¹⁵N isotope tracing reveals Beauveria bassiana colonization promotes nitrogen use efficiency of maize under elevated CO₂.

While the entomopathogenic fungus Beauveria bassiana is well known for its endophytic plant colonization and growth-promoting capabilities, its role in regulating plant nutrient dynamics, especially under climate change, remains poorly understood. This study investigated the interactive effects of B. bassiana colonization and elevated atmospheric carbon dioxide (CO₂) on nitrogen metabolism in maize. Open-top chambers were employed to simulate ambient and elevated (600 ppm) CO₂ conditions, combined with ¹ ⁵N isotope tracing to quantify nitrogen translocation, transformation and rhizosphere microbial responses. B. bassiana colonization significantly improved maize nitrogen uptake and assimilation under both CO₂ regimes, with more prominent benefits detected under elevated CO₂. The endophyte modulated internal nitrogen allocation, increased grain nitrogen concentration, and enhanced the activities of key enzymes involved in nitrogen metabolism (i.e., glutamine synthetase, GS; and nitrate reductase, NR). Meanwhile, B. bassiana inoculation reshaped rhizosphere microbial community assembly and regulated rhizosphere microenvironment, thereby facilitating nitrogen availability for plant uptake and reducing nitrogen loss. Notably, B. bassiana effectively alleviated the elevated CO₂-induced nitrogen dilution effect, resulting in greater plant biomass accumulation and higher nitrogen use efficiency (NUE). These findings highlight that beneficial B. bassiana can synergistically modulate plant nitrogen physiology under high-CO₂ scenarios, offering a promising microbial strategy to sustain crop productivity and reduce chemical fertilizer input under future climate change by optimizing plant nitrogen uptake, assimilation and allocation.