Evaluating Fermented and Non‐Fermented Lettuce‐Based Biostimulants Through a Design of Experiments Approach to Address Agronomic and Microbial Impacts in Strawberry Soilless Cultivation

ABSTRACT Intensive agriculture relies heavily on synthetic fertilizers, which contribute to green‐house gas emissions and to the degradation of soil quality and microbial biodiversity. In line with circular‐economy principles, the conversion of organic biomass into biostimulants through fermentation has emerged as a sustainable strategy to support plant performance. This study evaluated the use of solid‐state fermentation with lactic acid bacteria (LAB) to produce an experimental biostimulant from retail lettuce waste and assessed its agronomic and microbial effects on soilless‐grown strawberry. A design of experiments approach, a statistical, multivariate method for systematically planning experiments and modelling the relationships between process factors and measured responses, was used to model the fermentation process and to identify the key factors driving microbial growth. Strain selection was the main determinant, and fermentation with Lactiplantibacillus plantarum at 25°C for 24 h was identified as the optimal condition, reaching 8.6 Log CFU g −1 . When applied to strawberries, the fermented biostimulant enhanced plant growth, root development and fruit production in a dose‐ and stage‐dependent manner, with the highest dose increasing fruit number by approximately 3.7‐fold and markedly raising fruit fresh weight and phenolic content relative to the untreated control. Metabolic profiling of the rhizosphere revealed stage‐dependent shifts in carbon‐source utilization, while qPCR showed that the inoculated L. plantarum persisted in the rhizosphere for several weeks after application, despite a progressive decline in abundance. Overall, fermented lettuce biomass shows promise as a sustainable biostimulant capable of improving strawberry growth and fruit quality, offering a circular‐economy route for the valorization of retail vegetable waste. These findings should be regarded as preliminary and warrant further formulation and field‐scale validation.