The Carbon Budget of Land Conversion: Sugarcane Expansion and Implications for a Sustainable Bioenergy Landscape in Southeastern United States

ABSTRACT The expansion of sugarcane onto land currently occupied by improved (IMP) and semi‐native (SN) pastures will reshape the U.S. bioenergy landscape. We combined biometric, ground‐based and eddy covariance methods to investigate the impact of sugarcane expansion across subtropical Florida on the carbon (C) budget over a 3‐year rotation. With 2.3‐ and 5.1‐fold increase in productivity over IMP and SN pastures, sugarcane displayed a C use efficiency (CUE; i.e., fraction of gross C uptake allocated to plant growth) of 0.59, well above that of pastures (0.31–0.23). Sugarcane also had greater C allocation to aboveground productivity and hence, harvestable biomass relative to IMP and SN. Cane heterotrophic respiration over the 3‐year rotation (903 ± 335 gC m−2 year−1) was 1% and 14% higher than IMP and SN pastures, respectively. These soil C losses responded largely to disturbance over the first year after conversion (1510 ± 227 gC m−2 year−1) but declined in subsequent years to an average 599 ± 90 gC m−2 year−1—well below those of IMP (933 ± 140 gC m−2 year−1) and SN (759 ± 114 gC m−2 year−1) pastures—despite a significant 40%–61% increase in soil C inputs. Soil C inputs, however, shifted from root‐dominated in pastures to litter‐dominated in sugarcane, with only 5% C allocation to roots. Reduced decomposition rates in sugarcane were likely driven by changes in the recalcitrance and distribution rather than the size of the newly incorporated soil C pool. As a result, we observed a rapid shift in the net ecosystem C balance (NECB) of sugarcane from a large source immediately following conversion to approaching the net C losses of IMP pastures only 2 years after conversion. The environmental cost of converting pasture to sugarcane underscores the importance of implementing management practices to harness the soil C storage potential of sugarcane in advancing a sustainable bioeconomy in Southeastern United States.