Extraction Optimization, Multi-Dimensional Characterization, and Agricultural Application of Humic Acid from Protaetia brevitarsis Frass Derived from Straw Transformation

Agricultural straw, a massive lignocellulosic by-product, requires high-value utilization strategies, and larvae of <i>Protaetia brevitarsis</i> (a resource insect) can convert straw into two valuable products: insect protein and frass rich in humic acid (HA). In this study, we investigated the interactions among multiple parameters affecting HA extraction efficiency and optimized the extraction process. The resulting extract was characterized by elemental analysis to determine nutrient elements, trace elements, and heavy metals; by ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy to identify the main categories of bioactive molecules; and by pot experiments to evaluate its effects on plant growth and quality. The optimized extraction conditions yielded extracts with a total organic carbon (TOC) concentration of 46.8 g/L, meeting the Chinese standard for water-soluble humic acid fertilizers (NY 1106-2010). Elemental analysis indicated that the extract was rich in trace elements, and heavy metal contents met the limitation requirements of toxic and harmful substances in fertilizers (GB 38400-2019). ¹³C NMR analysis revealed that the extract was enriched in aliphatic and methoxyl carbons, while pot experiments with cherry radish demonstrated that application of the extract at appropriate dosages significantly promoted plant growth and improved crop quality. These findings provide scientific support for circular agriculture and arable land protection, highlighting both their academic significance and broad application prospects.