BOOK OF ABSTRACTS 203 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Hall A Poster Session / Soil Organic Matter, Caustobiolites, and Biochar vials containing the same medium amended with either HA or Al–HA complexes as used in the decolorization experiments, and CO2 emissions were measured approximately every two weeks using gas chromatography to estimate HA mineralization. Relative absorbance in cultures inoculated with C. consors was lower for free HA than for Al–HA complexes under both acidic (pH 4.0) and neutral (pH 6.8) conditions. In contrast, FDA activity in the presence of Al–HA complexes was not significantly lower than that in the absence of HA or in the presence of free HA, and was even enhanced at pH 4.0. In cultures inoculated with soil suspensions, relative absorbance was also lower for free HA than for Al–HA complexes under both pH conditions across all soil types. In particular, absorbance in the Al–HA treatments remained unchanged or slightly increased. CO2 emissions from both HA samples incubated with soil suspensions increased over time; however, emissions were consistently lower for Al–HA complexes than for free HA in both Cambisols and Andosols samples. These results indicate that Al complexation not only prevents structural changes in HA but also suppresses its complete decomposition into CO2, thereby enhancing its stability. Furthermore, because microbial activity was not suppressed during decolorization by C. consors, reduced microbial activity is unlikely to be the primary factor responsible for this stabilization. References 1. T. Takahashi, R. A. Dahlgren. Geoderma 263 (2016) 110–121. 2. Y. Yanagi, K. Yoshida, N. Matsuo, Book of Abstracts, 19th International Conference of Humic Substances and their Contribution to the Climate Change Mitigation, Bulgaria, 2018, p. 325-326. 3. T. Takahashi, M. Nanzyo, S. Hiradate. Soil Sci. Plant Nutr. 53 (2007) 115−124. Acknowledgement This work was supported by JSPS KAKENHI grants [26340005], [18K11622] and [25K15418].
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