Molecular composition of the Humeome extracted from different green composts and their biostimulation on early growth of maize

Background and aims The use of composted agricultural wastes as source of biostimulant compounds provides an added value to the recycling of biomasses. This study aims to expand the knowledge on the relationship between molecular composition and bioactivity of the Humeome extracted from green composts. Methods Humic acids (HA) were isolated from the following green composts: 1. artichoke (HA-CYN), 2. artichoke/fennel (HA-CYNF), 3. tomato (HA-TOM), 4. cauliflower (HA-CAV). The compost-extracted Humeome was characterized by solid- and liquid-state Nuclear Magnetic Resonance (NMR) Spectroscopy, Infrared Spectrometry, pyrolysis-gas chromatography/mass spectrometry (Pyr-GC/MS), elemental and thermal analyses (TGA, DSC), and the effects on maize (Zea mays L.) seedling growth (roots and shoots) and leaves chlorophyll content, investigated at three different concentration rates (25, 50 and 100 mg L-1 C). Results All Humeomes from green composts generally favored plant growth, with HA-CYN and HA-CAV being active at small concentrations, while HA-TOM and HA-CYNF at greater application rates. The response of HA-CYN and HA-CAV was attributed to their large hydrophobicity that may favor adhesion to roots surfaces and concomitant release of bioactive molecules, whereas the toxicity of phenolic moieties in HA-TOM and HA-CYNF could only be mitigated by tighter intermolecular aggregation at greater concentrations. Conclusion Bioactivity of Humeomes from green composts appeared to be related to their composition and consequent conformational structure in solution, whose flexibility determines the potential release of bioactive molecules that directly or indirectly influence plants development.