IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 170 Travel Awardee Presentations Friday, 28 August 2026 / Hall C TA5 Biostimulants Based on Humic Substances and Bacterial Consortia Modulate Soybean Microbiome and Growth Ildeane Silva de Oliveira¹*, Letícia Cespom Passos¹, Diego Vinicius de Sena Martins¹, Letícia Oliveira da Rocha¹, Daniel Basílio Zandonadi², Fabio Lopes Olivares¹ ¹ Cell and Tissue Biology Laboratory, Center of Biosciences and Biotechnology, Darcy Ribeiro North Fluminense State University, Campos dos Goytacazes, RJ, Brazil. ² Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil. * Corresponding author: ildeane1212@gmail.com Biostimulants based on humic substances and plant growth-promoting bacteria have shown significant potential to enhance plant growth and modulate microbiome assembly [1]. However, their integrated effects across the rhizosoil-root continuum remain poorly understood [2]. Two complementary experiments were conducted with soybean (Glycine max) under greenhouse conditions: (i) determination of the most efficient concentration of a commercial HS-enriched biofertilizer based on morphological responses; and (ii) evaluation of how this concentration, combined with a bacterial consortium (Herbaspirillum seropedicae strain HRC54, Bradyrhizobium japonicum strain BR102, and B. elkanii strain BR115), modulates bacterial diversity using 16S rRNA gene sequencing. Co-inoculation with H. seropedicae and HS has demonstrated significant efficacy in promoting vegetative growth and nodulation in this legume [3]. In Experiment I, a concentration of 8 mg C L⁻¹ yielded the highest accumulation of shoot fresh biomass (+131% vs. 8.2 g in the control) and enhanced morphophysiological performance, including +74% increase in leaf area (497.5 cm² plant⁻¹) and a 64% increase in shoot length (45.8 cm). In Experiment II, 16S rRNA gene sequencing analyses revealed an ecological filtering gradient characterized by higher soil richness and more pronounced specialization of microbial communities within the roots. These treatments facilitated the enrichment of taxa involved in nitrogen cycling and carbon turnover, driven by functional selection in the rhizosphere [2]. Collectively, these results indicate that HS-based biostimulants and bacterial consortia intensify ecological filtering along the soil-rhizosphere-root gradient [4], resulting in more specialized and functionally enriched microbial communities [5]. These findings validate the combined use of HS and bacterial consortia as a promising strategy for optimizing the soybean microbiome and promoting sustainable agricultural productivity.
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