogen content by 15.19 % and 25.81 %, respectively.Spectral analysis of model substrates (xylan and alkali lignin), further indicated that aromatic structures, promoted by aromatic amino acids, are critical for forming carbohydrate-derived AHA precursors. In kale pot trials, AHA liquid fertilizer enhanced slow-release nutrient characteristics, increased plant growth-promoting rhizobacteria, and lowered potential pathogens, thereby supporting improved plant growth and soil quality.