Stocks and annual fluxes of organic carbon in the mineral soil cover of Estonia

Annual cycling of soil organic carbon (SOC) is the main driving force in the formation and functioning of soil cover. Therefore knowledge about it forms the scientific base for sustainable management and ecologically sound soil protection. Systematized parameters of the mean annual cycling of SOC by soils are analysed on the basis of the SOC stock densities (Mg ha -1 ) of 16 mineral soil groups. The SOC stocks according to soil groups for the soil cover (solum) as a whole and for their epipedon were calculated on the basis of mean SOC densities and their distribution area of soil types. In the Estonian mineral soil cover (32 351 km 2 ) a total of 323 ± 46 Tg (10 12 g) SOC is retained; 42% of this is sequestered into stabilized humus, 40% into unstable raw humus, and 18% into forest (grassland) floor and shallow peat layers. Of the total SOC stock, 75% is situated in biologically active epipedons and 25% in subsoil. The annual SOC inputs and outputs in natural soils, which were calculated on the basis of annual productivity, ranged from 0.2 to 3.6 Mg ha -1 yr -1 . The influence of land management peculiarities on the annual cycling and balance of SOC has been demonstrated by our own experimental research, as well as by data published in the literature. In this work the pedo-ecological causal regularities of SOC sequestration in mineral soil cover (SOC concentration, soil thickness, moisture regime, texture, carbonate content), and agro-technological possibilities for its regulation (crops and their rotation, level of subsidization and soil amelioration) are discussed.