Long-Term Experimental Acidification Drives Watershed Scale Shift in Dissolved Organic Matter Composition and Flux

Abstract

Over the last several decades dissolved organic carbon concentrations (DOC) in surface waters have increased throughout much of the northern hemisphere. Several hypotheses have been proposed regarding the drivers of this phenomenon including decreased sulfur (S) deposition working via an acidity- change mechanism. Using fluorescence spectroscopy and data from two long-term (24+ years at completion of this study) whole watershed acidification experiments, that is, the Bear Brook Watershed in Maine (BBWM) and Fernow Experimental Forest in West Virginia (FEF) allowed us to control for factors other than the acidity-change mechanism (e.g., differing vegetation, shifting climate), resulting in the first study we are aware of where the acidity change mechanism could be experimentally isolated at the whole ecosystem and decadal scales as the driver of shifts in DOM dynamics. The multidecadal record of stream chemistry at BBWM demonstrates a significantly lower DOC concentration in the treated compared to the reference watershed. Additionally, at both BBWM and FEF we found significant and sustained differences in stream fluorescence index (FI) between the treated and reference watersheds, with the reference watersheds demonstrating a stronger terrestrial DOM signature. These data, coupled with evidence of pH shifts in upper soil horizons support the hypotheses that declines in S deposition are driving changes in the solubility of soil organic matter and increased flux of terrestrial DOC to water bodies.

A century of change: Reconstructing the biogeochemical history of Hubbard Brook

“…While this represents an important caveat to any conclusions about Al toxicity during this period, the small mass and charge of Al relative to major solutes means this assumption has very limited effects on our conclusions about major ion concentration and mass balance. Despite widespread reports linking declining freshwater DOC concentrations to acidification and rising DOC concentrations in regions recovering from acid rain(de Wit et al, 2016;SanClements et al, 2018), we were unable to detect any systematic change in DOC concentrations in stream water over the 47 year record at HBEF…”

Acidification and Climate Linkages to Increased Dissolved Organic Carbon in High‐Elevation Lakes

“…Disentangling the drivers of recent increases in freshwater DOC concentrations is difficult due to the confounding signals of climate change and recovery from acidification (Strang & Aherne, ). Several studies indicate that increasing DOC concentrations across the U.S. and northern Europe are driven by recovery from acidification (Evans et al, ; Monteith et al, ; SanClements et al, ; Sanclements et al, ; Sawicka et al, ). The underlying mechanism for an inverse relationship between and DOC in freshwaters is decreasing acidity, attributed to declining , and the subsequently diminishing ionic strength of soil solutions, which promotes DOC mobilization (Ekström et al, ; Evans et al, ; Hruška et al, ; SanClements et al, ).…”

Drivers of Dissolved Organic Carbon Mobilization From Forested Headwater Catchments: A Multi Scaled Approach

“…To overcome this uniqueness of place challenge, we suggest an investigative approach that does not seek direct links across scales but that instead evaluates larger-scale pattern and site-specific process in an iterative fashion, and we use reduced acid deposition as test case of regional disturbance for one such iteration. In this context, reduction in acidification in both wet and dry deposition has been hypothesized to have caused an increase in DOC concentrations from many Northern Hemisphere forested catchments (Driscoll et al, 2003;Burns et al, 2006;De Wit et al, 2007;Monteith et al, 2007;Hruška et al, 2009;Sanclements et al, 2018). However, trends in DOC partially contradict each other, which might be due to other and/or overlapping drivers, differences in record length, variable response of unique catchments and spatiotemporal variability.…”

“…Acidification (and ensuing reduction in acid deposition) is a good candidate for iterative pattern and process investigation in the context of disturbances. Large variation in forested catchment signals including increases in stream DOC exports are attributed reductions in acid deposition, but the pattern is not consistent (Driscoll et al, 2003;Burns et al, 2006;De Wit et al, 2007;Monteith et al, 2007;Hruška et al, 2009;Sanclements et al, 2018). For example, some catchments have shown increases in DOC concentration despite continued acid deposition (Oni et al, 2013), while others showed reduced deposition without attendant increases in DOC (Löfgren and Zetterberg, 2011).…”

“…Concurrent with the reduction in acid deposition, changes to the climate system have led to increasing precipitation, which is amplified by increasing frequency of extreme hydrological events such as heavy precipitation (Jentsch et al, 2007;Campbell et al, 2009;Arnone et al, 2011;Seneviratne et al, 2012). For example, in the case of catchments in the Northeastern US, increased heavy precipitation and reduced acid deposition could lead to increased exports of DOC, as shown in a long-term, paired catchment study (Sanclements et al, 2018) where one catchment received continuous acid treatment while the other was allowed to recover from acidification. The latter catchment exported significantly more DOC, indicating a strong effect of reduced acid deposition; however, the temporal variability in stream DOC concentrations for both catchments was high due to shifts in precipitation patterns.…”