IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 140 Soil Organic Matter, Caustobiolites, and Biochar Thursday, 27 August 2026 / Hall B This behavior was also observed in fluorescence analyses: up to intermediate treatment severity, heat progressively depletes the original fluorophores; beyond a certain threshold, especially in NM, the system does not simply continue to degrade, but rather reorganizes, giving rise to a new fluorescent signature increasingly dominated by aromatic/ conjugated domains. However, in agreement with Zhang et al. [3], maximum condensation did not coincide with maximum bio-redox functionality, which instead appeared to reach an optimum at intermediate heat severity, where aromaticity, phenolic character, electron-donating capacity, and accessability of redox-active sites coexist. This interpretation was also confirmed by the antioxidant capacity observed at higher termal treatments in the TEAC assay. The plant bioassays compared untreated HAs to 100°, 200°, and 300° C treated HAs at different exposuretime. While pants treated with 300-3H HAs showed the worst general growth response, intermediate treatments outperformed control plants, especially in root-related traits such as primary root elongation, lateral root formation, root hair development, early root biomass, and, consequently, overall seedling vigor and nutrient uptake. Interestingly, plant exposed to minimum heated HA (up to 100° and 30 min exposure) did not show significant differences when compared to controls, suggesting minor changes in the biostimulant ability of HAs when exposed to relatively mild heating conditions. The differences in the chemical composition among the ore sources investigated in this study were closely associated with the transformation induced by thermal treatment and ultimately reflected in the biostimulant response observed in plants. This confirmed that even HAs from apparently similar origin material may exhibit disticnt behaviors associated to their local geological formation [4]. Such variability, should be taken into account when interpreting the biostimulant outcomes. Altogether, these results provide an additional layer of insight into the interplay between HAs chemistry, thermal processing, and plant bioactivity, helping to furhter refine our understanding of HAs behavior as plant biostimulants. References 1. Bottomley, W. B., Annals of Botany, 34(135) (1920) 353-365. 2. Rusakova, M. A., Drozdova, O. Y., & Lapitskiy, S. A., . Moscow University Geology Bulletin 79(6) (2024) 872-877. 3. Zhang, Y., Xu, X., Cao, L., Ok, Y. S., & Cao, X., Chemosphere 211 (2018) 1073-1081. 4. Francioso, O., Montecchio, D., Gioacchini, P., & Ciavatta, C., Applied Geochemistry 20(3) (2005) 537-544.
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