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This study aimed to gain insight into the generation and fate of dissolved organic carbon (DOC) in organic layers. In a Free Air CO2 Enrichment Experiment at the alpine treeline, we estimated the contribution of C-13-depleted recent plant C to DOC of mor-type organic layers. In an additional laboratory soil column study with 40 leaching cycles, we traced the fate of C-13-labelled litter-DOC (22 and 45 mg l(-1)) in intact Oa horizons at 2 and 15 degrees C. Results of the field study showed that DOC in the Oa horizon at 5 cm depth contained only 20 +/- 3% of less than six-year-old C, indicating minor contributions of throughfall, root exudates, and fresh litter to leached DOC. In the soil column experiment, there was a sustained DOC leaching from native soil organic matter. Less than 10% of totally added litter-DOC was leached despite a rapid breakthrough of a bromide tracer (50 +/- 7% within two days). Biodegradation contributed only partly to the DOC removal with 18-30% of added litter-DOC being mineralized in the Oa horizons at 2 and 15 degrees C, respectively. This was substantially less than the potential 70%-biodegradability of the litter-DOC itself, which indicates a stabilization of litter-DOC in the Oa horizon. In summary, our results give evidence on an apparent 'exchange' of DOC in thick organic layers with litter-DOC being retained and 'replaced' by 'older' DOC leached from the large pool of indigenous soil organic matter. (C) 2009 Elsevier Ltd. All rights reserved.
