Perennial bioenergy crops provide biomass for renewable energy production, but also sequester atmospheric carbon (C) in the soil. Roots represent one of the most important soil C inputs-root length density (RLD, cm cm(-3)), root diameter and fine root biomass (FRB, Mg ha(-1)) in the top 1 m of soil were characterized for three woody (poplar, black locust, willow) and three herbaceous (giant reed, miscanthus, switchgrass) perennial crops in the same location. The vertical distribution of FRB and RLD was described by fitting the "beta" (beta) model to the experimental data. The herbaceous species had higher beta values for FRB and RLD than woody crops, suggesting that the former explore the deeper soil layers with a greater proportion of roots. In particular, 3.7 Mg ha(-1), or 43% of the whole root mass, was found below the ploughing soil layer (0.3 m) for the herbaceous species, while only 1.2 Mg ha(-1), or 26% of the whole root mass, was allocated by woody crops to the same soil layer. In all the species, the majority of the sampled roots (99.1%) had a diameter lower than 2 mm, and in the first 10 cm of the soil the woody species tended to produce roots with a smaller diameter than those of the herbaceous species. Overall, the herbaceous crops have a higher potential to contribute to C storage in the deep soil layers, while the woody species, have a greater potential to affect soil organic carbon in the top soil layer. (C) 2015 Elsevier Ltd. All rights reserved.