Nav: Home

Study says salt marshes have limited ability to absorb excess nitrogen

November 28, 2016

Add fertilizer to your garden and your plants will probably grow bigger and taller. Add fertilizer to a salt marsh and the plants may not get any bigger at all. That's according to a new study led by Dr. David Samuel Johnson of William & Mary's Virginia Institute of Marine Science.

The study results have important implications for the management and care of salt marsh habitats. These coastal resources have long thought to be "nutrient sponges" that soak up excess nitrogen through greater plant growth and other means, thus helping to prevent low-oxygen dead zones, fish kills, and harmful algal blooms in nearby waters. Lack of enhanced plant growth in the current study throws this assumption into doubt.

Says Johnson, "Our work underscores that we can't simply rely on salt marshes to clean up nutrient pollution. We need to do better job at keeping nutrients out of the water in the first place." Sources of excess nutrients include wastewater and runoff of agricultural and lawn fertilizers. Their presence in coastal waters can lead to over-fertilization or what scientists call "eutrophication."

Johnson and his team, including colleagues from Connecticut College, the Woods Hole Research Center, and Bryn Mawr College studied coastal eutrophication by conducting an unprecedented experiment in which they flooded football-fields worth of salt marsh in northeastern Massachusetts with fertilizer-rich water for almost a decade.

Scott Warren, a professor at Connecticut College and study co-author, says "When we were able to mimic a eutrophied estuary at an ecosystem scale--quite a challenge I must add--we found that salt marshes did not respond as you might have predicted from fertilization experiments done over the past half a century or so."

Despite the abundant supply of nitrogen, a key plant nutrient, plants in the fertilized marshes didn't grow much bigger than those in unfertilized marshes. "We were surprised at the mild responses, even after almost a decade of fertilization," says Johnson. Earlier salt marsh studies reported plants growing larger in response to adding fertilizer. Previous studies also found that fertilizer changed species composition, causing some species to outcompete others. "The species composition didn't budge during the entire experiment," Johnson says.

The mild response of plants doesn't mean that salt marshes are safe from eutrophication, however. Johnson notes that when it comes to understanding eutrophication's impact on salt marshes, the answer may lie beneath the surface. In an earlier paper from the same field study, published in Nature, the research team found that fertilizer treatments caused the marsh edges to collapse and erode away. Again, this is opposite of what they had predicted.

"We hypothesized that the grass would grow taller, which would trap more sediment and help the marsh grow," says Johnson. Instead they found that plants in fertilized marshes had fewer roots and rhizomes than those in non-fertilized ones, which may have contributed to the collapse.

One reason the team's results differed from previous studies may be their choice of fertilizer. "We used nitrate fertilizer, which is the most common form of nitrogen in eutrophied estuaries," says Johnson. "Much of the previous work used ammonium fertilizer. Those studies had different questions than ours; they weren't specifically looking to understand eutrophication." Wetland plants prefer ammonium to nitrate because it takes less energy to process, so bigger plants with application of ammonium would not be unexpected.

Another reason the plants may not have responded strongly was the way the fertilizer was delivered--with flooding tidal water, which meant that less fertilizer reached the plants compared to previous studies that had added fertilizer directly to the marsh surface.

Their work is published online in Ecological Applications. Funding for the research comes from the National Science Foundation, the Northeast Climate Science Center, and the Jean C. Tempel Professorship in Botany at Connecticut College.
-end-


Virginia Institute of Marine Science

Related Nitrogen Articles:

Fixing the role of nitrogen in coral bleaching
A unique investigation highlights how excess nitrogen can trigger coral bleaching in the absence of heat stress.
Universities release results on nitrogen footprints
Researchers have developed a large-scale method for calculating the nitrogen footprint of a university in the pursuit of reducing nitrogen pollution, which is linked to a cascade of negative impacts on the environment and human health, such as biodiversity loss, climate change, and smog.
A battery prototype powered by atmospheric nitrogen
As the most abundant gas in Earth's atmosphere, nitrogen has been an attractive option as a source of renewable energy.
Northern lakes respond differently to nitrogen deposition
Nitrogen deposition caused by human activities can lead to an increased phytoplankton production in boreal lakes.
Researchers discover greenhouse bypass for nitrogen
An international team discovers that production of a potent greenhouse gas can be bypassed as soil nitrogen breaks down into unreactive atmospheric N2.
Bacterial mechanism converts nitrogen to greenhouse gas
Cornell University researchers have discovered a biological mechanism that helps convert nitrogen-based fertilizer into nitrous oxide, an ozone-depleting greenhouse gas.
Going against the grain -- nitrogen turns out to be hypersociable!
Nitrogen is everywhere: even in the air there is four times as much of it as oxygen.
Soybean nitrogen breakthrough could help feed the world
Washington State University biologist Mechthild Tegeder has developed a way to dramatically increase the yield and quality of soybeans.
Trading farmland for nitrogen protection
Excess nitrogen from agricultural runoff can enter surface waters with devastating effects.
Measure of age in soil nitrogen could help precision agriculture
What's good for crops is not always good for the environment.

Related Nitrogen Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Digital Manipulation
Technology has reshaped our lives in amazing ways. But at what cost? This hour, TED speakers reveal how what we see, read, believe — even how we vote — can be manipulated by the technology we use. Guests include journalist Carole Cadwalladr, consumer advocate Finn Myrstad, writer and marketing professor Scott Galloway, behavioral designer Nir Eyal, and computer graphics researcher Doug Roble.
Now Playing: Science for the People

#529 Do You Really Want to Find Out Who's Your Daddy?
At least some of you by now have probably spit into a tube and mailed it off to find out who your closest relatives are, where you might be from, and what terrible diseases might await you. But what exactly did you find out? And what did you give away? In this live panel at Awesome Con we bring in science writer Tina Saey to talk about all her DNA testing, and bioethicist Debra Mathews, to determine whether Tina should have done it at all. Related links: What FamilyTreeDNA sharing genetic data with police means for you Crime solvers embraced...