Unveiling the impact of phosphorus availability on growth, root morphological and physiological traits of different phosphorus-tolerant alfalfa (Medicago sativa L.) in acidic soils in the karst region of southwest China

The cultivation of alfalfa ( Medicago sativa L.) in acidic soils in southern China poses significant challenges, primarily due to two key factors present in soils at low pH: phosphorus (P) deficiency and aluminum (Al) toxicity. This study aimed to assess the morphological and physiological responses of alfalfa roots under conditions of P limitation and low pH. A pot experiment compared the growth and root characteristics of an Al-sensitive cultivar (Longzhong) and anAl -tolerant cultivar (Trifecta) of alfalfa in acidic soil. This was achieved by supplying six P levels (0, 10, 20, 40, 80, and 120 mg P kg −1 soil) and utilizing two soil types (limestone soil pH 6.01 and yellow soil pH 5.46). Phosphorus application alleviated acid-aluminum stress and enhanced alfalfa growth, but aboveground growth did not further increase when the P supply exceeded 40 mg P kg −1 soil. Trifecta exhibited higher P uptake and P utilization efficiency, along with increased total root length, compared to Longzhong. In both soil types, the two alfalfa cultivars allocated more biomass to the roots to maximize P extraction from the soil. The superior root morphological traits observed in Al-tolerant alfalfa enhanced P uptake and biomass production in acidic soil conditions. The secretion of more oxalate in yellow loam soil is an effective strategy for alfalfa to improve P acquisition. Under the P application of 20 P kg-1, the oxalate secretion of the tolerant and sensitive genotypes was 1.6 times and 2.5 times that of the control. However, excessive application of acidic P fertilizers, such as calcium dihydrogen phosphate, can markedly decrease soil pH, increase the concentration of exchangeable aluminum, and ultimately inhibit plant growth. This suggests that tailoring phosphorus fertilizer application strategies to specific soil acidity can improve fertilizer use efficiency and enhance alfalfa yield and tolerance, not only providing breeding strategies for acid-Al -tolerant alfalfa development but also offering fertilization guidance for alfalfa cultivation in this region. • 20 mg P kg⁻¹ alleviates acid–Al stress and promotes alfalfa growth. • Al-tolerant cultivar invests more in roots to enhance P extraction. • Alfalfa secretes oxalate to improve P acquisition in acidic soils.