Land Use Controls on the Spatial Variability of Dissolved Black Carbon in a Subtropical Watershed

Rivers export roughly 250 Pg of dissolved organic carbon (DOC) to coastal oceans. DOC exported from rivers can be a reflection of watershed dynamics, and changes in land use can lead to shifts in the molecular composition and reactivity of riverine DOC. About 10% of DOC exported from rivers is dissolved black carbon (DBC), a collection of polycondensed aromatic compounds derived from the incomplete combustion of biomass and fossil fuels. While DOC and DBC export are generally coupled, the effects of watershed land use on DBC quality are not well understood. In this study, DBC samples were collected throughout the Altamaha River watershed in Georgia, USA. DBC was characterized using the benzenepoly(carboxylic acid) method and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). DBC had a more polycondensed character in areas of the watershed with less anthropogenic disturbance. Furthermore, FTICR-MS revealed that DBC became enriched with a lower molecular weight, heteroatomic signature in response to higher anthropogenic activity. As global land cover continues to change, this study demonstrates on a localized scale that watershed land use can influence the export and composition of DBC, which may have further implications for global carbon and nutrient cycling.