(266d) Estimation of Soil Carbon Change from Rotation Cropping of Rapeseed with Wheat in the Hydrotreated Renewable Jet Life Cycle

Rapeseed is being considered as a potential feedstock for Hydrotreated Renewable Jet (HRJ) fuel in the U.S. due to the agronomic benefits in rotation with wheat. The goal of this research is to determine the impact of soil C changes, induced through rotation of rapeseed with wheat, and effects it would have on greenhouse gas emissions from the rapeseed HRJ fuel pathway. The Intergovernmental Panel on Climate Change (IPCC) (Tier 1) method was used to determine the annual change of soil C influenced by changes in management practices of wheat-wheat-rapeseed (WWR) rotation and the reference wheat-wheat-fallow (WWF) rotation for 20 years of cultivation in 10 U.S. states. The results showed that growing rapeseed as a rotation crop with wheat could increase C sequestration in soil through the use of effective management practices, in which rapeseed provides a greater residue input than wheat with the adoption of decreased tillage. Conversely, the addition of a lower residue input from rapeseed than wheat caused soil C loss, but essentially no worse than WWF. The annual changes of soil C for rapeseed rotations in multiple locations in U.S. states were in the range of ±0.03 - ±0.09 Mg C ha^-1 yr^-1 over 20 years. When a fallow period was replaced by rapeseed in a wheat rotation, the preliminary analysis results of greenhouse gas (GHG) emissions for rapeseed HRJ using the displacement allocation method in multiple locations had ranges from state to state from 8 to 33 g CO₂ eq/MJ, overall due to  the benefits of nutrient utilizations and management activities of a rotation crop. The GHG savings compared to conventional jet fuel were in the range of 69-91%. Based on the results using the IPCC method, rotation cropping of rapeseed with wheat through practice of no tillage with high residue input for rapeseed has the potential to increase soil C compared to fallow. However, the IPCC method had limitations for predicting changes in soil C stocks for crop rotations. Biogeochemical based models that have a potential to capture processes of C dynamic in soil are expected to be better suited to quantify regional variations on soil C changes for the rotation cropping of rapeseed with wheat.