Method for estimating soil carbon stock changes in Finnish mineral cropland and grassland soils

This study presents a method for estimating the soil organic carbon (SOC) stock changes in mineral agricultural soils developed for the Finnish GHG inventory. SOC stock changes in mineral cropland and grassland soils from 1990 to 2013 were calculated by combining agricultural statistics and national conversion factors to estimate the organic inputs to soil, along with the Yasso07 soil carbon model. The effects of selected key assumptions on the simulation results were studied. The method yielded SOC change estimates closer to the observed SOC change than were the results of the previously used Tier 1 method. The SOC stocks of croplands in 1-m soil profiles were slightly decreasing in most regions of the country. At the national level, the decrease was on average 0.05 Mg C ha(-1) year(-1) (0.01%). Selection of climate data (annual vs. long-term mean) and the initialization procedure had large impacts on the simulated SOC stock and change results, whereas the simulations at regional and subnational levels provided similar results. The method was found to be suitable for the GHG inventory and preferable to the Tier 1 method. The modular structure of the system allows for continuous improvements when more information and data are gathered.