Variation Characteristics of Root Traits of Different Alfalfa Cultivars under Saline-Alkaline Stress and their Relationship with Soil Environmental Factors

Soil salinization is the main factor that threatens the growth and development of plants and limits the increase of yield. It is of great significance to study the key soil environmental factors affecting plant root traits to reveal the adaptation strategies of plants to saline-alkaline-stressed soil environments. In this study, the root biomass, root morphological parameters and root mineral nutrient content of two alfalfa cultivars with different sensitivities to alkaline stress were analyzed with black soil as the control group and the mixed saline-alkaline soil with a ratio of 7:3 between black soil and saline-alkaline soil as the saline-alkaline treatment group. At the same time, the correlation analysis of soil salinity indexes, soil nutrient indexes and the activities of key enzymes involved in soil carbon, nitrogen and phosphorus cycles was carried out. The results showed that compared with the control group, the pH, EC, and urease (URE) of the soil surrounding the roots of two alfalfa cultivars were significantly increased, while soil total nitrogen (TN), total phosphorus (TP), organic carbon (SOC), and α-glucosidase activity (AGC) were significantly decreased under saline-alkaline stress. There was no significant difference in root biomass and root morphological parameters of saline-alkaline tolerant cultivar GN under saline-alkaline stress. The number of root tips (RT), root surface area (RS) and root volume (RV) of AG were reduced by 61.16%, 44.54%, and 45.31%, respectively, compared with control group. The ratios of K⁺/Na⁺, Ca²⁺/Na⁺ and Mg²⁺/Na⁺ of GN were significantly higher than those of AG (p < 0.05). The root fresh weight (RFW) and dry weight (RDW), root length (RL), RV and RT of alfalfa were positively regulated by soil SOC and TN, but negatively regulated by soil pH, EC, and URE (p < 0.01). Root Ca²⁺/Na⁺ ratio was significantly positively correlated with soil TN, TP and SOC (p < 0.01). The absorption of Mg and Ca ions in roots is significantly negatively regulated by soil β-glucosidase activity (BGC) and acid phosphatase activity (APC) (p < 0.05). This study improved knowledge of the relationship between root traits and soil environmental factors and offered a theoretical framework for elucidating how plant roots adapt to saline-alkaline stressed soil environments.