Influence of soil frost on the character and degradability of dissolved organic carbon in boreal forest soils

Recent studies suggest that increases in extent and duration of winter soil frost increases dissolved organic carbon (DOC) concentrations in boreal riparian soils and connected aquatic systems during the subsequent spring and summer. However, little is known about the impact of frost on DOC character and its degradability. We applied three experimental treatments to riparian soils in northern Swedenshallow soil frost (insulated), deep soil frost (snow removed) and control plotsto test the effect of different soil frost regimes on the chemical characteristics and degradability of soil DOC. Soil pore water samples were analyzed using excitation-emission fluorescence (parallel factor analysis) combined with biological and photochemical degradation experiments. We found that the absolute bacterial metabolic rates were significantly lower in samples from the shallow soil frost treatments, compared with the other treatments. Explorative multivariate analyses indicate that increasing soil frost is contributing to increased protein-like fluorescence and to increased biological degradability of the DOC. Our study shows that decreases in riparian soil frost due to climate warming may not only contribute to decreased riparian DOC concentrations but also lead to shifts in the DOC composition, resulting in decreased biodegradability (yet similar photodegradability) of the DOC that is exported from riparian soils to streams.