Use of a fibrous solid substrate in pellet form in a method of cultivating fungal cells and/or spores; as a fertilizer; as a soil improver; and/or as litter for animals; wherein said fibrous solid substrate in pellet form is obtained by a method comprising the steps of a. providing a biomass material comprising solid and liquid parts from a biogas fermenter following an anaerobic fermentation and biogas production, b. subjecting the fermented biomass material of step a. to one or more separation steps resulting in the provision of i) a fibrous solid fraction comprising organic and inorganic nitrogen parts and having a reduced content (w/w) of water, and comprising one or more macromolecular nutrient constituents selected from the group consisting of cellulose, hemicellulose, lignin and lignocellulose, and ii) at least one liquid fraction comprising solid and liquid organic and inorganic phosphor-containing parts, c. subjecting the fibrous solid fraction of step b. to a sanitation treatment, said treatment comprising the steps of i) heating the fibrous solid fraction to a temperature of more than 70°C, optionally under alkaline pH conditions, and optionally ii) subjecting the fibrous solid fraction comprising organic and inorganic nitrogen parts to a pressure of more than 1 bar, wherein said treatment i) reduces or eliminates viable microorganisms present in the fibrous solid fraction, and/or ii) reduces the contents of volatile nitrogen-containing compounds and/or precursor volatile compounds present in the fibrous solid fraction, and d. obtaining a fibrous solid fraction comprising organic and inorganic nitrogen parts having a reduced content of volatile nitrogen-containing compounds, e. optionally subjecting said fibrous solid fraction to one or more of heating, drying, evaporation, pressure, and/or alkaline pH conditions, f. optionally adding to said fibrous solid fraction one or more solid and/or liquid supplemental nutrient substrate compositions, and g. compressing said fibrous solid fraction of step d., e. and/or f., thereby generating a fibrous solid substrate in pellet form. a. providing a biomass material comprising solid and liquid parts from a biogas fermenter following an anaerobic fermentation and biogas production, b. subjecting the fermented biomass material of step a. to one or more separation steps resulting in the provision of i) a fibrous solid fraction comprising organic and inorganic nitrogen parts and having a reduced content (w/w) of water, and comprising one or more macromolecular nutrient constituents selected from the group consisting of cellulose, hemicellulose, lignin and lignocellulose, and ii) at least one liquid fraction comprising solid and liquid organic and inorganic phosphor-containing parts, i) a fibrous solid fraction comprising organic and inorganic nitrogen parts and having a reduced content (w/w) of water, and comprising one or more macromolecular nutrient constituents selected from the group consisting of cellulose, hemicellulose, lignin and lignocellulose, and ii) at least one liquid fraction comprising solid and liquid organic and inorganic phosphor-containing parts, c. subjecting the fibrous solid fraction of step b. to a sanitation treatment, said treatment comprising the steps of i) heating the fibrous solid fraction to a temperature of more than 70°C, optionally under alkaline pH conditions, and optionally ii) subjecting the fibrous solid fraction comprising organic and inorganic nitrogen parts to a pressure of more than 1 bar, wherein said treatment i) reduces or eliminates viable microorganisms present in the fibrous solid fraction, and/or ii) reduces the contents of volatile nitrogen-containing compounds and/or precursor volatile compounds present in the fibrous solid fraction, and d. obtaining a fibrous solid fraction comprising organic and inorganic nitrogen parts having a reduced content of volatile nitrogen-containing compounds, e. optionally subjecting said fibrous solid fraction to one or more of heating, drying, evaporation, pressure, and/or alkaline pH conditions, f. optionally adding to said fibrous solid fraction one or more solid and/or liquid supplemental nutrient substrate compositions, and g. compressing said fibrous solid fraction of step d., e. and/or f., thereby generating a fibrous solid substrate in pellet form.