Changes in Ecosystem Nitrogen and Carbon Allocation with Black Mangrove ( Avicennia germinans ) Encroachment into Sparti
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Changes in Ecosystem Nitrogen and Carbon Allocation with Black Mangrove (Avicennia germinans) Encroachment into Spartina alterniflora Salt Marsh Aaron Macy,1,2,6*
Michael J. Osland,3 Just Cebrian1,2,5
Julia A. Cherry,4
and
1
Dauphin Island Sea Laboratory, Marine Environmental Sciences Consortium, Dauphin Island, Alabama 36528, USA; 2Department of Marine Sciences, University of South Alabama, Mobile, Alabama 36688, USA; 3U.S. Geological Survey, Wetland and Aquatic Research Center, Lafayette, Louisiana 70506, USA; 4Department of Biological Sciences, University of Alabama, New College, Box 870229, Tuscaloosa, Alabama 35487, USA; 5Northern Gulf Institute, Mississippi State University, 1021 Balch Blvd., Stennis Space Center, Starkville, Mississippi 39529, USA; 6Present address: Coastal Research and Extension Center, Mississippi State University, 1815 Popps Ferry Rd, Biloxi, Mississippi 39532, USA
ABSTRACT Increases in temperature are expected to facilitate encroachment of tropical mangrove forests into temperate salt marshes, yet the effects on ecosystem services are understudied. Our work was conducted along a mangrove expansion front in Louisiana (USA), an area where coastal wetlands are in rapid decline due to compounding factors, including reduced sediment supply, rising sea level, and subsidence. Marsh and mangrove ecosystems are each known for their ability to adjust to sealevel rise and support numerous ecosystem services, but there are some differences in the societal benefits they provide. Here, we compare carbon and nitrogen stocks and relate these findings to the expected effects of mangrove encroachment on nitrogen filtration and carbon sequestration in coastal wetlands. We specifically evaluate the
Received 22 November 2019; accepted 11 September 2020 Authors Contributions AM, MJO, JAC, JC conceived or designed study; AM performed research; AM, JC analyzed data; JAC, MJO contributed new methods or models; AM, MJO, JAC, JC wrote the paper. *Corresponding author; e-mail: [email protected]
implications of black mangrove (Avicennia germinans) encroachment into Spartina alterniflora-dominated salt marsh. Our results indicate that black mangrove encroachment will lead to increased aboveground carbon and nitrogen stocks. However, we found no differences in belowground (that is, root and sediment) nitrogen or carbon stocks between marshes and mangroves. Thus, the shift from marsh to mangrove may provide decadalscale increases in aboveground nitrogen and carbon sequestration, but belowground nitrogen and carbon sequestration (that is, carbon burial) may not be affected. We measured lower pore water nitrogen content beneath growing mangroves, which we postulate may be due to greater nitrogen uptake and storage in mangrove aboveground compartments compared to marshes. However, further studies are needed to better characterize the implications of mangrove encroachment on nitrogen cycling, storage, and export to the coastal ocean.
A. Macy and others Key words: Wetlands; Coastal; Sequestration; Louisiana; Tropicali
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