Purpose

Life cycle assessment (LCA) is widely used to evaluate the carbon footprint (CFP) of milk production. Soil organic carbon (SOC) stock is a key component impacting agricultural greenhouse gas (GHG) emissions. However, a standardized methodology for integrating SOC stock changes into agricultural LCAs is still debated. This study integrated SOC balance into LCA of the CFP of milk production at farm level based on data from a research dairy farm in Helsinki.

Methods

The CFP of milk production was analyzed for 2022 and 2023. Flux measurements of CO2 by eddy covariance (EC) technique and field management data were used to calculate the SOC balance, which was further explored using the DeNitrification-DeComposition (DNDC) model and the IPCC Tier 1 method.

Results and discussion

Measurements demonstrated substantial SOC loss, 607 and 2926 kg C ha− 1 in 2022 and 2023, from the grassland and subsequent annual cropland, respectively. Incorporation of those results increased the CFP of milk production by 41% during two-year period. Based on DNDC modelling, the SOC loss exceeded the measured results in 2022 but was lower than the measured in 2023, while the IPCC Tier 1 method showed increase in SOC stock in 2022 and lower than measured SOC loss in 2023. The observed emissions fluctuation between the two years was related to the conversion of perennial grass to annual crop, with a distinctly low grass biomass production during 2022 followed by an autumn ploughing prior to the crop establishment.

Conclusion

This study underscores the importance of including SOC changes in agricultural LCAs. While on-field measurements may have practical limitations, a more profound understanding of on-farm SOC dynamics linking agricultural management effects and better calculation of SOC stock changes are crucial to minimize bias in CFP estimations.