Majdi, Hooshang (Swedish University of Agricultural Sciences, P. O. Box 7072, Uppsala, SE-75007, SWEDEN; Phone: 004618672428; Fax: 004618673430; Email: Hooshang.Majdi@eom.slu.se)

Carbon Pools and Dynamics in Mineral Soil Horizons of Boreal Forests Along a Climatic Gradient in Sweden

H. Majdi *, D. Berggren, M. Olsson, G. Agren

The carbon stocks as a function of carbon fluxes in mineral soil horizons (A, E and B) of three Spodosols were studied along a S-N climatic gradient in Norway spruce stands in Sweden (Asa, Knottåsen and Flakaliden) during 2001-2003. Fine root litter production and dissolved organic carbon (DOC) were measured to investigate carbon fluxes. Minirhizotrons were used to estimate fine root longevity (yr) and soil cores to estimate standing biomass. Annual fine root litter production (g m ^-2yr ^-1) was calculated by dividing fine root standing biomass by fine root turnover (the inverse of root longevity). DOC concentrations were measured weekly in bulk deposition, throughfall and soil solutions in polyethylene funnels. Annual DOC fluxes (g m ^-2 ; bulk deposition and throughfall) was calculated by multiplying measured DOC concentrations with measured water volumes and DOC for soil solutions by multiplying measured DOC concentrations with simulated water fluxes using the COUP model. The steady state carbon stocks were assessed with the Q model based on the C influx data and a continuous description of soil fine-root carbon and DOC qualities during the decomposition process Annual fine root litter production in mineral soil horizons (0-40 cm) was 65, 50 and 47 g m^-2in the Asa, Knottåsen and Flakaliden stands, respectively. The corresponding values for DOC retention in the mineral soil was 24, 9.4 and 11 g m ^-2. The steady state stocks predicted by Q model were reasonable consistent with measured data, thus explaining larger carbon stocks in a warmer climate as a function of more root litter and DOC. It is concluded that root litter production and DOC retention are contributing about equally to the C build-up in the mineral soil, when accounting for the higher recalcitrance of DOC compared to root litter.