Including a one-year grass ley increases soil organic carbon and decreases greenhouse gas emissions from cereal-dominated rotations – A Swedish farm case study

Increased soil organic carbon (SOC) content has been shown to increase soil fertility and carbon sequestration, but SOC changes are frequently neglected in life cycle assessment (LCA) studies of crop production. This study used a novel LCA application using simulated SOC changes to examine the greenhouse gas (GHG) impact of a combined food and energy crop production from a crop rotation perspective. On a case pig farm, introduction of one year of grass ley into a cereal-dominated crop rotation was simulated. The grass and pig manure were used for biogas production and the digestion residues were used as fertiliser on the farm. This crop rotation shift increased the SOC stocks by an estimated 27 and 49% after 50 years and at steady state, respectively. The estimated corresponding net wheat yield increase due to higher SOC was 8–16% and 16–32%, respectively, indicating that initial loss of low-yield oat production can be partly counterbalanced. Net SOC increase (corresponding to 2 t CO₂-eq ha⁻¹ a⁻¹) was the single most important variable affecting the GHG balance. When biogas replaced fossil fuels, GHG emissions of the combined energy-food crop rotation were approx. 3 t CO₂-eq ha⁻¹ a⁻¹ lower than for the current food crop rotation. Sensitivity analyses led to variation of only 2–9% in the GHG balance. This study indicates that integrated food and energy crop production can improve SOC content and decrease GHG emissions from cropping systems. It also demonstrates the importance of including SOC changes in crop production-related LCA studies.