Accumulation of carbon and nitrogen in Swedish forest soils over stand age

Abstract: Forest disturbance and harvesting regimes may affect the carbon (C)amounts stored in forest soils. The amount of soil C decreases after harvest and recovers gradually. Earlier chronosequence studies have suggested 50% decrease of the forest floor within the first 15 years after harvest. Increased decomposition rates and decreased litter input are considered to be the main factors that cause reduction of C pools in the soils. The study utilized data from the Swedish National Forest Inventory and the National Forest Soil Inventory from all forest land in Sweden. The aim was to estimate the rate of gains and losses of soil organic C and nitrogen (N) over the forest stand age and to compare the results with data from literature. In the analysis climate, tree species composition, site index and soil moisture was considered to be the major variables that affect litter input and the decomposition rate. I separated the data in two categories for each variable, namely; south-north, spruce-pine, high-low and moist-dry for each variable respectively. Comparisons of the two categories of each variable was made and the amounts of soil organic C in the O horizon of northern sites, pine dominant forests, low SI and dry soils were 44%, 24%, 30 % and 10% lower compared to southern, spruce, high SI and moist soils respectively. The same comparison showed that the N amount were 64%, 44%, 60% and 12% lower for the respective groups. Third order polynomial regressions were used to evaluate the effects of the variables by category. All categories followed the same time trend with decreasing amounts of C and N in the O horizon until a stand age between 29 to 50 years. However, the magnitude of the change varied between 9 to 30% of the initial carbon amount. Thereafter the amount increased until the stand reached 100 years of age. Climate and tree composition were found to be key factors for determining the variation in soil organic C and N over stand age in Swedish forest soils.