BOOK OF ABSTRACTS 269 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Hall A Poster Session / Technologies and Applications P5.7 Sustainable Low-Energy Valorization of Plant Residues into Humic-Like Compounds with Biostimulant Activity Isabel Ortega1,2, Javier Erro1,2, José María García-Mina1,2 1 Universidad de Navarra, Instituto de Biodiversidad y Medioambiente BIOMA, Irunlarrea 1, 31008 Pamplona, España. icortegab@unav.es / jerrogar@unav.es 2 Universidad de Navarra, Facultad de Ciencias, Departamento de Biología Ambiental, Grupo Biología y Química Agrícola, Irunlarrea 1, 31008 Pamplona, España The transformation of vegetable residues into value-added products is a key strategy within circular economy frameworks. In this study, humic-like substances (HLS) were synthesized from a wide range of organic feedstocks, including broccoli, thistle, coffee and coffee husk, asparagus, orange peel and pear, using compost derived as an HLS control. The approach combined different chemical processes to promote the formation of recalcitrant, humic-like structures. This HLS fraction was obtained by the IHSS extraction methodology to be structurally characterized and agronomically evaluated. [1][2] The resulting solid products were characterized using UV–vis spectroscopy, fluorescence analysis, Fourier-transform infrared spectroscopy (FTIR) and total carbon content to assess the structural properties [3] [4], and germination index assays were used to study the biostimulant effect. Spectroscopic analyses revealed clear differences depending on the treatment pathway, indicating that the chemical nature of the precursor fractions strongly influences the formation and degree of humification of the final materials. In particular, variations in aromaticity and molecular complexity reflected distinct optical signatures associated with humic-like structures. From an agronomic perspective, the germination index varied across samples, highlighting the influence of both feedstock type and hydrolysis conditions on phytotoxicity and biostimulant potential. Several materials exhibited germination responses indicative of reduced toxicity and potential plant growth-promoting effects. Overall, this study demonstrates that the low consuming energy tested reactions constitute a promising route for producing humic-like substances from diverse organic residues. The biostimulant effect of the proved wastes depends on both the residue´s composition and the humification degree. In addition, the type of chemical treatment strongly influences the structural characteristics and agronomic performance of the resulting materials [1], opening new opportunities for the development of sustainable soil amendments.
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