Efficient Co-hydrothermal conversion of corn straw and pig manure into nitrogen-enriched soluble humic acid...

Valorization of agricultural waste into bio-based fertilizers is of great interest. Here, we synthesized nitrogen-enriched humic acids (NHAs) via co-hydrothermal treatment of corn straw (CS) and pig manure (PM) using 5-30% KOH under varying raw-material ratios. The addition of 20 g/L KOH optimally increased the yield of CS-derived humic acid (HA) by 81.5%. During the co-processing of CS and PM (ratios from 9:1 to 1:9), elemental analysis revealed that PM dramatically enhanced HA nitrogenation. Specifically, the N content of HA in the CSPM 1:9 treatment was 2.17 times higher than that in the CSPM 9:1 group. Furthermore, FT-IR and XPS analyses confirmed that manure-rich HA possessed more amide/amine and conjugated aromatic groups. These findings point to a synergistic mechanism: proteins and nutrients from PM drive HA nitrogenation, while lignin from CS constructs a stable aromatic backbone. Importantly, pot experiments demonstrated that the synthesized NHAs act as superior biostimulants, significantly increasing maize seedlings fresh weight by 119.8% compared to a commercial HA treatment. In summary, NHAs effectively accumulate carbon- and nitrogen-rich substances to promote plant growth, highlighting a sustainable route to convert agricultural wastes into value-added soil amendments.