The influence of dissolved organic matter composition on microbial degradation and carbon dioxide production in pristine subarctic rivers

Dissolved organic matter (DOM) degradation in freshwaters plays an important role in the global carbon cycle, yet there is limited understanding of how the origin and composition of DOM regulate the production of riverine greenhouse gases. We investigated the molecular composition of DOM using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and measured the potential carbon dioxide (CO2) production in pristine subarctic rivers of Finnish Lapland. During 21-day incubations, dissolved organic carbon (DOC) was effectively mineralized into CO2 in the clearwater river associated with mineral soils. The high degradability of mineral soil-derived DOM was supported by a high presence of aliphatic and peptide-like compounds. Significantly lower CO2 production per DOC was observed in the brown-water river, likely due to a large number of less biodegradable, vascular plant-derived compounds from surrounding peatlands. These findings highlight the significance of biolabile molecular compounds in the DOM degradation dynamics of subarctic catchments.