Enhanced Formation and Stability of Water-Stable Aggregates in Rhizosphere Soil over Bulk Soil with Exopolysaccharide from Rhizobium tropici: Insights from a Pot Study

This study investigated the effects of exopolysaccharide (EPS) addition on the formation and stability of water-stable aggregates in rhizosphere and bulk soils. A pot experiment was conducted using soils treated with EPS concentrations of 0.00‰, 0.25‰, 0.50‰, and 1.00‰. Soil aggregates were fractionated into four fractions, namely >2000 μm, 250–2000 μm, 53–250 μm, and <53 μm, and their stability was evaluated using mean weight diameter (MWD), geometric mean diameter (GMD), and fractal dimension (D). Results showed that EPS addition significantly increased the proportions of larger and macro aggregates (>2000 μm and 250–2000 μm) while reducing smaller particles (<53 μm), with rhizosphere soil exhibiting a stronger response compared to bulk soil. Aggregate stability indices (MWD and GMD) improved consistently with increasing EPS concentrations, while D decreased, indicating enhanced aggregates stability. Moderate EPS concentrations (0.25‰ and 0.50‰) were most effective in improving aggregate formation and stability and moderately enhanced plant biomass, particularly root biomass. Pearson correlation analysis revealed that macro-aggregate fractions (>2000 μm and 250–2000 μm) were positively correlated with each other but showed weak or non-significant relationships with plant biomass parameters. In particular, the 250–2000 μm fraction exhibited a weak negative correlation with total biomass (r = −0.37, p ≤ 0.05). These findings highlight the potential of moderate EPS concentrations to enhance soil structure and stability, particularly in rhizosphere soils, providing insights into its application for sustainable soil management.