Comparative Efficacy of Seed and Soil Inoculation in Soybean Unveiling the Dominant Role of Soil Microbial Activity in Drought Mitigation

Abstract

Drought stress severely negatively affects the growth and yield of soybean (Glycine max L.) by causing oxidative damage and suppressing physiological functions. While plant growth-promoting rhizobacteria (PGPR) alleviates these effects, the comparative effectiveness of different inoculation techniques has not been adequately investigated. This study investigated the physiological, biochemical, and soil microbiological responses of drought-stressed soybean (50% field capacity) to two Bradyrhizobium japonicum inoculation methods (conventional seed inoculation (A1) versus direct soil inoculation (A2)). Drought significantly reduced plant biomass, relative water content (RWC), and chlorophyll levels while aggravating markers of oxidative stress (H₂O₂, MDA, and electrolyte leakage). Both inoculation methods reduced drought-induced damage; however, soil inoculation (A2) showed significant superiority. Compared to uninoculated stressed plants, A2 treatment increased fresh and dry weight by 91.7% and 104.3%, respectively, while maximally suppressing lipid peroxidation and preserving cell membrane integrity. Moreover, soil inoculation profoundly increased rhizosphere microbial activity, increasing soil respiration (CO₂) and dehydrogenase activity (DHA) by 110.6% and 86.7%, respectively. In particular, Random Forest machine learning analysis identified soil DHA as the most critical determinant in predicting plant biomass under stress, far outweighing internal oxidative stress indicators. These findings suggest that direct soil inoculation optimizes rhizosphere enzymatic activity and offers a more robust agronomic strategy than seed inoculation for sustainable soybean cultivation in drought-prone regions.