Urbanization can switch floodplain wetland soils from a sink to a source of greenhouse gases

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This article is part of the Fall 2015 issue of the Earth Science Matters Newsletter. 

dry floodplain in Difficult Run, Virginia

Floodplain soils in Difficult Run, Virginia are losing soil carbon through emissions of carbon dioxide to the atmosphere in this urbanizing watershed.  The floodplain is dry most of the year, as seen here, with occasional and brief flooding.

(Credit: Greg Noe, USGS. Public domain.)

Carbon cycling and sequestration in ecosystems helps regulate climate and offset anthropogenic carbon emissions.  Soils store a large proportion of the Earth’s carbon, and disturbances can switch soils from a carbon sink (where inputs are greater than emissions) to a carbon source (where emissions are greater than inputs).  Wetland soils, in particular, hold very large amounts of carbon.  Wetland soils are often large sinks of atmospheric carbon dioxide (CO2), reducing greenhouse gas effects on climate because of inputs from plant growth and slow release from decomposition in waterlogged soils.   However, some wetlands also release the potent greenhouse gases methane (CH4) and nitrous oxide (N2O), possibly offsetting the climate benefits of CO2 uptake.

The role of floodplain soils in carbon cycling and sequestration are less well understood compared to other wetlands.  Floodplains are widespread along streams and rivers and could potentially intercept carbon eroded from watersheds.  Prior studies suggested that floodplain soils in the U.S. could have very large rates of carbon sequestration through sedimentation inputs, but great uncertainty exists on the magnitude, sources, and fate of that carbon.  Greater inputs of carbon from sedimentation could be offset by increased rates of greenhouse gas release from floodplain soils.  The balance of carbon inputs and outputs from floodplain soils could also be disrupted by changing land use in watersheds, including urbanization.

flooded soils in Difficult Run, Virginia

Floodplain soils in Difficult Run, Virginia are losing soil carbon through emissions of carbon dioxide to the atmosphere in this urbanizing watershed.  The floodplain is dry most of the year with occasional and brief flooding, as seen in this image.

(Credit: Greg Noe, USGS. Public domain.)

U. S. Geological Survey researchers recently published results of a new study that aimed to measure the potential for carbon sequestration in floodplain soils and to contrast carbon inputs from sedimentation and vegetation with losses from greenhouse gas release and bank erosion.  Carbon inputs, outputs, and storage from floodplain soils were measured throughout Difficult Run, a suburban watershed in the Washington, DC metro region that is part of the Piedmont region of the Chesapeake Bay watershed.  Annual rates of vegetative litterfall production, sedimentation, bank erosion, soil CO2, CH4, and N2O gas emissions, and soil carbon pool size were measured from the headwaters to the mouth of the watershed.

The researchers found that soil CO2, CH4, and N2O emissions did not increase in proportion to carbon input from sedimentation, suggesting that floodplains with greater sedimentation are capable of sequestering more carbon. However, soil CO2 releases were greater than soil carbon inputs from sedimentation and vegetation, indicating that this floodplain is losing soil carbon regardless of the sedimentation rate. On average, the floodplain is releasing 2% of stored soil carbon to the atmosphere per year. The loss of floodplain soil carbon is likely due to long-term drying from watershed urbanization as indicated by decreased stream water levels during low-flow periods over the past 80 years. This research highlights the potential impact of watershed urbanization on floodplain soil carbon cycling that can cause a reversal from carbon sequestration to a carbon source to the atmosphere.

This paper, published in Journal  of Geophysical Research: Biogeosciences, is available at  https://doi.org/10.1002/2014JG002817.

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