From Agro-Industrial By-Products to Microbial Soil Conditioners by Bioconversion of Olive and Grape Pomace

The bioconversion of agro-industrial waste represents a promising strategy for the valorisation of residual biomass. However, the chemical complexity of these matrices and the presence of potentially inhibitory compounds limit their direct use in several bioprocesses. In this study, a quantitative, time-resolved method was used to select bacteria for bioconverting agro-industrial by-products. The growth dynamics of bacterial strains were screened using olive and grape pomace at different compositions (up to 15%) and formulations. An integrated scoring approach (0–1) was used to compare strain behaviour across experimental conditions. The results revealed strain-dependent variability, with a matrix concentration of 10% defined as the growth-limiting concentration, with approximately 50% positive results. Among the tested strains, Bacillus subtilis BL showed a consistent and reproducible response across different by-products and formulations, maintaining stable spore viability over time, particularly in formulations supplemented with calcium carbonate (on average 10 9 UFC/mL after 144 h). Mixed agro-industrial matrices promoted a more homogeneous and stable microbial response than individual components (reaching 10 9 CFU/mL after approximately 100 h), supporting their direct use in a real operating environment. Overall, this work proposes a transferable quantitative approach to selecting microorganisms suitable for bioconversion of agro-industrial by-products, providing a methodological basis for developing more reliable and reproducible formulations.