IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 202 Poster Session / Soil Organic Matter, Caustobiolites, and Biochar Hall A P2.8 Effect of Aluminum Complexation on the Stability of Humic Acid: Evidence from Decolorization and Mineralization Yukiko Yanagi1, Miho Kimoto2, Cocona Shimizu2, Ayaka Ide2 1 Graduate School of Sciences and Technology for Innovation, Yamaguchi University; Yoshida 1677-1, Yamaguchi, 7538501, JAPAN; E-mail: yyanagi@yamaguchi-u.ac.jp 2 Faculty of Agriculture, Yamaguchi University Soil organic matter (SOM) is the largest carbon pool in terrestrial ecosystems, and its sequestration contributes to improved soil quality and the mitigation of global climate change. The stability of SOM is closely linked to soil carbon sequestration. The formation of aluminum (Al) complexes with SOM (Al–humus complexes) is considered a key factor in carbon accumulation and stabilization in Andosols [1]. Although correlations between organic carbon content and Al complexation or carbon mineralization rates have been widely reported, the direct effects of Al complexation on the degradability of humic substances, key components of SOM, and the mechanisms underlying SOM stabilization remain unclear. The objective of this study was to evaluate the stability of humic acid (HA) complexed with Al (Al–HA complexes). Previous studies [2] demonstrated the stability of synthetic Al–HA complexes against decolorization, an indicator of degradation, primarily in liquid culture systems, focusing on the activity of the humic acid–decolorizing fungus Coriolus consors, as well as inorganic oxidizing agents and enzymes. In the present study, we examined the stability of Al–HA complexes against decolorization by C. consors in a solidstate culture system, along with changes in microbial activity associated with decolorization. In addition, decolorization experiments using soil microorganisms were conducted, and mineralization was evaluated to further assess the stability of the complexes. Powdered HA was prepared from the A horizon of non-allophanic Andosols using the IHSS method, and Al–HA complexes were synthesized following a previous study [3]. Decolorization experiments were conducted on perlite supplemented with liquid medium and 0.01 g of either Al–HA complex or free HA (powder). C. consors NBRC 9078 or soil suspensions (three Cambisols and three Andosols samples) were used as inocula. After incubation, residual HA was extracted with 0.1 mol L⁻¹ NaOH, and absorbance at 600 nm was measured to evaluate decolorization. Microbial activity was assessed by fluorescein diacetate hydrolysis (FDA) activity in C. consors cultures. In addition, soil suspensions (from Cambisols and Andosols samples) were incubated in small-scale syringe
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