Conversion of arable land to perennial bioenergy crops increases soil organic carbon stocks on the long term

Perennial C4 bioenergy crops can combine high productivity and low input requirements. However, their impact on soil organic carbon (SOC) stocks remains uncertain. The aim of this study was to assess the long-term impact of converting arable land to perennial bioenergy crops on SOC stocks for two crop species (miscanthus and switchgrass) and several crop management practices (nitrogen fertilization, harvest date and irrigation). We analyzed two long-term experiments located in northern and southern France. Both sites were sampled initially and after 12 or 13 years. SOC stocks were calculated at equivalent soil mass in each site and δ 13 C measurements were used to calculate changes in “new” and “old” SOC stocks. SOC stocks in the old ploughed layer increased significantly in both sites but most of the SOC storage occurred in the topsoil layer (∼0–5 cm). SOC storage rate was fairly similar between miscanthus and switchgrass but was much greater in the southern site than in the northern site (0.96 vs 0.26 t C ha −1 yr −1 ). This larger storage rate was mainly explained by higher carbon inputs, as suggested by the higher accumulation rate of new SOC (1.41 vs 0.86 t C ha −1 yr −1 ). No significant effect of the management practices on the SOC change rate could be detected, but early harvest systematically reduced SOC storage compared to late harvest (by 33 % for miscanthus and 12 % for switchgrass). Higher carbon inputs due to late harvest or irrigated conditions were partly compensated by a higher old SOC decrease. • SOC stock changes under miscanthus and switchgrass were measured in 2 experiments. • SOC stocks increased after 12–13 years in both sites, mainly in the 0–5 cm layer. • SOC storage rates did not differ between crop species but differed between sites. • Late harvest resulted in a greater accumulation of “new” SOC than early harvest. • The decay rate of the old SOC was stimulated by the magnitude of new SOC storage.