Abstract
Over the past two decades, headwater streams of the northern hemisphere have shown increased amounts of dissolved organic carbon (DOC), coinciding with decreased acid deposition. The exact nature of the mechanistic link between precipitation composition and stream water DOC is still widely debated in the literature. We hypothesize that soil aggregates are the main source of stream water DOC and that DOC release is greater in organic rich, riparian soils versus hillslope soils. To test these hypotheses, we collected soils from two main landscape positions (hillslope and riparian zones) from the acid-impacted Sleepers River Research Watershed in northeastern Vermont. We performed aqueous soil extracts with solutions of different ionic strength (IS) and composition to simulate changes in soil solution. We monitored dynamic changes in soil particle size, aggregate architecture and composition, leachate DOC concentrations, dissolved organic matter (DOM) characteristics with fluorescence spectroscopy and trends in bioavailability. In low IS solutions, extractable DOC concentrations were significantly higher, particle size (by laser diffraction) was significantly smaller and organic material was separated from mineral particles in scanning electron microscope observations. Furthermore, higher DOC concentrations were found in Na+ compared to Ca2+ solutions of the same IS. These effects are attributed to aggregate dispersion due to expanding diffuse double layers in decreased IS solutions and to decreased bridging by divalent cations. Landscape position impacted quality but not quantity of released DOC. Overall, these results indicate that soil aggregates might be one important link between Critical Zone inputs (i.e. precipitation) and exports in streams.
Highlights
Over the last few decades increased levels of dissolved organic carbon (DOC) have been observed in forested streams across the northern hemisphere (Monteith et al, 2007; Porcal et al, 2009) and several processes have been proposed to explain observed patterns
Aggregate stability decreases in such environments due to expanding of diffuse double layers and a shift from divalent to monovalent cations reduces the propensity for cation bridging (Tisdall and Oades, 1982; Six et al, 2004)
Because surface water DOC is largely sourced from organic soils (Yallop and Clutterbuck, 2009) and because soil C is typically stabilized in aggregates (Six et al, 2000) we hypothesized that the breakup of soil aggregates plays an important role in increasing DOC during recovery from acidification
Summary
Over the last few decades increased levels of dissolved organic carbon (DOC) have been observed in forested streams across the northern hemisphere (Monteith et al, 2007; Porcal et al, 2009) and several processes have been proposed to explain observed patterns. Recovery from acidification is put forth as a general driver for observed changes in stream water DOC (Evans and Monteith, 2001; De Wit et al, 2007; Monteith et al, 2007; Hruška et al, 2009; Kang et al, 2018) The latter is consistent with the observation of a significant reduction in acid anion concentrations (i.e., sulfate) in many catchments that show increases in DOC (Monteith et al, 2007). Only a few studies have focused on the precise mechanisms of DOC release (Monteith et al, 2007; Haaland et al, 2010), making it difficult to investigate the link between Critical Zone input (i.e., precipitation) and stream water export
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