A New Strategy for Discovering Structurally Clear Plant Biostimulants

Structurally clear compounds have advantages for quality control in plant biostimulant production and application processes. Humic acid (HA) is a biostimulant that significantly affects plant growth, and soil-dwelling Protaetia brevitarsis larva (PBL) can rapidly convert agricultural waste into HA. In this study, we use PBL as a model to investigate HA formation and screen for HA-related structurally clear plant biostimulant compounds. The dephasing magic angle spinning nuclear magnetic resonance (13C DD–MAS NMR) analysis indicated HA structural changes during PBL digestion; the metabolic profile detected seven HA-related aromatic ring-containing compounds. Plant experiments identified six compounds with stimulating effects, with piperic acid being reported for the first time in this study. Thus, this study not only provides insights into the insect-mediated transformation of HA but also illustrates a new strategy for discovering structurally clear plant biostimulant compounds.