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Prized wildlife habitat may also benefit water quality

Already cherished as wildlife habitat, red oak bottomlands in the Mississippi River floodplain may offer another benefit to the environment and to people. These forested wetlands accrue significant amounts of nitrogen flowing downstream, says new research, indicating they could help remove nitrogen from the river and reduce the load entering the Gulf of Mexico.

The study, which was recently published in the Soil Science Society of America Journal, was conducted by a team of scientists and graduate students in the College of Forest Resources at Mississippi State University (MSU).

Researcher standing in flooded forest

The idea that wetlands trap nutrients is nothing new. But precisely how much nitrogen accumulates in the bottomlands of the Mississippi Alluvial Valley (MAV)—known as the Delta at its southern end—hasn’t been well studied, says soil scientist Jeff Hatten (now at Oregon State University), who led the MSU research.

In the group’s investigation, nitrogen derived from eroded soil moving downriver accounted for about 20% of the total nitrogen inputs to the red oak bottomland sites. In other words, eroded soil appears to be a major nitrogen source for these ecosystems, Hatten says.

What this suggests is that restoration of red oak bottomland forests in states like Louisiana, Mississippi, Missouri, Arkansas, and Tennessee, could help improve Gulf of Mexico water quality—although curbing erosion and nutrient loss from farms is still the best way to cut the nitrogen load in the Gulf.

“This isn’t going to be a miracle fix-all,” Hatten says. “But,” he adds, “it would help.”

The findings also offer an additional reason to conserve these ecosystems. Historically, bottomlands in the Mississippi River floodplain stretched across 8.5 million hectares, but today approximately 80% of them have been cleared for agriculture and development. Remaining forests, especially those dominated by red oaks, are now invaluable sources of hardwood timber, as well as prized habitat for migrating and wintering waterfowl. In fact, Hatten and his colleagues set out originally to examine the factors controlling production of red oak acorns—an important fall and winter food for ducks. Only later did they decide to broaden the purpose.

“This study looked at what other ecosystem services these sites, which are already important to wildlife, are providing,” Hatten says. “We measured nitrogen, carbon, sediment deposition, and the soil characteristics to see if anything popped out.”

Red oak bottomland forest

The team worked in six red oak-dominated sites in five states, including three sites that were well-connected to the Mississippi River and its tributaries, and three that were cut off from river flow by dams and levees.

They also measured the %carbon (%C) and C:N ratio of organic matter deposited in the sites to distinguish two types: Material that originated as eroded sediment upstream, and material that floated in from elsewhere in the floodplain.

What the scientists found is that sediments deposited in the sites cut off from local rivers contained fresh organic matter, such as fallen leaves or twigs, with high %C values and high C:N ratios. Fresh organic matter is rich in carbon, Hatten explains, because microbes have had little time to decompose the carbon.

In contrast, sediments deposited in the well-connected red oak bottomland sites tended to have low %C values and C:N ratios, suggesting these sediments came from upstream and were breaking down in the river for some time. The three bottomland sites with connectivity to the rivers—and, thus, greater inundation by floodwaters—also derived more of their deposited nitrogen from sediment from upriver than did sites with little or no connection.

“So by restoring the connections to the rivers,” Hatten says, “we can potentially mitigate nitrogen in the watershed even more.”

Before that conclusion can be solidly drawn, however, other questions need attention. First, Hatten and his colleagues examined red oak-dominated sites because of their value to waterfowl. But to know the bottomlands’ true capacity to capture nitrogen, the MAV’s other diverse forest types need to be characterized as well, such as willow- or cottonwood-dominated riverfront sites, white oak and elm-ash-sugarberry mixtures on flats, and cypress-tupelo swamps, to name a few.

What happens to the nitrogen after it’s deposited also needs study. If the river carries nitrogen onto the bottomlands only to wash it downstream again with the next flood, little is gained. “But if that material is mineralized and then denitrified [to nitrogen gas], we could potentially see an effect on the nitrogen getting down to the Gulf,” Hatten says.

Another caveat, he adds, is that while more frequent and longer flooding of red oak bottomlands increased the amount of deposited nitrogen, this also reduced the production of acorns for ducks. “So, it’s a little bit of a conundrum,” he cautions. That said, Hatten hopes scientists, land managers, and the public will begin paying serious attention to all that ecosystems have to offer, instead of focusing on one pet benefit at a time.

“Everyone has their own charge in mind. Some people are worried about ducks, some people are worried about water quality, and they’re often two different segments of the population,” he says. “But if you can draw from both those segments, then you get a broad base for acquiring the resources to protect these areas and restore them.”

Funding for this project was provided by the U.S. Forest Service, the U.S. Fish and Wildlife Service, and the Forest and Wildlife Research Center at Mississippi State University.