| Summary: | This study aimed at valorizing digestate through <i>Trichoderma</i> spp. solid-state fermentation (SSF) to produce a potentially ameliorated fertilizer combined with fungal biomass as a value-added bioproduct. Plant-growth-promoting <i>Trichoderma atroviride</i> Ta13, <i>T. reesei</i> RUT-C30, <i>T. asperellum</i> R, and <i>T. harzianum</i> T-22 were tested on different SSF substrates: whole digestate (WD), digestate dried up with wood sawdust (SSF1), and digestate enriched with food waste and dried up with wood sawdust (SSF2). The fungal biomass was quantified by using a qPCR assay. The growth of the four <i>Trichoderma</i> spp. was only observed on the SSF2 substrate. The highest quantity of mycelium was produced by <i>T. reesei</i> RUT-30 (689.80 ± 80.53 mg/g substrate), followed by <i>T. atroviride</i> Ta13, and <i>T. asperellum</i> R (584.24 ± 13.36 and 444.79 ± 91.02 mg/g substrate). The germination of <i>Lepidium sativum</i> seeds was evaluated in order to assess the phytoxicity of the <i>Trichoderma</i>-enriched substrate. The treatments with 7.5% SSF2-R, 3.75% SSF2-T-22, and 1.8% SSF2-Ta13 equally enhanced the root elongation in comparison to the non-fermented SSF-2. This study demonstrated that digestate, mixed with agro-food waste, was able to support the cultivation of <i>Trichoderma</i> spp., paving the way to the valorization of fermented digestate as a proper biofertilizer.
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