THE ROLE SOIL AGGREGATES PLAY IN THE GENERATION OF DISSOLVED ORGANIC CARBON: A CASE STUDY AT SLEEPERS RIVER RESEARCH WATERSHED

DOC is extensively monitored across the globe because it is typically quite labile and easily accessed by microorganisms that can transform organic molecules into the greenhouse gas CO₂. Over the past two decades, headwater streams of the northern hemisphere have shown increased amounts of DOC coinciding with decreased acidic wet deposition (Skjelkvåle et al., 2007). The issue is widely debated in the literature but a mechanistic link has not yet been confirmed. We hypothesize that soil aggregates are the main source of stream water DOC and performed a combined lab and field investigation on how solution chemistry impacts aggregate destabilizing mechanisms: dispersion due to decreased ionic strength (IS), dispersion due to a lack of cation bridging, and a reduction in competition for sorption sites as less sulfate input decreases the amount of DOC released into solution. Soils were collected at the USGS Sleepers River Research Watershed, Vermont, from riparian and hillslope zones and were extracted with laboratory mixed solutions of varying IS, composition, and pH. In low IS solutions, DOC concentration was significantly higher and particle size was significantly smaller compared to high IS extracts, suggesting aggregate breakup and DOC release. Furthermore, higher DOC concentrations were found in Na⁺ compared to Ca²⁺ dominated solutions, indicating some impact of cation bridging in limiting the amount DOC release. Competitive sorption between sulfate and DOC was less important. The substantial effect of IS on soils suggests a strong control of diffuse double layer dynamics, possibly driven by the presence of clays and oxides in the case of Sleepers River. Moreover, these results propose that DOC release might indeed depend on the main aggregate stabilizing mechanisms, which may vary by location.

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Skjelkvåle, B.L., Monteith, D.T., Stoddard, J.L., Evans, C.D., de Wit, H.A., Forsius, M., Høgåsen, T., Jefferies, D.S., Kopácek, J., Vesely, J., Vuorenmaa, J., Wilander, A., 2007, Increases in DOC in remote lakes and rivers: A signal of climate change or return to pre-acidification conditions?: ICP Waters Report 87, p. 39-49.