Characterization of soil dissolved organic matter using optical techniques in boreal catchments

Abstract

Dissolved organic matter (DOM) is a critical medium for controlling the storage and mobilization of carbon within watersheds. With climate change threatening to alter carbon characteristics within boreal forests, understanding both the quantity and quality of DOM in boreal catchment soils, and exported to low-order headwater streams, is important in order to investigate processing and overall watershed dynamics. Headwater streams contain largely terrestrially derived DOM, and thus, gaining a better understanding into DOM within catchment soils and streams, allows for an understanding into the transport of carbon between the two systems. Utilizing optical techniques has become an increasingly common way to characterize DOM within aquatic systems. However, limited knowledge on optical properties within DOM in soil water, creates uncertainty surrounding patterns and controls on soil water DOM, and the ability to utilize optical properties to characterize soil water DOM. Furthermore, although dissolved organic carbon (DOC) concentration in streams has been linked to catchment type and landscape features, it is unclear how catchment characteristics influences DOM quality in boreal headwater streams. This study investigated DOM quality in soil water and stream water in the same watershed, using optical indices, and EEM-PARAFAC modelling. Soil water DOM was characterized by three terrestrial humic-like, one microbial humic-like, and one protein-like component. Optical properties of soil water DOM saw similarities with that of other boreal aquatic systems, indicating the potential for optical techniques to be used to characterize DOM quality in catchment soils. PARAFAC modelling and optical indices characterized organic soils as more aromatic, more humified, and with a higher molecular weight relative to mineral soils, as well as implied a high degree of processing within the soil environment. Stream water DOM within the same watershed was characterized by four terrestrial humic-like PARAFAC components, and differences in optical properties between soil water and stream water, indicated the selective loss of low molecular weight aliphatic compounds along the soil-stream continuum. Stream water DOM from forest draining streams was seen to be more microbially derived, with fresher carbon and a lower molecular weight, relative to wetland draining streams. Aromatic content seen through SUVA254 values, were seen to be a poor predictor of wetland coverage in this study, and suggested that additional factors such as wetland vegetation type, might need to be considered when predicting quality indicators based on wetland coverage. Intensive discharge events following dry periods were also seen to influence DOM in both forested and wetland draining streams. Therefore, optical techniques could prove to be an effective tool in order to characterize DOM quality in boreal watersheds, increasing understanding into the quality of DOM that both remains in catchment soils, as well as that which is transported to the stream. This has implications for better understanding carbon fate within watersheds, and the potential for optical techniques to be utilized, in order to investigate changes in carbon storage and transportation across boreal catchments.NSERC; Research ManitobaMaster of Science in Environmental and Social Chang