© 2015, Springer International Publishing Switzerland. Background and aims: The productivity of lucerne (Medicago sativa L.) is limited by phosphorus (P) deficiency and drought on alkaline soils in northwest China. The aim of this study was to identify the morphological and physiological responses of roots to P- and moisture-limited conditions in an alkaline soil. Methods: A pot experiment compared the growth and root characteristics of an introduced (Arkaxiya) and a locally-selected (Longzhong) lucerne cultivar grown in alkaline soil, with an initial available soil P of 6.9 μg P g−1 dry soil and a pH of 8.3, with four applied-P rates (0, 4.2, 8.4 and 16.8 μg P g−1 dry soil) and three soil moisture treatments [maintained at 75–90 %, 45–55 % and 30–35 % of field capacity (FC)]. Results: At high soil P and high soil water content (SWC), high total root length contributed to high P uptake, high P-use efficiency, and greater plant growth in Arkaxiya compared to Longzhong. However at low SWC, Longzhong had a higher specific root length (thinner roots) and higher total root length than Arkaxiya. At low SWC greater biomass was allocated to roots, at low P-supply the specific root length increased, while the combination of low soil P and low SWC induced greater release of rhizosphere carboxylates. Conclusions: The results suggest that the introduced cultivar has traits that will benefit lucerne production in all but the driest and low-phosphate alkaline soils.