Sequential Drought–Waterlogging Treatments Regulate Root–Nodule Function, Nitrogen Acquisition, and Forage Productivity in Alfalfa (Medicago sativa L.)

Unpredictable shifts between water deficit and excess water are increasingly relevant to forage systems, yet the extent to which prior water conditions shape later responses in alfalfa remains insufficiently resolved. We compared seven water-treatment sequences to assess plant growth, root and nodule performance, nitrogen acquisition, leaf physiology, photosynthesis, forage quality, and rhizosphere properties in alfalfa (Medicago sativa L.). Individual drought and waterlogging treatments impaired several growth and physiological traits, but the order of the two stresses produced distinct outcomes. Relative to continuously watered plants (C→C), waterlogging followed by drought (W→D) reduced total dry biomass, root activity, and nitrogen accumulation by 45.82%, 55.00%, and 54.38%, respectively. Drought followed by waterlogging (D→W), by contrast, caused the largest declines in nitrogenase activity and leghemoglobin content, by 66.27% and 52.92%, respectively. Recovery after stress removal was incomplete for several traits, indicating that the preceding water regime continued to influence plant performance. These results show that the sequence of water events is an important determinant of short-term root–nodule function, nitrogen status, and forage productivity under fluctuating water availability.