Screening of Salt-Tolerant Arbuscular Mycorrhizal Fungi and Their Effects on the Growth of Two Leguminous Forage Grasses

Frequent agricultural and pastoral activities in northern China’s agro-pastoral ecotone have resulted in severe soil salinization. Although phytoremediation is currently the recommended remediation strategy, plant growth is often hindered by microbial deficiencies, nutrient limitations, and high salt content. Arbuscular mycorrhizal fungi (AMF) are prevalent in saline soils and have been shown to facilitate the growth of various forage grasses. However, highly efficient “AMF strain–leguminous forage grass” remediation systems are currently lacking in this area. This study examined the impact of nine AMF strains sourced from the Bank of Glomeromycota in China (BGC) on the growth and salt tolerance mechanisms of Medicago sativa and Astragalus adsurgens via pot experiment. The results showed that all the strains, except Funneliformis mosseae BGC NM04A (Fm-2), exhibited significant promotion of the growth of both leguminous forage species. Specifically, the growth of M. sativa increased by 157% to 354%, and that of A. adsurgens increased by 15.2% to 252%. The impact of different strains on plant-soluble sugar and protein content was found to vary, with Rhizophagus intraradices BGC BJ09 (Rin) and Rhizophagus irregularis MUCL 43194 (Rir-1) having particularly pronounced effects. Most of the AMF strains enhanced the uptake of phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in the shoots of both plant species. Additionally, all strains except for Fm-2 exhibited a significant increase in the K+/Na+ ratio in M. sativa shoots. In contrast, only Claroideoglomus etunicatum BGC GZ03C (Ce-1) and Rir-1 demonstrated a substantial increase in the K+/Na+ ratio in A. adsurgens. Following a comprehensive assessment, several highly effective combinations were identified: M. sativa in combination with Ce-1, F. mosseae BGC HUN01A (Fm-1), or Rir-1; and A. adsurgens with Rir-1 or Fm-1. These “forage–AMF strain” combinations have been demonstrated to alleviate salt stress by enhancing mycorrhizal effects, regulating ion homeostasis, and optimizing osmotic regulatory substances. Our findings have clarified the intraspecific and interspecific differences in AMF in mitigating salt stress, emphasizing the potential of AMF as a sustainable strategy to improve the stress resistance and productivity of leguminous forages in the saline agro-pastoral ecotone.