The Genesis and Exodus of Vascular Plant DOM from an Oak Woodland Landscape

Abstract

Evaluating the collective impact of small source inputs to larger rivers is a constant challenge in riverine biogeochemistry. In this study, we investigated the generation of dissolved organic matter (DOM) in a small oak woodland catchment in the foothills of northern California, the subsequent transformation in lignin biomarkers and chromophoric DOM (CDOM) parameters during transport through the landscape to an exporting stream, and finally the overall compositional impact on the larger receiving stream and river. Our study included a natural leaching experiment in which precipitation passing through oak, pine, and grass litter and duff samples was collected after each of a series of storms. Also included were soil trench samples to capture subsurface lateral flow, stream samples along with point-source reservoir inputs, and samples of canopy throughfall, stemflow, and gopher hole (bypass) flow. The litter/duff leaching study demonstrated changing DOM fractionation patterns throughout the season, as evidenced by changing lignin compositions in the leachates with each successive storm. This adds a necessary seasonal component to interpreting lignin compositions in streams, as the source signatures are constantly changing. Released DOM from leaching was modified extensively during transit through the subsurface to the stream, with preferential increases in aromaticity as evidenced by increases in carbon-normalized absorbance at 254 nm, yet preferential decreases in lignin phenols, as evidence by carbon-normalized lignin yields in the headwater stream that was less than half that of the litter/duff leachates. Our extensive number of lignin measurements for source materials reveals a much more complex perspective on using lignin as a source indicator, as many riverine values for syringyl:vanillyl and cinnamyl:vanillyl ratios that have previously been interpreted as degraded lignin signatures are also possible as unmodified source signatures. Finally, this study demonstrated that the impact of numerous small headwater streams can significantly overprint the DOM signatures of much larger rivers over relatively short distances spanning several to tens of kilometers. This finding in particular challenges the assumption that river studies can be adequately conducted by focusing only on the main tributaries.

Dissolved Organic Matter Compositional Change and Biolability During Two Storm Runoff Events in a Small Agricultural Watershed

“…Collectively, these findings suggest that rainfall patterns, spatially shifting source signatures, and flow path all strongly influenced DOM composition and concentration. Moreover, the observed changes in DOM composition reflect the response of different segments of the watershed, rather than an overprinting of upstream DOM source signatures by those of downstream land uses (Eckard et al, ; Hernes et al, ).…”

Where Carbon Goes When Water Flows: Carbon Cycling across the Aquatic Continuum

“…However, processes such as selective leaching of plantderived compounds from litterfall and soils (Hernes et al, 2007) or sorption/desorption of organic molecules in transit (Aufdenkampe et al, 2001) complicate interpretations of these signatures. For example, differences in the ratio of specific lignin phenols (i.e., common terrestrial plant biomarkers) are often attributed to differences in vegetation sources and degradation state, but these signatures can be greatly altered by selective fractionation while moving through the aforementioned flow paths (Hernes et al, 2017). Development of robust biomarker proxies for organic matter provenance, prior processing, and paleoclimate requires a quantitative understanding of how organic molecules evolve along the entire terrestrial-aquatic continuum.…”

Convergence of Terrestrial Dissolved Organic Matter Composition and the Role of Microbial Buffering in Aquatic Ecosystems

“…Journal of Geophysical Research: Biogeosciences carbonate (0.01% w/v) was added to all treatments to buffer pH due to respiration. Prior studies involving these plant tissues indicate that both inorganic and organic nitrogen as well as phosphate are also leached in significant quantities (Chow et al, 2009;Hernes, Spencer, et al, 2017); thus, no further nutrient amendments were made.…”

“…As plant species distribution shifts, so too will litter‐derived biomolecules that enter neighboring rivers and streams, altering the sources of dissolved organic matter (DOM) that provide energy to heterotrophic bacteria and drive the base of the aquatic food web. Since the distinct chemical signatures of plant litters are reflected in the DOM that is leached (Hernes et al, ; Textor et al, ), climate change‐driven plant succession has the potential to significantly impact adjacent aquatic systems. Yet once DOM is released from plants, early diagenetic processes (both microbially mediated and photochemically assisted; Obernosterer & Benner, ) rapidly and extensively alter its initial compositional signature (Benner & Kaiser, ; Hansen et al, ).…”