Microbial biostimulant reshapes carbon and nitrogen metabolism in olive trees: dendrochronological insights into enhanced growth and climate adaptation

Microbial biostimulants are increasingly proposed as sustainable tools to enhance crop performance and resilience under climate change. However, their ecophysiological effects and underlying mechanisms remain insufficiently understood, particularly in woody perennial species. This study investigated the effects of MICOSAT F ® microbial biostimulant on growth and ecophysiological traits of two-year-old Olea europaea L. cv. Leccino plants under controlled greenhouse conditions. For the first time, we integrated agronomic measurements with dendrochronological analyses and intra-annual assessments of carbon and nitrogen concentration (C%, N%) and their stable isotope composition (δ¹³C, δ¹ 5 N) in tree rings. Biostimulant-treated trees exhibited significantly larger stem diameter, height, lateral branching, and total biomass compared with controls. Treated trees showed lower stem wood C%, suggesting increased C allocation to non-structural carbohydrates and belowground symbionts. Furthermore, treated trees displayed significantly depleted δ¹³C values with reduced interannual variation, indicating enhanced stomatal conductance and more stable photosynthetic discrimination. Depleted δ¹ 5 N signatures reflected a shift toward microbially-mediated N acquisition pathways rather than increased absolute N availability. These findings demonstrate that MICOSAT F ® biostimulant fundamentally alters plant C and N metabolism, promoting growth while enhancing physiological stability - key traits for potential climate resilience in sustainable olive cultivation systems.