A carbon budget of forest biomass and soils in southeast Norway calculated using a widely applicable method

Growing stocks of trees in Europe have increased in a magnitude that is significant in terms of carbon (C) sink strength. Estimates of the soil C sink strength that this increased stock of trees may have induced on a regional scale are scarce, uncertain and difficult to compare. This illustrates the need for a widely applicable calculation method. Here, we calculate a C budget of productive forest in southeast Norway based on forest inventory information, biomass expansion factors (BEF), biomass turnover rates and the dynamic soil model Yasso. We estimate a 29% increase (112–145 Tg) of C in biomass between 1971 and 2000, and estimate the associated increase of C in soils (including dead wood) to be 4.5% (181–189 Tg). The C sink strengths in biomass and soils (including dead wood) in 1990 are 0.38 and 0.08 Mg ha−1 yr−1, respectively. Estimated soil C density is 58 Mg C ha−1 or ca 40% of measured soil C density in Norwegian forest soils. A sensitivity analysis – using uncertainty estimates of model inputs and parameters based on empirical data – shows that the underestimation of the soil C stock can be due to overestimation of decomposition rates of recalcitrant organic matter in the soil model and to including only trees as a source of litter. However, uncertainty in these two factors is shown to have a minimal effect on soil sink estimates. The key uncertainty in the soil sink is the initial value of the soil C stock, i.e. the assumed steady state soil C stock at the start of the time series in 1970. However, this source of uncertainty is reduced in importance for when approaching the end of the data series. This indicates that a longer time series of forest inventory data will decrease the uncertainty in the soil sink estimate due to initialisation of the soil C stock. Other, less significant, sources of uncertainty in estimates of soil stock and sink are BEF for fine roots and turnover rates of fine roots and foliage. The used method for calculation of a forest C budget can be readily applied to other regions for which similar forest resource data are available.