The Optical, Chemical, and Molecular Dissolved Organic Matter Succession Along a Boreal Soil-Stream-River Continuum

Soils export large amounts of organic matter to rivers, and there are still major uncertainties concerning the composition and reactivity of this material and its fate within the fluvial network. Here we reconstructed the pattern of movement and processing of dissolved organic matter (DOM) along a soil-stream-river continuum under summer baseflow conditions in a boreal region of Quebec (Canada), using a combination of fluorescence spectra, size exclusion chromatography and ultrahigh resolution mass spectrometry. Our results show that there is a clear sequence of selective DOM degradation along the soil-stream-river continuum, which results in pronounced compositional shifts downstream. The soil-stream interface was a hot spot of DOM degradation, where biopolymers and low molecular weight (LMW) compounds were selectively removed. In contrast, processing in the stream channel was dominated by the degradation of humic-like aromatic DOM, likely driven by photolysis, with little further degradation of either biopolymers or LMW compounds. Overall, there was a high degree of coherence between the patterns observed in DOM chemical composition, optical properties, and molecular profiles, and none of these approaches pointed to measurable production of new DOM components, suggesting that the DOM pools removed during transit were likely mineralized to CO2. Our first order estimates suggest that rates of soil-derived DOM mineralization could potentially sustain over half of the measured CO2 emissions from this stream network, with mineralization of biopolymers and humic substances contributing roughly equally to these fluvial emissions. Plain Language Summary Our study focused on the types of organic matter from soils that end up in streams and rivers and what happens to them once they are there. Streams and rivers emit carbon dioxide to the atmosphere. Organic matter in soils plays a role in this, but we do not fully understand how soil organic matter becomes carbon dioxide. To explore this, we collected water from rivers, streams, and soils in the Canadian subarctic in Quebec and analyzed the organic matter dissolved in the water. Since organic matter is a very complex mixture of different compounds, we combined several advanced analyses to describe it and how it changed from the soils to the streams. We found large changes in organic matter between the soils and the streams, and the properties of the organic matter that disappeared were consistent with what we would expect to be broken down by microorganisms. The organic matter that made it to streams and flowed downstream was further changed, and the missing organic matter here had likely broken down by light. The organic matter broken down to carbon dioxide between soils and streams and that which broke down within streams contributed roughly equal amounts of carbon dioxide to the atmosphere.