Morphological and biomechanical responses of Lespedeza bicolor turcz. roots to long-term phosphorus addition at high altitudes

Plant root morphology and mechanics are key determinants of slope stability. Soil phosphorus availability is typically limited in high-altitude regions subjected to engineering disturbances. Understanding how phosphorus supply shapes root traits is therefore crucial for enhancing vegetation restoration and slope stabilization in disturbed landscapes. A seven-year phosphorus addition experiment was conducted to examine the responses of Lespedeza bicolor Turcz. to five phosphorus supply levels (0, 2, 4, 6, and 8 g m^-2 yr^-1). The morphological traits (length, diameter, number), tensile strength, and chemical composition (cellulose, hemicellulose, lignin contents) were analyzed in taproots, first-, and second-order lateral roots. Most root traits exhibited a unimodal response to phosphorus supply, while lignin content showed an inverse trend. At a phosphorus supply level of 2 g m^-2 yr^-1, the root biomass, length, and number of L. bicolor reached their maximum values, likely due to adaptive responses under mild phosphorus deficiency. Under severe phosphorus deficiency or excess, root growth was inhibited because of weakened metabolism or phosphorus toxicity. Root tensile strength correlated positively with cellulose content but negatively with lignin content, suggesting the critical role of chemical composition in modulating root tensile strength. Overall, L. bicolor grown at the phosphorus supply level of 2 g m^-2 yr^-1 displayed optimal growth performance and erosion-reducing potential, with the greatest root biomass, fine root proportion, and tensile strength.