Climate Forcing of Bioenergy Feedstocks: Insights FromCarbon and Energy Flux Measurements

Bioenergy derived from biofuels can help slow the rise of atmospheric CO₂ by displacing fossil fuel consumption. Yet, cultivating bioenergy feedstocks requires substantial land area. In the United States, the recent growth of maize-based ethanol has entailed environmental trade-offs, motivating interest in alternative feedstocks. Many of these candidates have been chosen partly for characteristics linked to ecosystem services and may therefore deliver environmental gains beyond simple fossil-fuel substitution. We proposed that bioenergy cropping systems could also generate direct climatic cooling by altering carbon exchange and radiative energy fluxes (e.g., via surface albedo). To evaluate this proposition, we quantified the potential cooling influence of five current or prospective bioenergy feedstocks using multi-year eddy-covariance tower datasets. Perennial systems functioned as carbon sinks, with annual mean net ecosystem carbon balance (NECB) of −2.7 ± 2.1 Mg C ha⁻¹ for miscanthus, −0.8 ± 1.1 Mg C ha⁻¹ for switchgrass, and −1.4 ± 0.7 Mg C ha⁻¹ for prairie. By contrast, annual rotations were generally carbon sources, with annual mean NECB of 2.6 ± 2.4 Mg C ha⁻¹ for maize–soy and 3.2 ± 2.1 Mg C ha⁻¹ for sorghum–soy. Using maize–soy as the reference system, conversion to the alternative feedstocks increased albedo and produced additional cooling. This radiative effect was largest for miscanthus (−3.5 ± 2.0 W m⁻²) and smallest for sorghum (−1.4 ± 1.4 W m⁻²). When carbon- and albedo-driven impacts were compared using carbon-equivalent metrics, carbon exchange emerged as the dominant ecosystem effect, reinforcing the importance of perennial species as effective carbon sinks. Overall, these results demonstrate that feedstock selection strongly shapes ecosystem processes and should be considered an integral component of bioenergy land-conversion strategies.