Nitrogen Boosts Wetland Reed

The common reed, which is invasive in North American wetlands, is flourishing thanks to its appetite for heightened nitrogen levels, US scientists have concluded

The result suggests that the reed, known as  Phragmites australis , will continue to thrive as nitrogen fertilizers are flushed from farms, lawns and other sources upstream. But its rapid growth may also have a benefit, the scientists say, as the species’ biomass could elevate soils, protecting against future rises in sea levels.

"It is clear from our results and others that  Phragmites  is able to capitalize on nitrogen more strongly than common native saltmarsh species," said Joshua Caplan of Bryn Mawr College, US. "If we want to retain native marsh communities, it’s going to be hard to do without reducing the amount of nitrogen coming from the watersheds upstream. That said, there may be places where it won’t make sense to try and maintain native marshes, because they will eventually be drowned by rising seas."

Phragmites  is thought to have been introduced to North American wetlands from Eurasia in the 1800s. In recent decades it has become highly invasive. Given rising concentrations of carbon dioxide and nitrogen – both of which act as fertilizers – Caplan and colleagues wanted to find out how the presence of the reed would affect the accumulation of biomass in coastal regions in years to come.

Led by Thomas Mozdzer of Bryn Mawr College, the researchers built 12 clear chambers over a tidal marsh in Edgewater in Maryland, US, with each containing about one-third  Phragmites . The chambers were either left alone, or given increased levels of carbon dioxide, nitrogen, or both. By measuring the size of the plants at the end of the growing season, the researchers calculated the net primary productivity (NPP) – essentially the amount of carbon retained by the plant through a year.

Next, Caplan and colleagues used measurements from the field to build a computer simulation of  Phragmites  growth. Based on that simulation’s output, they calculated the gross primary productivity (GPP) – the amount of carbon taken in by the plants before some of it is later respired away.

In the control chambers, and in those provided with extra carbon dioxide,  Phragmites  had an NPP twice as great as native marsh vegetation. In the chambers with added nitrogen, however, the reed’s NPP was more than three times greater.

Under all conditions, the NPP was about half the GPP, which is typical of plants. This indicated that the NPP was high simply because the overall GPP was high, not because  Phragmites  had an improved ability to retain carbon. The GPP appeared to be particularly high at the beginning and at the end of the normal growing season.

The results present a dilemma for coast management, Caplan believes. On the one hand, controlling the accelerating spread of  Phragmites  is necessary to conserve native plant and animal communities. On the other hand, a boosted production of biomass is known to raise soil levels, potentially offering a defence against rising sea levels.

"We didn't look at data on rates of marsh surface rise directly, so we can’t (yet) say conclusively that  Phragmites  is capable of preventing marsh loss," said Caplan. "It does look likely though."

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