U.S. Department of Energy

Pacific Northwest National Laboratory

How Shoreline Vegetation Protects Sediment-Bound Carbon

A graphic from the paper.

The Science

Soils and nearshore sediments comprise a reservoir of carbon (C) 3.2 times larger than all the carbon stored in the atmosphere. Terrestrial C (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 C 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 C inputs along the terrestrial-aquatic interface change the mechanisms and pace of C processing. Their research also sheds light on how some of the C along shorelines remains in place for millennia. 

The Impact

This research provides ultra-high resolution data to infer new mechanisms of C 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.

Summary

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 C inputs. In this study, researchers compared the amount of C contained within sediments, the rate of metabolism, and the metabolic pathways associated with C loss in each type of terrain. 

Contrary to the prevailing ‘priming’ paradigm of C loss in soils, the data indicates 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 C from longer-term stored C pools. A unifying principle in both environments, however, seems to be the use of thermodynamically favorable C as a preferred substrate pool, providing a starting point for modelling the influences of C 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 C loss in changing land conditions.

Funding

This research was supported by the U.S. Department of Energy’s Office of Biological and Environmental Research (BER), as part of Subsurface Biogeochemical Research Program’s Scientific Focus Area (SFA) at the Pacific Northwest National Laboratory (PNNL). This research was performed using Institutional Computing at PNNL. 

Publications

E. Graham, et al. “Carbon inputs from riparian vegetation limit oxidation of physically-bound organic carbon via biochemical and thermodynamic processes.” JGR Biogeosciences. 2017 doi:10.1101/105486.

Related Links

https://www.pnnl.gov/science/highlights/highlight.asp?groupid=753&id=4589

https://phys.org/news/2016-12-microbial-hyporheic-zone.html

http://sbrsfa.pnnl.gov/docs/highlights/Highlight_Stegen_Microbial_Communities_11282016.pdf

http://sbrsfa.pnnl.gov/docs/highlights/Highlight_Stegen_10-8-2015.pdf

http://sbrsfa.pnnl.gov/docs/highlights/Highlight_Stegen_et_al_2015.pdf

Date: 
April 2017
| Pacific Northwest National Laboratory