Estimation of soil carbon change from rotation cropping of rapeseed with wheat in the hydrotreated renewable jet life cycle

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© 2015, Springer-Verlag Berlin Heidelberg. Purpose: Rapeseed is being considered as a potential feedstock for hydrotreated renewable jet (HRJ) fuel in the USA through its cultivation in rotation with wheat. The goal of this research was to determine the impact of soil C changes, induced through replacing the fallow period with rapeseed in rotation with wheat, and the effects it would have on emission of greenhouse gases (GHG) of rapeseed HRJ. Methods: The Intergovernmental Panel on Climate Change (IPCC) (Tier 1) method was used with modifications to determine the changes in soil C of wheat–wheat–rapeseed (WWR) relative to the reference wheat–wheat–fallow (WWF) rotation for 20 years of cultivation. The 27 case scenarios were conducted to study the impacts of changes in management practices (tillage practice and residue input) on changes in soil C for WWR rotation in multiple locations in 10 US states. The CO2 emissions resulting from soil C changes were incorporated into the rapeseed HRJ pathway in order to evaluate the GHG emissions. Results and discussion: Introducing rapeseed to replace the fallow period with wheat could either increase or decrease changes in soil C, depending on management practices. Soil C is predicted to increase with increased residue input and reduced tillage. The greatest gain of soil C was found when using high residue input for wheat and rapeseed under no tillage, resulting in the best management practice. Conversely, adding low residue input to both crops with full tillage created the highest loss of soil C, referring to as the worst management practice. Soil C changes varied across locations from −0.22 to 0.32 Mg C ha−1 year−1. Consequently, the GHG emissions of rapeseed HRJ ranged from 4 to 70 g CO2 eq./MJ, comparing to 46 g CO2 eq./MJ for excluding soil C change. The rapeseed HRJ exhibited the GHG savings of 65–96 % for the best practice and 20–42 % for the worst practice when compared to petroleum jet fuel. Conclusions and recommendations: Based on results using the modified IPCC method, adoption of high residue input with no tillage for the rotation cropping of rapeseed with wheat had the potential to increase soil C. However, the method has limitations for predicting soil C changes regarding crop management practices. Biogeochemical-based models that have a potential to capture processes of C and N dynamics in soil and yield may be better suited to quantify regional variations in soil C changes for the rotation cropping of rapeseed with wheat.

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International Journal of Life Cycle Assessment