Extensification and afforestation of cultivated mineral soil for climate change mitigation in Finland

Abstract Offsetting nation-wide CO2 emissions by carbon sinks from land use change (LUC), e.g. agricultural fields extensification and afforestation, is considered as a major climate change mitigation option. We evaluated the LUC potential for reducing emissions and creating annual soil and ecosystem carbon sinks in Finland. We used agricultural statistics, the forest growth model MOTTI, the soil carbon model Yasso07, and the RCP8.5 climate scenario. The soil carbon stock (SOC) of extensified grasslands showed on average less carbon loss than cropland, thus reducing future carbon emissions by LUC between 0.17 Mg ha−1 y-1, initially, and 0.08 Mg ha−1 y-1 after 50 years. The annual rate of such carbon gain was in comparison to SOC between 1.4‰ and 0.7‰ which is lower than proposed by the Paris 4‰ initiative for offsetting global anthropogenic CO2 emissions. Furthermore, after afforestation, estimated SOC is expected to increase above pre-LUC levels with 30 years lag. Estimated SOC sink from afforestation when compared to continuous cultivation varied depending on dominant tree species and soil fertility from between 0.19 Mg ha−1 y-1 (1.7‰ for spruce in medium fertile soil) to 0.46 Mg ha−1 y-1 (3.7‰ for silver birch in highly fertile soil). Future total soil and biomass carbon sink attributed to afforestation ranged between 1.65 and 2.44 Mg ha−1 y-1. Combined carbon sinks created by the present LUC could with 30 years lag offset annually between 0.01 and 4% of the present national net CO2 emissions in Finland. The long delay and a small scale of potential future carbon emission reduction by the LUC highlighted the importance of employing additional tools to reach the national neutrality targets due in next 15 or 30 years.