Sugarcane is poised to become a key bioenergy crop in the Southeastern U.S., where its expansion may replace extensive pasturelands. Our findings show that such land-use transitions may incur significant carbon losses – beyond harvest or fire removals – led by rapid mineralization of legacy biomass and limited stabilization upon conversion. Yet, sugarcane’s high metabolic efficiency, strong productivity, and effective allocation to harvestable biomass position it as a strong candidate for biofuel production in subtropical regions. A shift from root- to litter-dominated inputs, however, may have implications for long-term soil carbon stabilization. These results highlight both the trade-offs and potential of sugarcane-based bioenergy systems.