The contribution of fresh litter to dissolved organic carbon leached from a coniferous forest floor M. F RO ¨ BERG a *, D. B ERGGREN KLEJA a & F. HAGEDORN b a Department of Soil Sciences, Swedish University of Agricultural Science, PO Box 7014, SE-75007 Uppsala, Sweden, and b Swiss Federal Institute of Forest, Snow and Landscape Research, Zu ¨ rcher Str. 111, CH-8903 Birmensdorf, Switzerland Summary The relative contributions of litter and humified organic matter as the source of dissolved organic carbon (DOC) leached from organic layers of forest soils are poorly understood. In the present investigation, 13 C labelled spruce litter was used to study the role of recent litter in the leaching of DOC from a conifer- ous forest floor in southern Sweden, while litterbags were used to quantify the total loss of C from the labelled litter. The labelled litter applied on bare lysimeters released considerable amounts of DOC dur- ing the first weeks, but the concentration of DOC originating from labelled litter decreased gradually from 176 mg litre –1 during the first sampling period in May to 5 mg litre –1 in the last sampling period in October. Only a moderate flush of DOC from the labelled litter occurred under the Oe and Oa hori- zons, with concentrations of 20 and 6 mg litre –1 from labelled litter, equal to 19 and 9% of the total DOC flux, respectively, during the first sampling period. Total flux of DOC from labelled litter from May to September was 16 g m –2 , whereas only 2.2 and 0.9 g m –2 were captured under the Oe and Oa horizons, respectively. The almost complete loss of new DOC implies that DOC leached from the Oe and Oa horizons consists not of recent litter-derived carbon, but of DOC produced in these two hori- zons themselves. Water-extractable organic carbon from labelled litter left in litterbags in the field for 4 months consisted of about one-third native carbon from external sources at the experimental site and two-thirds of the labelled litter. In contrast, the 13 C content of the bulk litter from the litterbags was not changed by the incubation in the field. We suggest that the soluble native carbon in water extracts originated from throughfall DOC that had been assimilated by microorganisms in the litterbags. Introduction Fluxes of dissolved organic carbon (DOC) from the litter layer represent a small but potentially important loss of carbon (C) (Neff & Asner, 2001). The sorption of DOC in the mineral soil might contribute significantly to the accumulation and preser- vation of organic C (Kaiser & Guggenberger, 2000). Dissolved organic C contains numerous organic compounds, from simple sugars to complex fulvic and humic acids. It is produced princi- pally by microbial activity, root exudation, and leaching from litter and humus in the soil (Kalbitz et al., 2000; Yano et al., 2000; Kaiser et al., 2001). However, the fate of DOC in soils is controlled not only by its production, but also by its consump- tion through microbes, hydrologically driven transport and physico-chemical retention (Hagedorn et al., 2000; Kalbitz et al., 2003). The net production of DOC is thus a result of a complex interaction between abiotic and biotic factors, with the relative importance of each process not yet known. Mechanistic models of DOC dynamics require information on both the origin of DOC and the turnover of DOC in soils (Michalzik et al., 2003). In several laboratory studies (e.g. Huang & Schoenau, 1996; Magill & Aber, 2000), recent litter has been found to have a large potential to produce DOC. The greatest amounts of DOC have consistently been found under the litter horizon in many field studies, with no further increase in DOC concentrations and fluxes in the lower parts of the O layer (Qualls & Haines, 1991; Huang & Schoenau, 1998; Michalzik & Matzner, 1999). However, recent tracer studies using 13 C (Hagedorn et al., 2002, 2004) and 14 C isotope techniques (Fro¨berg et al., 2003; Michalzik et al., 2003) have identified humified organic matter as the major source of DOC in leachates from the forest floor and mineral soil. Based on laboratory experiments using soil columns and a field Correspondence: M. Fro¨berg. E-mail: froebergmj@ornl.gov *Present address: Environmental Sciences Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, Tennessee 37831– 6422, USA. Received 2 September 2004; revised version accepted 7 February 2006 European Journal of Soil Science, February 2007, 58, 108–114 doi: 10.1111/j.1365-2389.2006.00812.x 108 # 2006 The Authors Journal compilation # 2006 British Society of Soil Science