Modeling approaches to incorporate soil C changes in life cycle assessment of dairy farm rotations with varying duration and proportion of grassland

Abstract Purpose Despite its importance for climate change mitigation, soil organic carbon (SOC) is still insufficiently accounted for in cattle life cycle assessment (LCA) studies. Thus, this study evaluated how different methods for estimating SOC changes affect LCA results for mixed dairy systems with varying grassland proportions and durations. Methods Two SOC modeling approaches (Petersen et al. (2013) and rCTOOL) were applied, along with two methods for estimating C inputs to soil: allometric functions and fixed belowground biomass coefficients. The analysis covered three Danish dairy farm types ( conventional low stocking , conventional high stocking , and organic ) and three grassland scenarios: current practice ( Now ), extended grass area and duration ( More ), and current grass proportion with longer duration ( Equal ). Results and discussion Despite variation in the SOC changes estimates per hectare of farmland, the two SOC modeling approaches exhibited similar overall patterns. Using fixed belowground biomass coefficients resulted in higher SOC changes estimates than allometric functions, especially in organic systems, which had a higher proportion of grassland. Across all farms, scenarios, and modeling approaches, SOC changes contributed only modestly to climate impacts per kg energy-corrected milk (ECM) over a 100-year time horizon, largely due to the choice of initial SOC level. For conventional farms, More scenarios resulted in the lowest climate impacts per kg ECM, driven by positive effects on SOC changes and direct land use changes. For organic farms, the most favorable outcome was provided by the Equal scenario, which supported both farm productivity and SOC sequestration. Conclusions Overall, the study underscores the need for improved methods for the estimation of C inputs when modeling SOC dynamics in LCA. The climate change implications of grassland management scenarios are particularly relevant for mixed dairy farms with rotational grasslands.