Salinity and waterlogging are major abiotic stresses constraining soybean productivity, with their combined effects often surpassing individual impacts. However, the nature of these combined effects-whether additive, synergistic, or antagonistic-remains unclear. In this study, we quantitatively demonstrated that combined salinity-waterlogging stress exerts additive inhibitory effects on soybean (Glycine max L. cv. Qihuang 34) growth, surpassing individual stresses without reaching synergistic severity. Physiological analysis revealed additive inhibition, as reflected by a 38.8 % reduction in leaf area, 36.8 % decrease in dry matter accumulation, and a 63.4 % decline in net photosynthetic rate under combined stress conditions, closely aligning with theoretical additive predictions (e.g., observed vs. predicted values: 38.8 % vs. 40.8 % for leaf area
36.8 % vs. 37.7 % for biomass). Integrated transcriptomic and metabolomic analysis identified hypoxia-induced ATP depletion as a key driver, disrupting Na