The short version
One hope for restoring old cranberry bogs is that the new wetland will strip nitrogen out of the water before it reaches the coast. Researchers from the University of Connecticut and the Woodwell Climate Research Center tested where that happens in two restored bogs on Falmouth’s Coonamessett River. Soils at groundwater seeps, where nitrate-rich groundwater bubbles up, had by far the highest capacity to remove nitrogen, followed by low, wet, plant-rich spots. Releases of nitrous oxide, a greenhouse gas, were mostly small. But by the authors’ rough estimate, the restored bogs removed well under 1% of the nitrogen the river carried.
What the researchers did
The study took place in the Upper (3.8 ha) and Middle (6.3 ha) bogs, farmed for more than a century, retired in 2013 and restored before the 2020 growing season with a new winding channel, filled ditches and deliberately bumpy ground.
- In 20 random plots they sampled soil and plants at the highest and lowest point, in August of the second and third growing seasons after restoration.
- They found four groundwater seeps along side channels with a thermal camera and paired each with a nearby non-seep spot.
- In the lab they measured each soil’s potential to convert nitrate to gas (denitrification), and in the field they measured nitrous oxide coming off the ground.
What they found
- Seeps are hot spots. Seep soils had about ten times the nitrogen-removal potential of the bog surface and 100–300 times more nitrate than typical low, wet soils, because groundwater delivers the nitrate right there.
- Low spots beat high spots. The lowest, wettest points had more than twice the removal potential of the highest points, and removal rose with the number of plant species.
- Little greenhouse gas in the field. Lab tests suggested much of the gas made could be nitrous oxide, but actual emissions were small across the bogs. They were higher at seeps but varied widely, and seeps cover only a small area.
- Sandy soils hold it back. Decades of sanding left 35 cm to more than a meter of sand over the old wetland soil. Low soil carbon limited the microbes, and even seep rates were about ten times lower than in a natural wetland with organic soils.
- A small dent so far. The authors estimate the bogs removed 14–20 kg of nitrogen a year this way, much less than 1% of the roughly 6,000 kg of inorganic nitrogen flowing through the restored reach. Even if every square meter worked like a seep, it would be under 5%.
What it means for Waquoit Bay today
The Coonamessett drains to Great Pond, not Waquoit Bay, but the same kind of restoration is planned or under way on our side, including Red Brook and the Childs River, and old bogs line the Quashnet. This paper fits with early river data from the same project, which did not show the restored reach lowering the river’s nitrogen. The authors conclude that bog restoration on its own is likely to remove only a relatively small portion of total watershed N loading
(Klionsky et al., 2024).
These projects still bring back cold-water fish habitat and native wetlands, and removal may grow as soils build up carbon. The authors also suggest designs that keep groundwater in contact with low, plant-rich ground for longer. But for Waquoit Bay’s nitrogen problem, restored bogs can help modestly; they don’t replace sewering and better septic systems. See also our summary of the Red Brook culvert feasibility study.
Read the paper
Klionsky, S. M., Neill, C., Helton, A. M., & Lawrence, B. (2024). Groundwater seeps are hot spots of denitrification and N2O emissions in a restored wetland. Biogeochemistry, 167(8), 1041–1056. https://doi.org/10.1007/s10533-024-01156-w
License: This article is open access under a Creative Commons Attribution 4.0 (CC BY 4.0) license. The PDF below is reproduced unmodified from the publisher, Springer Nature. The card image is the article’s first page.
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