Analysis on DOC transformation in a forested catchment using stable carbon isotope values

In order to elucidate the production, consumption and transformation mechanisms of dissolved organic carbon (DOC) in temperate forested catchment , time and spatial variations in several parameters which characterize the DOC quality were surveyed in various hydrological processes in a headwater catchment in central Japan. Rain, through fall, soil waters in various depths, groundwater and streamwater were sampled since June 2006 to November 2008. Concentration, δ13C, fluorescence spectrum and UV absorbance of DOC were measured. We measured the δ13C-DOC values using an IRMS with a CO2 purifying system connected to a TOC analyzer. Combined discussions on the profiles of concentration, δ13C and fluorescence characteristics provided following findings: 1) Microbial decomposition of DOC was commonly predominant for consumption in relatively shallower soil horizons (0 to 30 cm in depth) regardless of the soil moisture condition among the soil profiles at the different parts in hillslope, while adsorption was significant in the relatively dry soil profile. 2) During this process, persistent portions remained preferentially in the soil solution. 3) In the groundwater body, two different processes caused; protein-like dissolved organic carbon was added in the relatively oxygen rich part and the δ13C value decreased with the anoxic DOC decomposition in the oxygen poor part, suggesting the methanogenic activity related DOC production (Figure 1). Stream DOC characteristics were determined by relative contributions of subsurface water and groundwater. Then, it received fresh DOC with high fulvic fluorescence peak from bank side or riparian zones again. These indicated that the stream DOC characteristics were influenced not only by in stream and streamside organic supply, but also terrestrial DOC formation through the groundwater discharge. Figure 1. Relationship between UV/DOC and δ13C-DOC of soil water at three different soil profiles. G1, G34 and G27 were located in the lower, middle and upper part of hillslope in a headwater catchment.