Spent Mushroom Substrate Supplemented with Streptomyces gardneri MC1-2 Improves Tomato Seedling Growth and Its Establishment in Soil Application

Spent mushroom substrate (SMS), a lignocellulosic by-product of mushroom cultivation, represents a promising renewable carrier for microbial bioinoculants as well as agricultural waste valorization. This study evaluated SMS as a carrier for the plant growth-promoting actinobacterium Streptomyces gardneri MC1-2 and assessed its effects on tomato growth and soil bacterial communities. Three formulations were prepared: SMS alone (F1), SMS supplemented with 1% (w/w) carboxymethyl cellulose (CMC) (F2), and talc supplemented with 1% CMC and 1.5% CaCO₃ (F3). S. gardneri MC1-2 exhibited in vitro traits commonly associated with plant-beneficial activity, including indolic compound production, siderophore production, and inorganic phosphate solubilization. During 30 days of storage at 4, 30, and 40 °C, both SMS-based formulations maintained viable populations at a 3 Log CFU reduction, whereas the talc-based formulation lost viability within seven days. Among the formulations, F2 showed the best overall performance and was selected for the pot experiment. Direct inoculation with S. gardneri MC1-2 promoted tomato growth comparably to chemical fertilizer, while F2 strongly supported growth in sterile soil, where S. gardneri MC1-2 established and dominated the bacterial community. In non-sterile soil, reduced bacterial establishment suggested that competition with native microbiota limited inoculant performance. These findings demonstrate that SMS can serve as an effective carrier for Streptomyces-based bioinoculants with potential applications for sustainable crop production.