Production of artificial humic acid from corn straw acid hydrolysis residue with biogas slurry impregnation for fertilizer application.

This study investigated hydrothermal humification of corn straw acid hydrolysis residue with biogas slurry impregnation, aiming at producing water-soluble artificial humic acid fertilizer for fertilizer application and soil remediation. Hydrothermal humification parameters, including potassium hydroxide concentration (1-3 mol/L), retention time (2-6 h), and temperature (140-180 °C), were investigated using water as the liquid phase. The selected hydrothermal humification condition was 1.5 mol/L potassium hydroxide at 180 °C for 4 h. Moreover, biogas slurry impregnation (0-30 days) was evaluated to improve humic acid yield without introducing additional chemicals or energy input. Biogas slurry as the liquid phase increased the humic acid production by 73.24% with 5 days of impregnation compared to the control due to the alkalinity. The humic acid concentration was sufficient for China's national standard of water-soluble humic acid fertilizers in such conditions. The organic components in biogas slurry were involved in artificial humification as a precursor, forming C-N bonds with humic acid. The product with fortified nitrogen-containing functional groups enhanced the nutrient slow-release characteristics and water retention capabilities. The pot experiment further confirmed that artificial humic acid prepared in this study not only promoted the growth of plants but also achieved soil remediation.

Bioaugmentation on humification during co-composting of corn straw and biogas slurry.

In order to increase the nutrients and humic acid (HA) contents of corn straw (CS) derived organic fertilizer and recover resources from biogas slurry (BS) simultaneously, the co-composting of CS and BS was carried out with the addition of biochar and microbial agents including lignocellulose degrading and ammonia assimilating bacteria. The results showed that 1 kg straw could treat 2.5 L BS by recovering nutrients and bio-heat introduced evaporation. The bioaugmentation strengthened both the polyphenol and Maillard humification pathways by promoting the polycondensation of precursors (reducing sugars, polyphenols, and amino acids). HA obtained in the microbial-enhanced group (20.83 g/kg), biochar-enhanced group (19.34 g/kg), and combined-enhanced group (21.66 g/kg) were significantly higher than that in the control group (16.26 g/kg). The bioaugmentation achieved directional humification and reduced the loss of C and N by promoting the CN formation of HA. The humified co-compost had nutrient slow-release effect in agricultural production.