To investigate the responses of alfalfa root morphological traits to phosphorus supply， a total of 31 plant materials were selected comprising 30 varieties of Medicago sativa L. and one variety of Medicago falcata L.， and three levels of phosphorus addition treatments were set up at P₅ （5 mg·kg^-1 KH₂PO₄）， P₂₀ （20 mg·kg^-1 KH₂PO₄）， and P₅₀ （50 mg·kg^-1 KH₂PO₄）. To screen low phosphorus tolerant alfalfa varieties with high phosphorus use efficiency， we measured the aboveground and belowground biomass and root morphological indices for the 31 alfalfa varieties. The results indicated that the aboveground and belowground biomass of alfalfa were lowest under P₅ treatment. With increasing phosphorus supply， the aboveground biomass of alfalfa significantly increased， reaching its highest level under the P₅₀ treatment. Under the P₂₀ and P₅₀ treatments， the root biomass of alfalfa was significantly higher than that under P₅， but there was no significant difference between P₂₀ and P₅₀. However， the root morphology showed significant variation among different varieties. Among them， ‘HD’‘Algonguin’‘Magna Graze II’ and ‘Longmu 801’ had higher biomass than other varieties， and exhibited greater root morphological plasticity with higher root length， root volume， root surface area， and root diameter， which demonstrated tolerance to low phosphorus condition. The results provide a scientific basis for optimizing alfalfa cultivation and improving phosphorus fertilizer utilization efficiency.

Phosphorus level  /  Alfalfa  /  Root morphology  /  Phosphorus-efficient varieties