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 ecophys- iological 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 (d¹³C, d¹5N) 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 d¹³C values with reduced interannual vari- ation, indicating enhanced stomatal conductance and more stable photosyn- thetic discrimination. Depleted d¹5N signatures reflected a shift toward microbially-mediated N acquisition pathways rather than increased absolute N availability. These findings demonstrate that MICOSAT F® biostimulant funda- mentally alters plant C and N metabolism, promoting growth while enhancing physiological stability - key traits for potential climate resilience in sustainable olive cultivation systems.