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In the near future, climate change will increasingly affect export of carbon (C) from peatlands. Terminal mineralization of dissolved organic carbon (DOC) is a source of greenhouse gases (carbon dioxide, CO2, and methane, CH4) for the atmosphere, and may contribute to climatic warming. Quantitative understanding of sources, dispersion pathways, and degradation of DOC produced in peat bogs is important for predictions of global change dynamics. Here we explore the potential of combining C isotope ratios of isotopically stratified, Pb-210-dated peat deposits and C isotope ratios of DOC exported via runoff to determine the depth and age of the predominant DOC source. In a poorly drained, Sphagnum-derived peat bog in the Czech Republic, delta C-13 values of bulk peat increased downcore to a depth of 10 cm. The C isotope signatures of fresh DOC and bulk peat differ only insignificantly, because C isotope changes in maturating peat require multiple microbial modifications of the organic matter taking place over relatively long time periods. Based on 41 samplings of runoff, 70% of exported DOC originated from peat substrate 1 to 10 years old, 2-10 cm deep. Recently photosynthesized DOC (< 1 year old) contributed about 5% to runoff DOC, whereas approximately 25% of exported DOC was derived from peat layers older than 10 years. Because biogenic methane at the study site contained isotopically extremely light C (delta C-13 values of -64.2 to -53.2 parts per thousand), it would seem reasonable to expect residual DOC following partial decomposition to become isotopically heavier. A short-term laboratory incubation of wet peat samples from four depths at mid-summer temperatures indicated more complicated delta C-13 systematics. The initial limited amount of DOC in the leachate was isotopically heavy, compared to bulk peat, but after 18 days delta C-13(DOC) became lower, converging to the C isotope signature of bulk peat. Such delta C-13 systematics can be explained by multi-step isotope effects associated with different reaction rates during substrate decomposition. Limitations of using the combination of within-bog delta C-13 systematics and Pb-210 dating of recent peat accretion are discussed. Horizontal heterogeneity in substrate C isotope signatures is one of the main sources of uncertainty of this approach to DOC source apportionment in peaty soils.