Soil C balances in Swedish agricultural soils 1990-2004, with preliminary projections

Swedish agricultural land comprises about 3 Mha and its topsoil contains about 270 Mt C (0-25 cm depth). Based on daily climate data, annual yield data and a soil database, we calculate the topsoil C dynamics for Swedish agricultural land 1990-2004, using a soil C balance model, ICBM. Losses from high C (organic) soils are calculated from subsidence, which in turn is calculated from soil properties, cropping system and weather conditions. We also present scenarios and projections into the future. Mineral soils are close to balance in all of the eight agricultural regions investigated. Average soil C mass roughly increases from South to North, since the lower yields and thus C inputs in Northern regions are more than balanced by the higher decomposition rates due to warmer climate in the South. The higher proportion of grass leys in the North also contributes to higher C mass. High C soils (> 7% C, corresponding to 12% soil organic matter content) lose 2-6 t C ha(-1) year(-1), depending on weather and cropping system, and total annual loss from Swedish agricultural high-C soils is about 1 Mt year(-1). This loss is discussed in the context of plant production and remedial actions. Projections into the future, assuming that a temperature increase leading to increased decomposition rates also will lead to higher yields, indicate a potential to at least maintain soil C mass in Swedish agricultural mineral soils. Growing crops with residues more resistant towards decomposition would be an efficient way to increase soil C mass. See also http://www-mv.slu.se/vaxtnaring/olle.