In this study, we examined changes in isotopic (¹³C and ¹⁴C) and spectroscopic (UV and ¹³C NMR) properties of dissolved organic carbon (DOC) in relation to soil organic matter (SOM) to elucidate the sources and sinks of DOC as water percolates through the soils of two contrasting upland coastal California ecosystems—a redwood forest and a coastal prairie. Despite differences in the distribution of C stocks and litter chemistry at these two sites, we found similar shifts in DOC chemistry with soil depth. DOC concentrations dropped rapidly with increasing depth, with an accompanying decrease in the C:N ratio, an increase in the δ¹³C value and an decrease in specific UV adsorption. In the grassland soil, Δ¹⁴C values declined from current atmospheric values (+70‰) in the surface horizon to −75‰ at 100 cm. In the redwood soil, the Δ¹⁴C value of 111‰ in O horizon leachates was indicative of OM with a residence time of 8–10 yrs, with a decrease in Δ¹⁴C values to −80‰ at 100 cm. Solid-state CP/MAS ¹³C NMR spectra were generally most similar to highly humified OM, with a general decrease in the relative abundance of aromatic compounds and an increase in the alkyl C/O-alkyl C ratio with increasing depth. All of these trends are consistent with the shifts in SOM properties with increasing depth, which are interpreted to mean a shift from fresh plant material to older, highly altered OM. In this Mediterranean climate, we found distinct seasonal shifts in the quantity and composition of DOC found in soil solution during the winter rainy period that was also consistent with a shift from recent labile substrates to older, highly altered OM. These results fit in with a growing body of literature suggesting that the source of much of the DOC within mineral soils is the local soil OM, and the ¹⁴C data, in particular, indicate that DOC at depth is not simply the fraction of surficial leachates that have not been adsorbed or decomposed. Rather, exchange reactions with a portion of the more stabilized SOM pool exert the strongest control on both the concentration and composition of DOC found in these soils.