Effective retention of litter-derived dissolved organic carbon in organic layers

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 13C-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 13C-labelled litter–DOC (22 and 45 mg l−1) in intact Oa horizons at 2 and 15 °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 °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.