Background and aims: Drunken horse grass (DHG, Achnatherum inebrians) is a perennial grass widely distributed in the grasslands of northwestern China, forming a symbiosis with the Epichloë endophyte, and has a strong growth advantage in the community. However, the mechanism of its competitive success remains unclear.Epichloë gansuensis-infected (EI) and endophyte-free (EF) DHG were employed in greenhouse experiments. EI plants served as competitive species, while EF plants were the target species. Aggressivity was employed to quantify the competitiveness. The effects of endophyte, plant density, and three different nitrogen levels in the pot on the competitiveness of plants were investigated.This study indicates that under 0.1 mM and 1 mM nitrogen stress, E. gansuensis enhances its competitiveness by improving the growth performance, light utilization capacity, and nitrogen absorption efficiency of the host plant, with a relative increase of 15.0% and 5.2%, 72.0% and 23.0%, 21.5% and 17.2%. Notably, the gain effect of E. gansuensis on the competitiveness of DHG gradually weakened with the decrease in the number of EI plants.The E. gansuensis enhances DHG competitiveness under nitrogen limitation but diminishes as soil nitrogen content increases. These results provide insights into the competitive solid advantage of EI DHG and the high infection rate of DHG in natural grasslands with harsh habitats.Methods: Drunken horse grass (DHG, Achnatherum inebrians) is a perennial grass widely distributed in the grasslands of northwestern China, forming a symbiosis with the Epichloë endophyte, and has a strong growth advantage in the community. However, the mechanism of its competitive success remains unclear.Epichloë gansuensis-infected (EI) and endophyte-free (EF) DHG were employed in greenhouse experiments. EI plants served as competitive species, while EF plants were the target species. Aggressivity was employed to quantify the competitiveness. The effects of endophyte, plant density, and three different nitrogen levels in the pot on the competitiveness of plants were investigated.This study indicates that under 0.1 mM and 1 mM nitrogen stress, E. gansuensis enhances its competitiveness by improving the growth performance, light utilization capacity, and nitrogen absorption efficiency of the host plant, with a relative increase of 15.0% and 5.2%, 72.0% and 23.0%, 21.5% and 17.2%. Notably, the gain effect of E. gansuensis on the competitiveness of DHG gradually weakened with the decrease in the number of EI plants.The E. gansuensis enhances DHG competitiveness under nitrogen limitation but diminishes as soil nitrogen content increases. These results provide insights into the competitive solid advantage of EI DHG and the high infection rate of DHG in natural grasslands with harsh habitats.Results: Drunken horse grass (DHG, Achnatherum inebrians) is a perennial grass widely distributed in the grasslands of northwestern China, forming a symbiosis with the Epichloë endophyte, and has a strong growth advantage in the community. However, the mechanism of its competitive success remains unclear.Epichloë gansuensis-infected (EI) and endophyte-free (EF) DHG were employed in greenhouse experiments. EI plants served as competitive species, while EF plants were the target species. Aggressivity was employed to quantify the competitiveness. The effects of endophyte, plant density, and three different nitrogen levels in the pot on the competitiveness of plants were investigated.This study indicates that under 0.1 mM and 1 mM nitrogen stress, E. gansuensis enhances its competitiveness by improving the growth performance, light utilization capacity, and nitrogen absorption efficiency of the host plant, with a relative increase of 15.0% and 5.2%, 72.0% and 23.0%, 21.5% and 17.2%. Notably, the gain effect of E. gansuensis on the competitiveness of DHG gradually weakened with the decrease in the number of EI plants.The E. gansuensis enhances DHG competitiveness under nitrogen limitation but diminishes as soil nitrogen content increases. These results provide insights into the competitive solid advantage of EI DHG and the high infection rate of DHG in natural grasslands with harsh habitats.Conclusion: Drunken horse grass (DHG, Achnatherum inebrians) is a perennial grass widely distributed in the grasslands of northwestern China, forming a symbiosis with the Epichloë endophyte, and has a strong growth advantage in the community. However, the mechanism of its competitive success remains unclear.Epichloë gansuensis-infected (EI) and endophyte-free (EF) DHG were employed in greenhouse experiments. EI plants served as competitive species, while EF plants were the target species. Aggressivity was employed to quantify the competitiveness. The effects of endophyte, plant density, and three different nitrogen levels in the pot on the competitiveness of plants were investigated.This study indicates that under 0.1 mM and 1 mM nitrogen stress, E. gansuensis enhances its competitiveness by improving the growth performance, light utilization capacity, and nitrogen absorption efficiency of the host plant, with a relative increase of 15.0% and 5.2%, 72.0% and 23.0%, 21.5% and 17.2%. Notably, the gain effect of E. gansuensis on the competitiveness of DHG gradually weakened with the decrease in the number of EI plants.The E. gansuensis enhances DHG competitiveness under nitrogen limitation but diminishes as soil nitrogen content increases. These results provide insights into the competitive solid advantage of EI DHG and the high infection rate of DHG in natural grasslands with harsh habitats. [ABSTRACT FROM AUTHOR]