Arbuscular mycorrhizal fungi enhance Hordeum jubatum adaptation to nitrogen deposition and cold stress by regulating plant physiology, metabolism, and the rhizosphere soil environment

• Nitrogen deposition increased the sensitivity of Hordeum jubatum to cold stress. • AMF enhanced nitrogen deposition and cold stress tolerance of H. jubatum. • AMF colonization primarily enhances the adaptability of H. jubatum to nitrogen deposition and cold stress by promoting root organic acids and carbohydrate metabolism. • AMF colonization effectively improved the rhizosphere soil physicochemical properties and enzyme activities of H. jubatum under nitrogen deposition and cold stress. In natural environments, plants often rely on mycorrhizal symbiosis to mitigate abiotic stresses, including atmospheric nitrogen deposition with different NH 4 + :NO 3 − ratios and cold stress, which are particularly prevalent in northeastern regions. However, the specific mechanisms through which this symbiotic relationship enhances plant tolerance remain inadequately understood In this study, we applied two nitrogen deposition regimes with different NH 4 + :NO 3 − ratios (N 1 : 3:1 and N 2 : 1:1) and two temperatures (T 1 : 5 °C and T 2 : 25°C) to investigate how arbuscular mycorrhizal fungi (AMF) enhance the adaptation of Hordeum jubatum to nitrogen deposition and cold stress The results showed that nitrogen deposition did not significantly inhibit the growth of Hordeum jubatum , whereas its inhibitory effect became more pronounced under cold stress, particularly under the N 2 treatment (NH 4 + :NO 3 − =1:1). AMF inoculation increased the net photosynthetic rate and the activities of antioxidant enzymes in Hordeum jubatum subjected to combined nitrogen deposition and cold stress. AMF also promoted the accumulation of proline and soluble sugars while concomitantly reducing malondialdehyde content. Under cold stress, AMF inoculation altered 741 metabolites in the root system compared to the uninoculated treatment, primarily enhancing the tolerance of Hordeum jubatum through the metabolism of organic acids and carbohydrates. Additionally, AMF inoculation increased soil urease and acid phosphatase activities and improved soil physicochemical properties in the rhizosphere Therefore, we conclude that under the interactive context of nitrogen deposition and cold stress, AMF can enhance the environmental adaptability of host plants through multiple strategies, including the regulation of plant physiology, metabolism, and soil microenvironment.