Soil carbon stocks and water stable aggregates under annual and perennial biofuel crops in central Ohio

Biofuels have the potential to mitigate global warming by partially replacing fossil fuels. Long-term cultivation of bioenergy crops may affect soil quality and C exchange between the soil-plant-atmosphere continuum depending upon crop type and duration of cultivation. Thus, studies are needed to compare the impact different bioenergy crops have on soil organic carbon (SOC) stocks to estimate the net C mitigation potential of these crops. This study reports on changes in soil properties and C stocks after eight years of annual no-till (NT) corn (Zea mays L.), sorghum (Sorghum bicolor L.), perennial switchgrass (Panicum virgatum L.), miscanthus (Miscanthus x giganteus), and polyculture prairie cultivation in central Ohio. Perennial biofuel crops (PBCs) improved soil health by reducing soil bulk density, increasing soil porosity, and improving soil aggregate stability compared to those under annual bioenergy crops (ABCs). Changes were most prominent in the surface 0-10 cm soil layer. Bulk density decreased by 14.4% and 12.8% under PBCs and 6.0% and 5.7% under ABCs in the 0-10 cm and 10-20 cm depths, respectively. Water stable aggregate values in the PBC systems were 29.6% (0-10 cm) and 23.0% (10-20 cm) greater than those in the ABC systems with maximum and the minimum values observed under miscanthus (95.1%) and corn (70.2%). PBC systems retained 71% of overall aggregates within the 0.25 mm size fraction, compared to ABCs retaining 20% and 28% of total aggregates within these two aggregate size fractions, respectively. C and N accumulation were greater in macroaggregates ( 0.25 mm) compared to microaggregates (<0.25 mm). The SOC stock within the first eight years of the study decreased by 8.25 Mg C ha-1 (annual rate of -1.03 Mg C ha-1 y-1) for the ABC systems. In contrast, the SOC stock under PBCs for the first eight years increased by 7.94 Mg C ha-1 at an annual rate of 0.99 Mg C ha-1 y-1. These data indicate that PBCs enhanced SOC stocks and provided the dual benefit of increasing SOC stock and improving soil properties. The rate and cumulative amount of SOC sequestration were the highest under miscanthus and the least under sorghum. However, more information is needed on the C life-cycle of biofuel crops and profile C changes to evaluate the net C mitigation potential.