Optimizing Soybean Growth and Soil Microbial Attributes with Pseudomonas fluorescens: Dose-Dependent Responses and Exploratory Functional Profiles

To evaluate nonlinear responses of field-grown soybean and soil microbial attributes to increasing rates of a commercial formulation containing Pseudomonas fluorescens CCT7997. A randomized complete block experiment with four blocks compared 0, 2, 4, 6, and 8 L ha⁻¹ of the formulation. Soybean growth, leaf physiology, defense-related enzymes, and conventional soil microbial attributes were evaluated. Shotgun metagenomic sequencing was performed on one pooled soil sample per treatment and interpreted descriptively. Application rate affected root length ( P = 0.002), shoot length ( P = 0.049), fresh root mass ( P = 0.050), and fresh shoot mass ( P = 0.019). Quadratic models estimated maximum root length, shoot length, and fresh root mass at 3.01–3.39 L ha⁻¹. Physiological indices and grain number were unaffected. Microbial biomass carbon was greatest at 6 L ha⁻¹, whereas basal respiration was greatest at 4 L ha⁻¹. Fungal hyphal length was greater at 4 and 8 L ha⁻¹, while bacterial biovolume was greater at 2 and 4 L ha⁻¹ than at 8 L ha⁻¹. SOD activity was lower at all inoculated rates than in the control; CAT and PAL showed rate-specific responses. Exploratory metagenomic profiles varied among libraries, and P. fluorescens represented 10.66% of Pseudomonas -assigned sequences at 4 L ha⁻¹ versus 1.47% in the control. Intermediate application rates, particularly 4 L ha⁻¹, were associated with the strongest early soybean growth responses and distinct changes in soil microbial attributes. The metagenomic profiles revealed variation in taxonomic composition and functional potential among the composite libraries, including a greater representation of P. fluorescens sequences at 4 L ha⁻¹. These findings demonstrate the importance of application rate in shaping plant and soil microbial responses to P. fluorescens CCT7997 under field conditions.