Carbon sequestration rates in Swedish forest soils -: a comparison of three approaches

Carbon sequestration rates in forest soil can be estimated using the concept of calculable stable remains in decomposing litter. In a case study of Swedish forest land we estimated C-sequestration rates for the two dominant tree species in the forest floor on top of the mineral soil. Carbon sequestration rates were upscaled to the forested land of Sweden with 23 x 10(6) ha with Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (Karst.) L.). Two different theoretical approaches, based on limit-value for litter decomposition and N-balance for vegetation and SOM gave rates of the same magnitude. For the upscaling, using these methods, 17 000 grids of 5 x 5 km were used. The 'limit-value approach' gave a sequestration of 4.8 x 10(6) tons of C, annually sequestered in the forest floor, with an average of 180 kg C ha(-1) yr(-1) and a range from 40 to 410 kg C ha(-1) yr(-1). The 'N-balance approach' gave an average value of c. 96 kg ha(-1) yr(-1) and a range from -60 to 360 kg ha(-1) yr(-1). A method based on direct measurements of changes in humus depth over 40 years, combined with C analyses gave an average rate that was not very different from the calculated rates, viz. c. 180 kg ha(-1) yr(-1) and a range from -20 to 730 kg ha(-1) yr(-1). These values agree with forest floor C sequestration rate based on e.g. sampling of chronsequences but differ from CO2 balance measurements. The three approaches showed different patterns over the country and regions with high and low carbon sequestration rates that were not always directly related to climate.