Molecular Chemodiversity of Dissolved Organic Matter in Runoff from Central Canadian Boreal Forest Streams

Abstract:

The Canadian boreal forest accounts for 8% of the world’s forests and covers more than 50% of the province of Ontario. These essential forests have a significant global role in the absorption of atmospheric CO₂ and the sequestration of carbon in trees, foliage, and soils. Yet, detailed information into the carbon biogeochemistry and molecular diversity of dissolved carbon draining from Canadian boreal watersheds remains sparse. In this study, we use Fourier Transform Ion Cyclotron Mass Spectrometry (FT-ICR-MS) and absorbance spectroscopy metrics to examine the chemodiversity of dissolved organic matter (DOM). Thirteen first- and second-order boreal forest streams with varying physiographic characteristics in northwestern Ontario, Canada were sampled across late spring and fall seasons. Our results show greater DOM chemodiversity during the spring, with higher abundances of microbially processed N-containing aliphatics and oxidized lignins across sandy plain regions and exposed bedrock/shallow soil watersheds, respectively. The effect of landscape type on DOM composition is less pronounced during the fall, as reduced molecular chemodiversity is linked to greater abundances of low-oxidized lignins, ubiquitous across watersheds due to increased precipitation and hydrological connectivity within watersheds. Molecular level correlations with landscape properties, hydrological metrics and optical properties were observed among low-oxidized lignins, tree coverage and the length of runoff pathways within catchments, highlighting the importance of topmost soil layers and leaf leachates on stream DOM composition. This study underscores how landscape properties have a significant role in enhancing DOM chemodiversity during periods of limited hydrological connectivity, but that DOM chemodiversity in this region is reduced during wetter periods.