Effects of Supplementary Fertilization on Physiological Traits and Yield of Soybean Following Drought-Flood Abrupt Alternations of Varying Intensities

Drought-flood abrupt alternation (DFAA) events severely constrain soybean production, yet the physiological effects of supplementary fertilization and other measures following such events are unclear. This study investigated the effects of supplementary fertilization (SF) on the physiological characteristics and yield of soybean after different intensities of DFAA events. Seven treatments were set up in the experiment: drought rapidly followed by light, moderate, or heavy waterlogging (D-LW, D-MW, D-HW), SF after DFAA (D-LWF, D-MWF, D-HWF), and normal water supply (control, CK). Soybean growth, leaf physiological indicators, above-ground dry matter, yield, and yield components were measured throughout the experiment. The results showed that SF promoted stem diameter and leaf growth in soybeans under drought-to-waterlogging stress. At 35 days after fertilization, the D-HWF treatment significantly outperformed D-HW in stem diameter and leaf area. At 10 days after fertilization, compared with the corresponding unfertilized treatments, the D-L/-M/-HWF treatments significantly increased leaf relative water content (LRWC, by 13.55–33.39%) and SOD activity, while the D-M/-HWF treatments reduced the chlorophyll a/b ratio by 6.14–8.29%. Under uniform SF application, yield-related traits exhibited a statistically non-significant positive recovery trend under the D-MW and D-HW treatments (except for the effective grain number and total grain number). The D-MWF and D-HWF combinations significantly increased effective grain number and total grain number by 39.20–88.59% relative to their unfertilized controls. SF application tended to enhance soybean yield under DFAA, with D-MWF and D-HWF showing significant yield increases of 63.24% and 57.04% over D-MW and D-HW, respectively. The yield compensation rate rose substantially, with fertilized treatments recovering to 71.28–74.69% of the CK level. Collectively, these results indicate that SF alleviates yield loss under DFAA stress through a combination of optimized pod traits, increased grain number, alleviated oxidative damage during later recovery stages, and promoted regrowth. Under the present experimental conditions, greater compensatory effects were observed under moderate and heavy post-drought waterlogging than under light post-drought waterlogging, reflecting the combined influences of DFAA severity and the duration of pre-fertilization recovery intervals.