Production of dissolved organic carbon in forest soils along the north–south European transect

The aim of this study is to estimate the C loss from forest soils due to the production of dissolved organic C (DOC) along a north–south European transect. Dissolved organic matter (DOM) was extracted from the forest soils incubated at a controlled temperature and water content. Soils were sampled from forest plots from Sweden to Italy. The plots represent monocultures of spruce, pine and beech and three selected chronosequences of spruce and beech spanning a range of mean annual temperature from 2 to 14 °C. The DOM was characterized by its DOC/DON ratio and the C isotope composition δ¹³C. The DOC/DON ratio of DOM varied from 25 to 15 after 16 days of incubation and it decreased to between 16 and 10 after 126 days. At the beginning of incubation the δ¹³C values of DOC were 1‰ or 2‰ less negative than incubated soils. At the end of the experiment δ¹³C of DOC were the same as soil values. In addition to DOC production heterotrophic respiration and N mineralization were measured on the incubated soils. The DON production rates decreased from 30 to 5 μgN gC⁻¹ d⁻¹ after 16 days of incubation to constant values from 5 to 2 μgN gC⁻¹ d⁻¹ after 126 days at the end of experiment. The DIN production rates were nearly constant during the experiments with values ranging from 20 to 4 μgN gC⁻¹ d⁻¹. DOC production followed first-order reaction kinetics and heterotrophic respiration followed zero-order reaction kinetics. Kinetic analysis of the experimental data yielded mean annual DOC and respiration productions with respect to sites. Mean annual estimates of DOC flux varied from 3 to 29 g of C m⁻² (1–19 mg C g⁻¹ of available C), corresponding to mean DOC concentrations from 2 to 85 mg C L⁻¹.