Characterizing soil dissolved organic matter in boreal catchments using optical properties

Carbon fate in boreal regions, largely depends on the chemical composition of organic matter, with dissolved organic matter (DOM) acting as an essential vector by which carbon is transported from watersheds to streams. While optical properties of DOM have been extensively utilized in aquatic systems in order to characterize compositional features of DOM, there have been limited studies on optical properties of soil water DOM, limiting our ability to link watershed patterns and processes. This study investigated soil water DOM chemical composition in boreal forested catchments using absorbance and fluorescence spectroscopy and compared DOM optical properties amongst soil types. Optical indices and a five component fluorescence parallel factor analysis (PARAFAC) model, characterized organic soils as more aromatic, more humified, and with a higher molecular weight, relative to mineral soils. Furthermore, PARAFAC modelling, as well as fluorescence index (FI) and specific UV absorbance at 254nm (SUVA 254 ) values, indicated a large portion of the DOM was degraded. Soil water DOM PARAFAC components indicated consistencies with boreal aquatic models, suggesting the potential for linking surface water data to catchment soils. Dissolved organic carbon (DOC) concentration was found to be predictable from soil organic matter content, however, DOM optical indices were not well correlated with soil properties. Optical characterization of soil water DOM chemical composition is a promising technique for improving our understanding of transport and processing of carbon in boreal forested watersheds.