How Shoreline Vegetation Protects Sediment-Bound Carbon
A new study investigates the mechanisms and pace of carbon processing at the terrestrial-aquatic interface of a major river corridor.
Soils and nearshore sediments comprise a reservoir of carbon (C) 3.2 times larger than all the carbon stored in the atmosphere. Terrestrial carbon (e.g., from falling leaves and roots growing underground) is increasingly transported into aquatic systems due to significant changes in how land is used as the population increases, but little is known about the processing of carbon along terrestrial-to-aquatic continuums.
A new study led by ecologists Emily Graham and James Stegen at the Pacific Northwest National Laboratory takes a closer look at how carbon inputs along the terrestrial-aquatic interface change the mechanisms and pace of carbon processing. Their research also sheds light on how some of the carbon along shorelines remains in place for millennia.
This research provides ultrahigh-resolution data to infer new mechanisms of carbon oxidization along a terrestrial-aquatic boundary. The work will help protect watersheds by providing the underpinnings for a new conceptualization of biogeochemical function within models used to predict how river corridors function.
A bird’s eye view of the Columbia River in southeastern Washington State reveals varied ecological conditions ranging from dense vegetation to dry, rocky shoreline, and this variability leads to disparities in carbon inputs. In this study, researchers compared the amount of carbon contained within sediments, the rate of metabolism, and the metabolic pathways associated with carbon loss in each type of terrain.
Contrary to the prevailing “priming” paradigm of carbon loss in soils, the data indicate that vegetation “protects” the bound carbon already in nearshore sediments. Researchers learned that water-soluble and thermodynamically favorable organic carbon (OC) protects bound OC from oxidation in densely vegetated areas—presumably because it is easier to break down than the bound OC. Areas with sparse vegetation were more likely to metabolize bound OC, likely leading to the loss of carbon from longer-term stored carbon pools. A unifying principle in both environments, however, seems to be the use of thermodynamically favorable carbon as a preferred substrate pool, providing a starting point for modelling the influences of carbon character in heterogeneous landscapes.
“Another interesting data point is that contrasting metabolic pathways oxidize OC in the presence versus absence of vegetation,” said Graham. “Put simply, we have two different environments with distinct C inputs, C pools, and microbial communities. Each microbial community adapts to the resources available in their local environment and processes the C that returns the most energy back to them.”
These important discoveries are just the tip of the iceberg, Graham and Stegen say. More studies are needed to understand and model the patterns of carbon loss in changing land conditions.
Pacific Northwest National Laboratory
Pacific Northwest National Laboratory
This research was supported by the Subsurface Biogeochemical Research (SBR) program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, as part of SBR’s Scientific Focus Area (SFA) at the Pacific Northwest National Laboratory (PNNL). This research was performed using Institutional Computing at PNNL
Graham, E., et al. “Carbon inputs from riparian vegetation limit oxidation of physically bound organic carbon via biochemical and thermodynamic processes.” Journal of Geophysical Research: Biogeosciences 122(12), 3188–3205 (2017). [DOI:10.1101/105486].