Relationships between dissolved black carbon and dissolved organic matter in streams

Black carbon (BC) is a pyrolyzed product derived from incomplete combustion. A major fraction of BC produced by landscape fires is initially deposited onto onsite soils. Atmospheric deposition of soot is known to be an important source of soil BC, especially in watersheds that are not affected by landscape fires. The transport of the dissolved fraction of oxidized BC in soil, defined as dissolved black carbon (DBC), to streams is considered one of the important loss pathways of BC in soil, but the mechanism is not well documented. We measured the quantity and quality of DBC, determined by a benzenepolycarboxylic acid method, and the quantitative and qualitative parameters of bulk dissolved organic matter (DOM) in streams in Hokkaido, northern Japan, whose catchments were not affected by landscape fire for at least 110 years. DBC with relatively low polycondensed signatures occurred in the streams, irrespective of differences in watershed characteristics and seasons, suggesting that atmospheric deposition of soot into the catchment is probably a major source of stream DBC. The DBC concentration was linearly related to the dissolved organic carbon (DOC) concentration, irrespective of the differences in watershed characteristics and seasons. Furthermore, the polycondensation degree of DBC was observed to correlate with the qualitative parameters of bulk DOM. Such quantitative and qualitative relationships between DBC and bulk DOM imply that the transfer mechanism from soils to streams of soot-derived polycondensed DBC is linked with that of higher plant-derived, high-molecular-weight aromatic DOM.