Combining long-term observations with experiments to test hypotheses on stream water dissolved organic carbon dynamics at the Sleepers River Research Watershed

Many headwater streams of the northern hemisphere have shown increased amounts of dissolved organic carbon (DOC), coinciding with decreased acid deposition; however, a mechanistic link between precipitation composition and stream water DOC has not yet been proposed. We hypothesized that soil aggregate dispersion is the mechanistic link between reduced acid deposition and DOC increases in soil solution and streams because i) surface water DOC is largely sourced from organic soils, ii) soil carbon is typically stabilized in aggregates, and iii) aggregate stability is partially controlled by solution chemistry. Furthermore, we hypothesized that variations in soil composition by landscape position exacerbate this effect and that riparian soils, presumably containing highest amounts of carbon, release the most DOC. To test these hypotheses, we combined observations on long-term stream water fluxes (DOC and base cations since 1991) with experiments on soils collected from two main landscape positions (hillslope and riparian zones) from the acid-impacted Sleepers River Research Watershed in northeastern Vermont. From these soils we extracted DOC using solutions of different ionic strength (IS) and composition (simulation acidification and recovery) and monitored dynamic changes in soil particle size, aggregate architecture and composition and leachate DOC concentrations. In simulated recovery solutions (low IS), extractable DOC concentrations were significantly higher, particle size (by laser diffraction) was significantly smaller and organic and mineral particles were seen separated in scanning electron microscope observations, suggesting that DOC is indeed released from aggregates. In contrast, high IS solution, especially high Ca content, limited DOC release through this process, which is significant for riparian soil at Sleepers Rivers that receive Ca from upwelling ground water. Increasing trends in stream water Ca fluxes over time (and decreases of riparian zone Ca since 1996) are in agreement with a slow release of Ca and DOC from riparian soils into streams. The next step is to investigate more sites with different soil composition to test these process-based hypotheses using long-term data.