Land-use change can have significant impacts on soil and aboveground carbon (C) stocks and there is a clear need to identify sustainable land uses which maximize C mitigation potential. Land-use transitions from agricultural to bioenergy crops are increasingly common in Europe with one option being Short Rotation Forestry (SRF). Research on the impact on C stocks of the establishment of SRF is limited, but given the potential for this bioenergy crop in temperate climates, there is an evident knowledge gap. Here, we examine changes in soil C stock following the establishment of SRF using combined short (30 cm depth) and deep (1 m depth) soil cores at 11 sites representing 29 transitions from agriculture to SRF. We compare the effects of tree species including 9 coniferous, 16 broadleaved and 4 Eucalyptus transitions. SRF aboveground and root biomass were also estimated in 15 of the transitions using tree mensuration data allowing assessments of changes in total ecosystem C stock. Planting coniferous SRF, compared to broadleaved and Eucalyptus SRF, resulted in greater accumulation of litter and overall increased soil C stock relative to agricultural controls. Though broadleaved SRF had no overall effect on soil C stock, it showed the most variable response suggesting species-specific effects and interactions with soil types. While Eucalyptus transitions induced a reduction in soil C stocks, this was not significant unless considered on a soil mass basis. Given the relatively young age and limited number of Eucalyptus plantations, it is not possible to say whether this reduction will persist in older stands. Combining estimates of C stocks from different ecosystem components (e.g., soil, aboveground biomass) reinforced the accumulation of C under coniferous SRF, and indicates generally positive effects of SRF on whole-ecosystem C. These results fill an important knowledge gap and provide data for modelling of future scenarios of LUC.

contribute to greenhouse gas (GHG) emissions savings through reduced soil GHG fluxes and greater 19 soil C sequestration. If we are to predict the magnitude of any such GHG benefits a better understanding 20 is needed of the effect of land use change (LUC) on the underlying factors which regulate GHG fluxes. 21Under controlled conditions we measured soil GHG flux potentials, and associated soil physico-22 chemical and microbial community characteristics for a range of LUC transitions from grassland land 23 uses to SRF. These involved ten broadleaved and seven coniferous transitions. Differences in GHGs 24 and microbial community composition assessed by phospholipid fatty acids (PLFA) profiles were 25 detected between land uses, with distinctions between broadleaved and coniferous tree species. 26Compared to grassland controls, CO2 flux, total PLFAs and fungal PLFAs (on a mass of C basis), were 27 lower under coniferous species but unaffected under broadleaved tree species. There were no significant 28 differences in N2O and CH4 flux rates between grassland, broadleaved and coniferous land uses, though 29 both CH4 and N2O tended to have greater uptake under broadleaved species in the upper soil layer. 30Effect sizes of CO2 flux across LUC transitions were positively related with effect sizes of soil pH, total 31 PLFA and fungal PLFA. These relationships between fluxes and microbial community suggest that 32 LUC to SRF may drive change in soil respiration by altering the composition of the soil microbial 33 community. These findings support that LUC to SRF for bioenergy can contribute towards C savings 34 and GHG mitigation. 35 36