This study examines the growth performances, nutrient uptake and potential methane yield of six energy crops fertilized with the digestate liquid fraction (DLF) with or without arbuscular mycorrhizal fungi (AMF) inoculation. The 3-year-experiment involved the four perennial (Arundo donax L. Miscanthus × giganteus Greef et Deu, Heliantus tuberosus L. Lolium perenne L.) and two annual (Zea mays L. and Sorghum bicolor (L.) Moench × sudanense Stapf.) crops. The cumulative highest yield was observed for A. donax (128.1 ± 8.4 Mg ha−1), followed by M. × giganteus and S. bicolor (83.0 ± 8.0 Mg ha−1), H. tuberosus and Z. mays (66.2 ± 7.7 Mg ha−1) and L. perenne (31.4 ± 2.9 Mg ha−1). A. donax showed the highest estimated methane yield per hectare (11.030 Nm3 ha−1). After 3 years, the fertilization with DLF increased the initial soil Na+ content by 76.9%. During the experiment, only temporary and marginal effects were shown by AMF inoculation, which did not affect biomass production by any crop. Interestingly, AMF significantly increased the NO3-N concentration (+70%) while it reduced the NH4-N concentration in percolation water (−32.8%). DLF could be considered a viable organic fertilizer for biomass production, but the soil Na+ concentration should be carefully monitored.
