BOOK OF ABSTRACTS 137 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Thursday, 27 August 2026 / Hall C Soil Organic Matter, Caustobiolites, and Biochar SL66 Molecular Characteristics of Aggregate-Associated Humic Substances in CaO and SiO2 Amended Soils Na-Hyun Kwon1*, Dong-Jin Lee1, Chang-Dong Lee1, Seung-Hwan Lee1, Jae-Eun Jung1, Jong-Hoon Lim2, Chan-Young Lee2, Jeong-Gu Lee1,2 1 Kyungpook National University, Department of Applied Biosciences, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea, kwonna1@knu.ac.kr 2 Kyungpook National University, School of Applied Life Science, College of Agriculture and Life Sciences, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea Humic substances (HS) play important roles in soil fertility [1] and plant performance by improving soil structure, increasing nutrient availability, alleviating metal toxicity, and enhancing plant tolerance to abiotic stress [2]. Among soil amendments, lime (CaO) and silicate (SiO2) may further influence the humification and stabilization [3,4] of HS, yet their effects on HS quality across aggregate size classes remain poorly understood. This study evaluated the quantity and compositional characteristics of humic acid (HA) within aggregate size fractions under four amendment treatments, NPK, CaO, SiO2, and half-dose SiO2 (0.5 SiO2). Following incubation, mean weight diameter decreased by 90% in the 0.5 SiO2 treatment and by 60% in the NPK treatment relative to the preincubation soil, indicating aggregate disintegration. Across all treatments, the microaggregate fraction yielded the highest amount of extracted HA and contained higher nutrient concentrations than the largest and smallest aggregate fractions. In contrast, in the SiO2 treatment, the macroaggregate fraction showed the highest total C concentration of HA at 23 g kg-1, which was 27% and 52% higher than that in the microaggregate and smallest aggregate fractions, respectively (DELTA-Q, Thermo Fisher Scientific, USA, NFEC2025-03-304690). Water-dissolved C of HAs averaged 221 mg C g-1 in the SiO2 treatment and 130 mg C g-1 in the CaO treatment, both of which exceeded the value in the NPK treatment of the largest fraction. Overall, in intensive farming systems, silicate and lime amendments may be an effective strategy to increase humic matter content, promote aggregate formation, and enhance nutrient retention in soil [5]. References 1. U.B. Antu, T.K. Roy, T.I. Kulsum, P.R. Mitu, Z. Ismail, M. Arifin, M. Datta, S.A. Hossain, M.S. Islam, N.A. Mahiddin, A.A. Bakky, S. Hossin, S. Islam, A.M. Idris, International Journal of Biological Macromolecules 313 (2025) 144043. 2. L.P. Canellas, R.M. da Silva, J.G. Busato, F.L. Olivares, Chemical and Biological Technologies in Agriculture 11 (2024) 66.
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